Orographic Effect

This paper discusses the Sierra Madre Mountain Range of the Philippines and its associated influence on the intensity and distribution of rainfall during tropical cyclones. Based on Weather and Research Forecasting model simulations, a shift in rainfall was observed in different portions of the country, due to the reduction of the topography of the mountain. Besides increasing the rainfall along the mountain range, a shift in precipitation was observed during Tropical Storm Ondoy, Typhoon Labuyo, and Tