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Da-Lin Zhang

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Prof. Zhang received professional training from the Department of Meteorology, The Pennsylvania State University where he obtained his M.S. in 1981 and Ph.D. in 1985. From 1986 to 1988, he worked as a postdoctoral fellow at the National Center for Atmospheric Research (NCAR), which is sponsored by the National Science Foundation. After spending one year in the University of Toronto, he took a faculty position in the Department of Atmospheric and Oceanic Sciences at McGill University . He joined the Department of Atmospheric and Oceanic Science of University of Maryland in September 1996. Prof. Zhang works on the modeling and understanding of fundamental processes taking place in squall lines, mesoscale convective complexes, hurricanes and heavy rain- (or snow-) storms, tropical and extratropical cyclones, gravity waves, frontal circulations and topographically generated weather phenomena. His research involves simulating a variety of different severe convective systems and cyclones

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Papers by Da-Lin Zhang

The effect of parameterized ice microphysics on the simulation of vortex circulation with a mesoscale hydrostatic model

Tellus A, Mar 1, 1989

On the rapid intensification of Hurricane <i>Wilma</i> (2005). Part IV: Inner‐core dynamics during the steady radius of maximum wind stage

Quarterly Journal of the Royal Meteorological Society, Oct 1, 2018

This is an open access article under the terms of the Creative Commons Attribution-NonCommercial ... more

The Impact of Moist Frontogenesis and Tropopause Undulation on the Intensity, Size, and Structural Changes of Hurricane Sandy (2012)

Journal of the Atmospheric Sciences, Mar 1, 2017

This study examines the relative roles of moist frontogenesis and tropopause undulation in determ... more This study examines the relative roles of moist frontogenesis and tropopause undulation in determining the intensity, size, and structural changes of Hurricane Sandy using a high-resolution cloud-resolving model. A 138-h simulation reproduces Sandy's four distinct development stages: (i) rapid intensification, (ii) weakening, (iii) steady maximum surface wind but with large continued sea level pressure (SLP) falls, and (iv) reintensification. Results show typical correlations between intensity changes, sea surface temperature, and vertical wind shear during the first two stages. The large SLP falls during the last two stages are mostly caused by Sandy's northward movement into lower-tropopause regions associated with an eastward-propagating midlatitude trough, where the associated lower-stratospheric warm air wraps into the storm and its surrounding areas. The steady maximum surface wind occurs because of the widespread SLP falls with weak gradients lacking significant inward advection of absolute angular momentum (AAM). Meanwhile, three spiral frontogenetic zones and associated rainbands develop internally in the outer northeastern quadrant during the last three stages when Sandy's southeasterly warm current converges with an easterly cold current associated with an eastern Canadian high. Cyclonic inward advection of AAM along each frontal rainband accounts for the continued expansion of the tropical storm-force wind and structural changes, while deep convection in the eyewall and merging of the final two surviving frontogenetic zones generate a spiraling jet in Sandy's northwestern quadrant, leading to its reintensification prior to landfall. The authors conclude that a series of moist frontogenesis plays a more important role than the lower-stratospheric warmth in determining Sandy's size, intensity, and structural changes.

A 6-yr Cloud-to-Ground Lightning Climatology and Its Relationship to Rainfall over Central and Eastern China

Journal of Applied Meteorology and Climatology, Dec 1, 2015

The cloud-to-ground (CG) lightning climatology and its relationship to rainfall over central and ... more The cloud-to-ground (CG) lightning climatology and its relationship to rainfall over central and eastern China is examined, using data from 32 million CG lightning flashes and Tropical Rainfall Measuring Mission measurements during a 6-yr period covering 2008-13. Results show substantial spatial and temporal variations of flash density across China. Flash counts are the highest (lowest) in summer (winter) with the lowest (highest) proportion of positive flashes. CG lightning over northern China is more active only in summer, whereas in winter CG lightning is more active only in the Yangtze River basin. The highest CG lightning densities, exceeding 9 flashes per kilometer squared per year and more than 70 CG lightning days per year, are found in the northern Pearl River delta region, followed by the Sichuan basin, the Yangtze River delta, and the southeastern coast of China in that order. Lower-flash-density days occur over mountainous regions as a result of the development of short-lived afternoon storms, while higher-flash-density days, typically associated with nocturnal thunderstorms, appear over the north China plain and Sichuan basin. The highest number of CG lightning flashes is found in August whereas monthly convective rainfall peaks in May or July. Flash rates during the warm season are typically maximized in the afternoon hours in coincidence with a convective rainfall peak except for the Sichuan basin and its surrounding mountainous areas where a single late-night convective rainfall peak dominates. Much less lightning activity corresponds to a late-night to morning rainfall peak over the plains in eastern China because of the increased proportion of stratiform rainfall during that period.

Diurnal Variations of Presummer Rainfall over Southern China

Journal of Climate, 2017

In this study, the presummer diurnal cycle of rainfall (DCR) over southern China is examined usin... more In this study, the presummer diurnal cycle of rainfall (DCR) over southern China is examined using the merged 0.18-resolution gridded hourly rain gauge and satellite rainfall dataset and the National Centers for Environmental Prediction Final Global Analysis during April to June of 2008-2015. Results show pronounced diurnal variations in rainfall amount, frequency, and intensity over southern China, with substantially different amplitudes from southwestern to southeastern China, and from the pre-to postmonsoon-onset period. Southwestern China often encounters significant nocturnal-to-morning rainfall under the influence of enhanced nocturnal low-level southwesterly winds. Southeastern China is dominated by afternoon rainfall, as a result of surface heating, likely aided by local topographical lifting. Both the pre-and postmonsoon-onset periods exhibit two diurnal rainfall peaks: one in the early morning and the other in the late afternoon. But the latter shows the two peaks with nearly equal amplitude whereas the former displays a much larger early morning peak than that in the late afternoon. Three propagating modes accounting for the presummer DCR are found: (i) an eastward-or southeastward-propagating mode occurs mostly over southwestern China that is associated with enhanced transport of warm and moist air from tropical origin and the induced low-level convergence, (ii) a quasi-stationary mode over southeastern China appears locally in the warm sector with weak-gradient flows, and (iii) an inlandpropagating mode occurs during the daytime in association with sea breezes along the southern coastal regions, especially evident throughout the postmonsoon-onset period.

Analysis of Missed Summer Severe Rainfall Forecasts

Weather and Forecasting, Feb 26, 2016

Despite considerable progress in mesoscale numerical weather prediction (NWP), the ability to pre... more Despite considerable progress in mesoscale numerical weather prediction (NWP), the ability to predict summer severe rainfall (SSR) in terms of amount, location, and timing remains very limited because of its association with convective or mesoscale phenomena. In this study, two representative missed SSR events that occurred in the highly populated Great Lakes regions are analyzed within the context of moisture availability, convective instability, and lifting mechanism in order to help identify the possible causes of these events and improve SSR forecasts/nowcasts. Results reveal the following limitations of the Canadian regional NWP model in predicting SSR events: 1) the model-predicted rainfall is phase shifted to an undesired location that is likely caused by the model initial condition errors, and 2) the model is unable to resolve the echotraining process because of the weakness of the parameterized convection and/or coarse resolutions. These limitations are related to the ensuing model-predicted features: 1) vertical motion in the areas of SSR occurrence is unfavorable for triggering parameterized convection and grid-scale condensation; 2) convective available potential energy is lacking for initial model spinup and later for elevating latent heating to higher levels through parameterized convection, giving rise to less precipitation; and 3) the conversion of water vapor into cloud water at the upper and middle levels is underpredicted. Recommendations for future improvements are discussed.

The Relationship Between Anomalous Presummer Extreme Rainfall Over South China and Synoptic Disturbances

Journal Of Geophysical Research: Atmospheres, Apr 6, 2018

A spectral analysis of daily rainfall data has been performed to investigate extreme rainfall eve... more A spectral analysis of daily rainfall data has been performed to investigate extreme rainfall events in south China during the presummer rainy seasons between 1998 and 2015 (excluding 1999, 2006, 2011, and 2014). The results reveal a dominant frequency mode at the synoptic scale with pronounced positive rainfall anomalies. By analyzing the synoptic-scale bandpass-filtered anomalous circulations, 24 extreme rainfall episodes (defined as those with a daily rainfall amount in the top 5%) are categorized into "cyclone" (15) and "trough" (8) types, with the remaining events as an "anticyclone" type, according to the primary anomalous weather system contributing to each extreme rainfall episode. The 15 cyclone-type episodes are further separated into (11) lower-and (4) upper-tropospheric migratory anomalies. An analysis of their anomalous fields shows that both types could be traced back to the generation of cyclonic anomalies downstream of the Tibetan Plateau, except for two episodes of lower-tropospheric migratory anomalies originating over the South China Sea. However, a lower-tropospheric cyclonic anomaly appears during all phases in the former type, but only in the wettest phase in the latter type, with its peak disturbance occurring immediately beneath an upper-level warm anomaly. The production of extreme rainfall in the trough-type episodes is closely related to a deep trough anomaly extending from an intense cyclonic anomaly over north China, which in turn could be traced back to a midlatitude Rossby wave train passing by the Tibetan Plateau. The results have important implications for understanding the origin, structure, and evolution of synoptic disturbances associated with the presummer extreme rainfall in south China.

Facilitating International Collaboration on Climate Change Research

Bulletin of the American Meteorological Society, May 1, 2020

and the United Kingdom presented research results on a variety of important topics related to atm... more

Ground-based radar reflectivity mosaic of mei-yu precipitation systems over the Yangtze River–Huaihe River basins

Advances in Atmospheric Sciences, Oct 19, 2016

The 3D radar reflectivity produced by a mosaic software system, with measurements from 29 operati... more The 3D radar reflectivity produced by a mosaic software system, with measurements from 29 operational weather radars in the Yangtze River-Huaihe River Basins (YRHRB) during the mei-yu season of 2007, is compared to coincident TRMM PR observations in order to evaluate the value of the ground-based radar reflectivity mosaic in characterizing the 3D structures of mei-yu precipitation. Results show reasonable agreement in the composite radar reflectivity between the two datasets, with a correlation coefficient of 0.8 and a mean bias of −1 dB. The radar mosaic data at constant altitudes are reasonably consistent with the TRMM PR observations in the height range of 2-5 km, revealing essentially the same spatial distribution of radar echo and nearly identical histograms of reflectivity. However, at altitudes above 5 km, the mosaic data overestimate reflectivity and have slower decreasing rates with height compared to the TRMM PR observations. The areas of convective and stratiform precipitation, based on the mosaic reflectivity distribution at 3-km altitude, are highly correlated with the corresponding regions in the TRMM products, with correlation coefficients of 0.92 and 0.97 and mean relative differences of −7.9% and −2.5%, respectively. Finally, the usefulness of the mosaic reflectivity at 3-km altitude at 6-min intervals is illustrated using a mesoscale convective system that occurred over the YRHRB.

Numerical simulations of local circulation and its response to land cover changes over the Yellow Mountains of China

Journal of Meteorological Research, Aug 1, 2015

In this study, local circulations and their responses to land use and land cover (LULC) changes o... more In this study, local circulations and their responses to land use and land cover (LULC) changes over the Yellow Mountains of China are examined by using Weather Research and Forecast (WRF) model simulations of a selected case under weak-gradient synoptic conditions. The results show that mountain-valley breezes over the region are characterized by an intense upslope flow lasting for about 11 h (0600-1700 LST) along the northern slope during daytime. A convergence zone occurs at the mountain ridge and moves northwestward with time. During nighttime, wind directions are reversed, starting first at higher elevations. Three sensitivity experiments are conducted, in which the current land covers are replaced by grassland, mixed forest, and bare soil, respectively, while keeping the other model conditions identical to a control run. These sensitivity simulations are designed to represent the changes of LULC over the Yellow Mountains area during the past decades. The results show that changes in land cover could affect substantially land-surface and atmosphere interactions, the evolution of local circulations, and characteristics of the planetary boundary layer (PBL). Significant differences are noted in horizontal winds, and sensible and latent heat fluxes. On the other hand, when the surface is covered by mixed forest, slight variations in local winds and surface variables are identified. The results appear to have important implications to urban planning and constructions as well as the transport of air pollutants over mountainous regions.

On the Rapid Intensification of Hurricane Wilma (2005). Part I: Model Prediction and Structural Changes

Weather and Forecasting, Dec 1, 2011

In this study, a 72-h cloud-permitting numerical prediction of Hurricane Wilma (2005), covering i... more In this study, a 72-h cloud-permitting numerical prediction of Hurricane Wilma (2005), covering its initial 18-h spinup, an 18-h rapid intensification (RI), and the subsequent 36-h weakening stage, is performed using the Weather Research Forecast Model (WRF) with the finest grid length of 1 km. The model prediction uses the initial and lateral boundary conditions, including the bogus vortex, that are identical to the Geophysical Fluid Dynamics Laboratory's then-operational data, except for the time-independent sea surface temperature field. Results show that the WRF prediction compares favorably in many aspects to the best-track analysis, as well as satellite and reconnaissance flight-level observations. In particular, the model predicts an RI rate of more than 4 hPa h 21 for an 18-h period, with the minimum central pressure of less than 889 hPa. Of significance is that the model captures a sequence of important inner-core structural variations associated with Wilma's intensity changes, namely, from a partial eyewall open to the west prior to RI to a full eyewall at the onset of RI, rapid eyewall contraction during the initial spinup, the formation of double eyewalls with a wide moat area in between during the most intense stage, and the subsequent eyewall replacement leading to the weakening of Wilma. In addition, the model reproduces the boundary layer growth up to 750 hPa with an intense inversion layer above in the eye. Recognizing that a single case does not provide a rigorous test of the model predictability due to the stochastic nature of deep convection, results presented herein suggest that it is possible to improve forecasts of hurricane intensity and intensity changes, and especially RI, if the innercore structural changes and storm size could be reasonably predicted in an operational setting using highresolution cloud-permitting models with realistic initial conditions and model physical parameterizations.

Impact of Cyclone‐Cyclone Interaction on Lake‐Effect Snowbands: A False Alarm

Journal Of Geophysical Research: Atmospheres, Jan 14, 2023

In this study, the impacts of two cyclone interactions with the environmental flow and with each ... more In this study, the impacts of two cyclone interactions with the environmental flow and with each other on a heavy winter snowfall event over the Great Lakes region are examined by applying a recently‐developed diagnostic tool to a lake‐effect snowstorm. This winter snowfall event was a false alarm by the Canadian operational model, in which the predicted lake‐effect snowfall at the lee side of Lake Erie doubles the observed. The false alarm involves the interactions of two cyclones, whose relative locations are poorly predicted, leading to inaccurate prediction of their induced winds and interactions over Lake Erie at both low and upper levels. In particular, at 1,000 hPa, the cyclone‐induced winds and their interactions over Lake Erie caused colder and drier air over warm‐moist open‐water lake surface that is favorable for more latent heat flux deriving from the lake in the model than in the real atmosphere. At 500 hPa, the location errors of the model predicted cyclones reversed the cyclone‐induced winds over Lake Erie from mainly southerly to northerly, especially during the intense snowfall period. An analysis of the conditional symmetric instability (CSI) index over Lake Erie reveals that the operational model systematically over‐predicts CSI, more significantly during the intense snowfall period. As a result, the operational model prediction makes a false alarm of a lake‐effect snowfall event with large errors in precipitation intensity. Future improvements in operational model predictions are also suggested.

北京地区雷电灾害天气系统的动力&LT;Bold&GT;-&LT;/Bold&GT;微物理&LT;Bold&GT;-&LT;/Bold&GT;电过程观测研究

Zhongguo kexue, Oct 15, 2020

SAFIR-3000 Lightning Statistics over the Beijing Metropolitan Region during 2005–07

Journal of Applied Meteorology and Climatology, 2016

In this study, the spatiotemporal characteristics of cloud-to-ground (CG) and intracloud (IC) lig... more In this study, the spatiotemporal characteristics of cloud-to-ground (CG) and intracloud (IC) lightning flashes observed by Surveillance et Alerte Foudre par Interférometrie Radioélectrique (SAFIR)-3000 over the Beijing metropolitan region (BMR) during 2005–07 were investigated. The results showed the presence of 299 lightning days with 241 688 flashes, most of which were IC lightning flashes. Only 19% of the total flashes were CG lightning flashes; 14% of these CG flashes were positive. Most lightning activity occurred during the summer months (June–August), with a major diurnal peak around 1900 Beijing standard time (BST) and a secondary peak around 2300 BST. Spatial variations in flash density and lightning days both exhibited an obvious southeastwardly increasing pattern, with higher flash densities or more lightning days occurring in the southeastern plains and lower values distributed on the northwestern mountains. The Z ratio (IC/CG lightning flashes) exhibited a similar spat...

The Relationship Between Anomalous Presummer Extreme Rainfall Over South China and Synoptic Disturbances

Journal of Geophysical Research: Atmospheres, 2018

A spectral analysis of daily rainfall data has been performed to investigate extreme rainfall eve... more A spectral analysis of daily rainfall data has been performed to investigate extreme rainfall events in south China during the presummer rainy seasons between 1998 and 2015 (excluding 1999, 2006, 2011, and 2014). The results reveal a dominant frequency mode at the synoptic scale with pronounced positive rainfall anomalies. By analyzing the synoptic‐scale bandpass‐filtered anomalous circulations, 24 extreme rainfall episodes (defined as those with a daily rainfall amount in the top 5%) are categorized into “cyclone” (15) and “trough” (8) types, with the remaining events as an “anticyclone” type, according to the primary anomalous weather system contributing to each extreme rainfall episode. The 15 cyclone‐type episodes are further separated into (11) lower‐ and (4) upper‐tropospheric migratory anomalies. An analysis of their anomalous fields shows that both types could be traced back to the generation of cyclonic anomalies downstream of the Tibetan Plateau, except for two episodes of...

Genesis of Tropical Storm Debby (2006) within an African Easterly Wave: Roles of the Bottom-Up and Midlevel Pouch Processes

Journal of the Atmospheric Sciences, May 27, 2015

The ''bottom up'' generation of low-level vortices (LVs) and midlevel vortices (MVs) during the g... more The ''bottom up'' generation of low-level vortices (LVs) and midlevel vortices (MVs) during the genesis of Tropical Storm Debby (2006) and the roles of a midlevel ''marsupial pouch'' associated with an African easterly wave (AEW) are examined using an 84-h simulation with the finest grid size of 1.33 km. Results show that several MVs are generated in leading convective bands and then advected rearward into stratiform regions by front-to-rear ascending flows. Because of different Lagrangian storm-scale circulations, MVs and LVs are displaced along different paths during the early genesis stages. MVs propagate cyclonically inward within the AEW pouch while experiencing slow intensification and merging under the influence of converging flows. The MVs' merging into a mesovortex is accelerated as they come closer to each other in the core region. In contrast, the low-level Lagrangian circulation is opened as a wave trough prior to tropical depression (TD) stage, so the LVs tend to ''escape'' from the pouch region. Only after the low-level flows become closed do some LVs congregate and contribute directly to Debby's genesis. The TD stage is reached when the midlevel mesovortex and an LV are collocated with a convective zone having intense low-level convergence. Results also show the roles of upper-level warming in hydrostatically maintaining the midlevel pouch and producing mesoscale surface pressure falls. It is found that the vertically tilted AEW with a cold dome below is transformed to a deep warm-core TD vortex by subsiding motion. A conceptual model describing the key elements in the genesis of Debby is also provided.

A Piecewise Potential Vorticity Inversion Algorithm and Its Application to Hurricane Inner-Core Anomalies

Journal of the Atmospheric Sciences, Aug 1, 2010

An Examination of the Pressure–Wind Relationship for Intense Tropical Cyclones

Weather and Forecasting, Jun 1, 2010

In this study, the dynamical constraints underlining the pressure-wind relationship (PWR) for int... more In this study, the dynamical constraints underlining the pressure-wind relationship (PWR) for intense tropical cyclones (TCs) are examined with the particular focus on the physical connections between the maximum surface wind (VMAX) and the minimum sea level pressure (PMIN). Use of the Rankine vortex demonstrates that the frictional forcing in the planetary boundary layer (PBL) could explain a sizeable portion of the linear contributions of VMAX to pressure drops. This contribution becomes increasingly important for intense TCs with small eye sizes, in which the radial inflows in the PBL could no longer be neglected. Furthermore, the inclusion of the tangential wind tendency can make an additional contribution to the pressure drops when coupled with the surface friction. An examination of the double-eyewall configuration reveals that the formation of an outer eyewall or wellorganized spiral rainbands complicates the PWR. An analysis of a cloud-resolving simulation of Hurricane Wilma (2005) shows that the outer eyewall could result in the continuous deepening of PMIN even with a constant VMAX. The results presented here suggest that (i) the TC size should be coupled with VMAX rather than being treated as an independent predictor as in the current PWRs, (ii) the TC intensity change should be at least coupled linearly with the radius of VMAX, and (iii) the radial wind in the PBL is of equal importance to the linear contribution of VMAX and its impact should be included in the PWR.

A Statistical Analysis of Steady Eyewall Sizes Associated with Rapidly Intensifying Hurricanes

Weather and Forecasting, May 20, 2016

It is well known that hurricane intensification is often accompanied by continuous contraction of... more It is well known that hurricane intensification is often accompanied by continuous contraction of the radius of maximum wind (RMW) and eyewall size. However, a few recent studies have shown rapid and then slow contraction of the RMW/eyewall size prior to the onset and during the early stages of rapid intensification (RI) of hurricanes, respectively, but a steady state in the RMW (S-RMW) and eyewall size during the later stages of RI. In this study, a statistical analysis of S-RMWs associated with rapidly intensifying hurricanes is performed using the extended best-track dataset during 1990-2014 in order to examine how frequently, and at what intensity and size, the S-RMW structure tends to occur. Results show that about 53% of the 139 RI events of 24-h duration associated with 55 rapidly intensifying hurricanes exhibit S-RMWs, and that the percentage of the S-RMW events increases to 69% when RI events are evaluated at 12-h intervals, based on a new RI rate definition of 10 m s 21 (12 h) 21 ; both results satisfy the Student's t tests with confidence levels of over 95%. In general, S-RMWs tend to appear more frequently in more intense storms and when their RMWs are contracted to less than 50 km. This work suggests a new fruitful research area in studying the RI of hurricanes with S-RMWs.

An analytical model for the rapid intensification of tropical cyclones

Quarterly Journal of the Royal Meteorological Society, Jun 18, 2009

The nonlinearity and complexity of the primitive equations have been key obstacles to our underst... more The nonlinearity and complexity of the primitive equations have been key obstacles to our understanding of tropical cyclones (TCs), particularly in relation to the dynamical processes leading to their rapid intensification. In this study, an axisymmetric model, in which all nonlinear terms in the horizontal momentum equations are retained, is used to examine analytically the effects of organized deep convection on TC rapid intensification. By prescribing a vertical profile of the vertical motion with exponential growth in the core region, a class of exact time-dependent solutions for the primary circulations of TCs are obtained. The analytical solutions are shown to capture well many observed dynamical structures in both the core and outer regions and the rapid growth of TCs in terms of maximum winds and central pressure drops. The analytical solutions reveal that (1) the rotational flows in the inner-core region grow double-exponentially, and the central pressure drops occur at rates much faster than the rotational growth; (2) the amplification rates of the primary circulations differ profoundly from those of the secondary circulations; (3) the rotational flows tend to grow from the bottom upwards with the fastest growth occurring at the lowest levels; and (4) the TC growth rates depend critically on the vertical structure of tangential flows, with a faster rate for a lower-level peak rotation. The nonlinear dynamics are shown to play an important role in the rapid growth of TCs. It is demonstrated that the analytical solutions can also be used to construct dynamically consistent vortices for the initialization of TC models. Limitations and possible improvements of the analytical model are also discussed.

The effect of parameterized ice microphysics on the simulation of vortex circulation with a mesoscale hydrostatic model

Tellus A, Mar 1, 1989

On the rapid intensification of Hurricane <i>Wilma</i> (2005). Part IV: Inner‐core dynamics during the steady radius of maximum wind stage

Quarterly Journal of the Royal Meteorological Society, Oct 1, 2018

This is an open access article under the terms of the Creative Commons Attribution-NonCommercial ... more

The Impact of Moist Frontogenesis and Tropopause Undulation on the Intensity, Size, and Structural Changes of Hurricane Sandy (2012)

Journal of the Atmospheric Sciences, Mar 1, 2017

This study examines the relative roles of moist frontogenesis and tropopause undulation in determ... more This study examines the relative roles of moist frontogenesis and tropopause undulation in determining the intensity, size, and structural changes of Hurricane Sandy using a high-resolution cloud-resolving model. A 138-h simulation reproduces Sandy's four distinct development stages: (i) rapid intensification, (ii) weakening, (iii) steady maximum surface wind but with large continued sea level pressure (SLP) falls, and (iv) reintensification. Results show typical correlations between intensity changes, sea surface temperature, and vertical wind shear during the first two stages. The large SLP falls during the last two stages are mostly caused by Sandy's northward movement into lower-tropopause regions associated with an eastward-propagating midlatitude trough, where the associated lower-stratospheric warm air wraps into the storm and its surrounding areas. The steady maximum surface wind occurs because of the widespread SLP falls with weak gradients lacking significant inward advection of absolute angular momentum (AAM). Meanwhile, three spiral frontogenetic zones and associated rainbands develop internally in the outer northeastern quadrant during the last three stages when Sandy's southeasterly warm current converges with an easterly cold current associated with an eastern Canadian high. Cyclonic inward advection of AAM along each frontal rainband accounts for the continued expansion of the tropical storm-force wind and structural changes, while deep convection in the eyewall and merging of the final two surviving frontogenetic zones generate a spiraling jet in Sandy's northwestern quadrant, leading to its reintensification prior to landfall. The authors conclude that a series of moist frontogenesis plays a more important role than the lower-stratospheric warmth in determining Sandy's size, intensity, and structural changes.

A 6-yr Cloud-to-Ground Lightning Climatology and Its Relationship to Rainfall over Central and Eastern China

Journal of Applied Meteorology and Climatology, Dec 1, 2015

The cloud-to-ground (CG) lightning climatology and its relationship to rainfall over central and ... more The cloud-to-ground (CG) lightning climatology and its relationship to rainfall over central and eastern China is examined, using data from 32 million CG lightning flashes and Tropical Rainfall Measuring Mission measurements during a 6-yr period covering 2008-13. Results show substantial spatial and temporal variations of flash density across China. Flash counts are the highest (lowest) in summer (winter) with the lowest (highest) proportion of positive flashes. CG lightning over northern China is more active only in summer, whereas in winter CG lightning is more active only in the Yangtze River basin. The highest CG lightning densities, exceeding 9 flashes per kilometer squared per year and more than 70 CG lightning days per year, are found in the northern Pearl River delta region, followed by the Sichuan basin, the Yangtze River delta, and the southeastern coast of China in that order. Lower-flash-density days occur over mountainous regions as a result of the development of short-lived afternoon storms, while higher-flash-density days, typically associated with nocturnal thunderstorms, appear over the north China plain and Sichuan basin. The highest number of CG lightning flashes is found in August whereas monthly convective rainfall peaks in May or July. Flash rates during the warm season are typically maximized in the afternoon hours in coincidence with a convective rainfall peak except for the Sichuan basin and its surrounding mountainous areas where a single late-night convective rainfall peak dominates. Much less lightning activity corresponds to a late-night to morning rainfall peak over the plains in eastern China because of the increased proportion of stratiform rainfall during that period.

Diurnal Variations of Presummer Rainfall over Southern China

Journal of Climate, 2017

In this study, the presummer diurnal cycle of rainfall (DCR) over southern China is examined usin... more In this study, the presummer diurnal cycle of rainfall (DCR) over southern China is examined using the merged 0.18-resolution gridded hourly rain gauge and satellite rainfall dataset and the National Centers for Environmental Prediction Final Global Analysis during April to June of 2008-2015. Results show pronounced diurnal variations in rainfall amount, frequency, and intensity over southern China, with substantially different amplitudes from southwestern to southeastern China, and from the pre-to postmonsoon-onset period. Southwestern China often encounters significant nocturnal-to-morning rainfall under the influence of enhanced nocturnal low-level southwesterly winds. Southeastern China is dominated by afternoon rainfall, as a result of surface heating, likely aided by local topographical lifting. Both the pre-and postmonsoon-onset periods exhibit two diurnal rainfall peaks: one in the early morning and the other in the late afternoon. But the latter shows the two peaks with nearly equal amplitude whereas the former displays a much larger early morning peak than that in the late afternoon. Three propagating modes accounting for the presummer DCR are found: (i) an eastward-or southeastward-propagating mode occurs mostly over southwestern China that is associated with enhanced transport of warm and moist air from tropical origin and the induced low-level convergence, (ii) a quasi-stationary mode over southeastern China appears locally in the warm sector with weak-gradient flows, and (iii) an inlandpropagating mode occurs during the daytime in association with sea breezes along the southern coastal regions, especially evident throughout the postmonsoon-onset period.

Analysis of Missed Summer Severe Rainfall Forecasts

Weather and Forecasting, Feb 26, 2016

Despite considerable progress in mesoscale numerical weather prediction (NWP), the ability to pre... more Despite considerable progress in mesoscale numerical weather prediction (NWP), the ability to predict summer severe rainfall (SSR) in terms of amount, location, and timing remains very limited because of its association with convective or mesoscale phenomena. In this study, two representative missed SSR events that occurred in the highly populated Great Lakes regions are analyzed within the context of moisture availability, convective instability, and lifting mechanism in order to help identify the possible causes of these events and improve SSR forecasts/nowcasts. Results reveal the following limitations of the Canadian regional NWP model in predicting SSR events: 1) the model-predicted rainfall is phase shifted to an undesired location that is likely caused by the model initial condition errors, and 2) the model is unable to resolve the echotraining process because of the weakness of the parameterized convection and/or coarse resolutions. These limitations are related to the ensuing model-predicted features: 1) vertical motion in the areas of SSR occurrence is unfavorable for triggering parameterized convection and grid-scale condensation; 2) convective available potential energy is lacking for initial model spinup and later for elevating latent heating to higher levels through parameterized convection, giving rise to less precipitation; and 3) the conversion of water vapor into cloud water at the upper and middle levels is underpredicted. Recommendations for future improvements are discussed.

The Relationship Between Anomalous Presummer Extreme Rainfall Over South China and Synoptic Disturbances

Journal Of Geophysical Research: Atmospheres, Apr 6, 2018

A spectral analysis of daily rainfall data has been performed to investigate extreme rainfall eve... more A spectral analysis of daily rainfall data has been performed to investigate extreme rainfall events in south China during the presummer rainy seasons between 1998 and 2015 (excluding 1999, 2006, 2011, and 2014). The results reveal a dominant frequency mode at the synoptic scale with pronounced positive rainfall anomalies. By analyzing the synoptic-scale bandpass-filtered anomalous circulations, 24 extreme rainfall episodes (defined as those with a daily rainfall amount in the top 5%) are categorized into "cyclone" (15) and "trough" (8) types, with the remaining events as an "anticyclone" type, according to the primary anomalous weather system contributing to each extreme rainfall episode. The 15 cyclone-type episodes are further separated into (11) lower-and (4) upper-tropospheric migratory anomalies. An analysis of their anomalous fields shows that both types could be traced back to the generation of cyclonic anomalies downstream of the Tibetan Plateau, except for two episodes of lower-tropospheric migratory anomalies originating over the South China Sea. However, a lower-tropospheric cyclonic anomaly appears during all phases in the former type, but only in the wettest phase in the latter type, with its peak disturbance occurring immediately beneath an upper-level warm anomaly. The production of extreme rainfall in the trough-type episodes is closely related to a deep trough anomaly extending from an intense cyclonic anomaly over north China, which in turn could be traced back to a midlatitude Rossby wave train passing by the Tibetan Plateau. The results have important implications for understanding the origin, structure, and evolution of synoptic disturbances associated with the presummer extreme rainfall in south China.

Facilitating International Collaboration on Climate Change Research

Bulletin of the American Meteorological Society, May 1, 2020

and the United Kingdom presented research results on a variety of important topics related to atm... more

Ground-based radar reflectivity mosaic of mei-yu precipitation systems over the Yangtze River–Huaihe River basins

Advances in Atmospheric Sciences, Oct 19, 2016

The 3D radar reflectivity produced by a mosaic software system, with measurements from 29 operati... more The 3D radar reflectivity produced by a mosaic software system, with measurements from 29 operational weather radars in the Yangtze River-Huaihe River Basins (YRHRB) during the mei-yu season of 2007, is compared to coincident TRMM PR observations in order to evaluate the value of the ground-based radar reflectivity mosaic in characterizing the 3D structures of mei-yu precipitation. Results show reasonable agreement in the composite radar reflectivity between the two datasets, with a correlation coefficient of 0.8 and a mean bias of −1 dB. The radar mosaic data at constant altitudes are reasonably consistent with the TRMM PR observations in the height range of 2-5 km, revealing essentially the same spatial distribution of radar echo and nearly identical histograms of reflectivity. However, at altitudes above 5 km, the mosaic data overestimate reflectivity and have slower decreasing rates with height compared to the TRMM PR observations. The areas of convective and stratiform precipitation, based on the mosaic reflectivity distribution at 3-km altitude, are highly correlated with the corresponding regions in the TRMM products, with correlation coefficients of 0.92 and 0.97 and mean relative differences of −7.9% and −2.5%, respectively. Finally, the usefulness of the mosaic reflectivity at 3-km altitude at 6-min intervals is illustrated using a mesoscale convective system that occurred over the YRHRB.

Numerical simulations of local circulation and its response to land cover changes over the Yellow Mountains of China

Journal of Meteorological Research, Aug 1, 2015

In this study, local circulations and their responses to land use and land cover (LULC) changes o... more In this study, local circulations and their responses to land use and land cover (LULC) changes over the Yellow Mountains of China are examined by using Weather Research and Forecast (WRF) model simulations of a selected case under weak-gradient synoptic conditions. The results show that mountain-valley breezes over the region are characterized by an intense upslope flow lasting for about 11 h (0600-1700 LST) along the northern slope during daytime. A convergence zone occurs at the mountain ridge and moves northwestward with time. During nighttime, wind directions are reversed, starting first at higher elevations. Three sensitivity experiments are conducted, in which the current land covers are replaced by grassland, mixed forest, and bare soil, respectively, while keeping the other model conditions identical to a control run. These sensitivity simulations are designed to represent the changes of LULC over the Yellow Mountains area during the past decades. The results show that changes in land cover could affect substantially land-surface and atmosphere interactions, the evolution of local circulations, and characteristics of the planetary boundary layer (PBL). Significant differences are noted in horizontal winds, and sensible and latent heat fluxes. On the other hand, when the surface is covered by mixed forest, slight variations in local winds and surface variables are identified. The results appear to have important implications to urban planning and constructions as well as the transport of air pollutants over mountainous regions.

On the Rapid Intensification of Hurricane Wilma (2005). Part I: Model Prediction and Structural Changes

Weather and Forecasting, Dec 1, 2011

In this study, a 72-h cloud-permitting numerical prediction of Hurricane Wilma (2005), covering i... more In this study, a 72-h cloud-permitting numerical prediction of Hurricane Wilma (2005), covering its initial 18-h spinup, an 18-h rapid intensification (RI), and the subsequent 36-h weakening stage, is performed using the Weather Research Forecast Model (WRF) with the finest grid length of 1 km. The model prediction uses the initial and lateral boundary conditions, including the bogus vortex, that are identical to the Geophysical Fluid Dynamics Laboratory's then-operational data, except for the time-independent sea surface temperature field. Results show that the WRF prediction compares favorably in many aspects to the best-track analysis, as well as satellite and reconnaissance flight-level observations. In particular, the model predicts an RI rate of more than 4 hPa h 21 for an 18-h period, with the minimum central pressure of less than 889 hPa. Of significance is that the model captures a sequence of important inner-core structural variations associated with Wilma's intensity changes, namely, from a partial eyewall open to the west prior to RI to a full eyewall at the onset of RI, rapid eyewall contraction during the initial spinup, the formation of double eyewalls with a wide moat area in between during the most intense stage, and the subsequent eyewall replacement leading to the weakening of Wilma. In addition, the model reproduces the boundary layer growth up to 750 hPa with an intense inversion layer above in the eye. Recognizing that a single case does not provide a rigorous test of the model predictability due to the stochastic nature of deep convection, results presented herein suggest that it is possible to improve forecasts of hurricane intensity and intensity changes, and especially RI, if the innercore structural changes and storm size could be reasonably predicted in an operational setting using highresolution cloud-permitting models with realistic initial conditions and model physical parameterizations.

Impact of Cyclone‐Cyclone Interaction on Lake‐Effect Snowbands: A False Alarm

Journal Of Geophysical Research: Atmospheres, Jan 14, 2023

In this study, the impacts of two cyclone interactions with the environmental flow and with each ... more In this study, the impacts of two cyclone interactions with the environmental flow and with each other on a heavy winter snowfall event over the Great Lakes region are examined by applying a recently‐developed diagnostic tool to a lake‐effect snowstorm. This winter snowfall event was a false alarm by the Canadian operational model, in which the predicted lake‐effect snowfall at the lee side of Lake Erie doubles the observed. The false alarm involves the interactions of two cyclones, whose relative locations are poorly predicted, leading to inaccurate prediction of their induced winds and interactions over Lake Erie at both low and upper levels. In particular, at 1,000 hPa, the cyclone‐induced winds and their interactions over Lake Erie caused colder and drier air over warm‐moist open‐water lake surface that is favorable for more latent heat flux deriving from the lake in the model than in the real atmosphere. At 500 hPa, the location errors of the model predicted cyclones reversed the cyclone‐induced winds over Lake Erie from mainly southerly to northerly, especially during the intense snowfall period. An analysis of the conditional symmetric instability (CSI) index over Lake Erie reveals that the operational model systematically over‐predicts CSI, more significantly during the intense snowfall period. As a result, the operational model prediction makes a false alarm of a lake‐effect snowfall event with large errors in precipitation intensity. Future improvements in operational model predictions are also suggested.

北京地区雷电灾害天气系统的动力&LT;Bold&GT;-&LT;/Bold&GT;微物理&LT;Bold&GT;-&LT;/Bold&GT;电过程观测研究

Zhongguo kexue, Oct 15, 2020

SAFIR-3000 Lightning Statistics over the Beijing Metropolitan Region during 2005–07

Journal of Applied Meteorology and Climatology, 2016

In this study, the spatiotemporal characteristics of cloud-to-ground (CG) and intracloud (IC) lig... more In this study, the spatiotemporal characteristics of cloud-to-ground (CG) and intracloud (IC) lightning flashes observed by Surveillance et Alerte Foudre par Interférometrie Radioélectrique (SAFIR)-3000 over the Beijing metropolitan region (BMR) during 2005–07 were investigated. The results showed the presence of 299 lightning days with 241 688 flashes, most of which were IC lightning flashes. Only 19% of the total flashes were CG lightning flashes; 14% of these CG flashes were positive. Most lightning activity occurred during the summer months (June–August), with a major diurnal peak around 1900 Beijing standard time (BST) and a secondary peak around 2300 BST. Spatial variations in flash density and lightning days both exhibited an obvious southeastwardly increasing pattern, with higher flash densities or more lightning days occurring in the southeastern plains and lower values distributed on the northwestern mountains. The Z ratio (IC/CG lightning flashes) exhibited a similar spat...

The Relationship Between Anomalous Presummer Extreme Rainfall Over South China and Synoptic Disturbances

Journal of Geophysical Research: Atmospheres, 2018

A spectral analysis of daily rainfall data has been performed to investigate extreme rainfall eve... more A spectral analysis of daily rainfall data has been performed to investigate extreme rainfall events in south China during the presummer rainy seasons between 1998 and 2015 (excluding 1999, 2006, 2011, and 2014). The results reveal a dominant frequency mode at the synoptic scale with pronounced positive rainfall anomalies. By analyzing the synoptic‐scale bandpass‐filtered anomalous circulations, 24 extreme rainfall episodes (defined as those with a daily rainfall amount in the top 5%) are categorized into “cyclone” (15) and “trough” (8) types, with the remaining events as an “anticyclone” type, according to the primary anomalous weather system contributing to each extreme rainfall episode. The 15 cyclone‐type episodes are further separated into (11) lower‐ and (4) upper‐tropospheric migratory anomalies. An analysis of their anomalous fields shows that both types could be traced back to the generation of cyclonic anomalies downstream of the Tibetan Plateau, except for two episodes of...

Genesis of Tropical Storm Debby (2006) within an African Easterly Wave: Roles of the Bottom-Up and Midlevel Pouch Processes

Journal of the Atmospheric Sciences, May 27, 2015

The ''bottom up'' generation of low-level vortices (LVs) and midlevel vortices (MVs) during the g... more The ''bottom up'' generation of low-level vortices (LVs) and midlevel vortices (MVs) during the genesis of Tropical Storm Debby (2006) and the roles of a midlevel ''marsupial pouch'' associated with an African easterly wave (AEW) are examined using an 84-h simulation with the finest grid size of 1.33 km. Results show that several MVs are generated in leading convective bands and then advected rearward into stratiform regions by front-to-rear ascending flows. Because of different Lagrangian storm-scale circulations, MVs and LVs are displaced along different paths during the early genesis stages. MVs propagate cyclonically inward within the AEW pouch while experiencing slow intensification and merging under the influence of converging flows. The MVs' merging into a mesovortex is accelerated as they come closer to each other in the core region. In contrast, the low-level Lagrangian circulation is opened as a wave trough prior to tropical depression (TD) stage, so the LVs tend to ''escape'' from the pouch region. Only after the low-level flows become closed do some LVs congregate and contribute directly to Debby's genesis. The TD stage is reached when the midlevel mesovortex and an LV are collocated with a convective zone having intense low-level convergence. Results also show the roles of upper-level warming in hydrostatically maintaining the midlevel pouch and producing mesoscale surface pressure falls. It is found that the vertically tilted AEW with a cold dome below is transformed to a deep warm-core TD vortex by subsiding motion. A conceptual model describing the key elements in the genesis of Debby is also provided.

A Piecewise Potential Vorticity Inversion Algorithm and Its Application to Hurricane Inner-Core Anomalies

Journal of the Atmospheric Sciences, Aug 1, 2010

An Examination of the Pressure–Wind Relationship for Intense Tropical Cyclones

Weather and Forecasting, Jun 1, 2010

In this study, the dynamical constraints underlining the pressure-wind relationship (PWR) for int... more In this study, the dynamical constraints underlining the pressure-wind relationship (PWR) for intense tropical cyclones (TCs) are examined with the particular focus on the physical connections between the maximum surface wind (VMAX) and the minimum sea level pressure (PMIN). Use of the Rankine vortex demonstrates that the frictional forcing in the planetary boundary layer (PBL) could explain a sizeable portion of the linear contributions of VMAX to pressure drops. This contribution becomes increasingly important for intense TCs with small eye sizes, in which the radial inflows in the PBL could no longer be neglected. Furthermore, the inclusion of the tangential wind tendency can make an additional contribution to the pressure drops when coupled with the surface friction. An examination of the double-eyewall configuration reveals that the formation of an outer eyewall or wellorganized spiral rainbands complicates the PWR. An analysis of a cloud-resolving simulation of Hurricane Wilma (2005) shows that the outer eyewall could result in the continuous deepening of PMIN even with a constant VMAX. The results presented here suggest that (i) the TC size should be coupled with VMAX rather than being treated as an independent predictor as in the current PWRs, (ii) the TC intensity change should be at least coupled linearly with the radius of VMAX, and (iii) the radial wind in the PBL is of equal importance to the linear contribution of VMAX and its impact should be included in the PWR.

A Statistical Analysis of Steady Eyewall Sizes Associated with Rapidly Intensifying Hurricanes

Weather and Forecasting, May 20, 2016

It is well known that hurricane intensification is often accompanied by continuous contraction of... more It is well known that hurricane intensification is often accompanied by continuous contraction of the radius of maximum wind (RMW) and eyewall size. However, a few recent studies have shown rapid and then slow contraction of the RMW/eyewall size prior to the onset and during the early stages of rapid intensification (RI) of hurricanes, respectively, but a steady state in the RMW (S-RMW) and eyewall size during the later stages of RI. In this study, a statistical analysis of S-RMWs associated with rapidly intensifying hurricanes is performed using the extended best-track dataset during 1990-2014 in order to examine how frequently, and at what intensity and size, the S-RMW structure tends to occur. Results show that about 53% of the 139 RI events of 24-h duration associated with 55 rapidly intensifying hurricanes exhibit S-RMWs, and that the percentage of the S-RMW events increases to 69% when RI events are evaluated at 12-h intervals, based on a new RI rate definition of 10 m s 21 (12 h) 21 ; both results satisfy the Student's t tests with confidence levels of over 95%. In general, S-RMWs tend to appear more frequently in more intense storms and when their RMWs are contracted to less than 50 km. This work suggests a new fruitful research area in studying the RI of hurricanes with S-RMWs.

An analytical model for the rapid intensification of tropical cyclones

Quarterly Journal of the Royal Meteorological Society, Jun 18, 2009

The nonlinearity and complexity of the primitive equations have been key obstacles to our underst... more The nonlinearity and complexity of the primitive equations have been key obstacles to our understanding of tropical cyclones (TCs), particularly in relation to the dynamical processes leading to their rapid intensification. In this study, an axisymmetric model, in which all nonlinear terms in the horizontal momentum equations are retained, is used to examine analytically the effects of organized deep convection on TC rapid intensification. By prescribing a vertical profile of the vertical motion with exponential growth in the core region, a class of exact time-dependent solutions for the primary circulations of TCs are obtained. The analytical solutions are shown to capture well many observed dynamical structures in both the core and outer regions and the rapid growth of TCs in terms of maximum winds and central pressure drops. The analytical solutions reveal that (1) the rotational flows in the inner-core region grow double-exponentially, and the central pressure drops occur at rates much faster than the rotational growth; (2) the amplification rates of the primary circulations differ profoundly from those of the secondary circulations; (3) the rotational flows tend to grow from the bottom upwards with the fastest growth occurring at the lowest levels; and (4) the TC growth rates depend critically on the vertical structure of tangential flows, with a faster rate for a lower-level peak rotation. The nonlinear dynamics are shown to play an important role in the rapid growth of TCs. It is demonstrated that the analytical solutions can also be used to construct dynamically consistent vortices for the initialization of TC models. Limitations and possible improvements of the analytical model are also discussed.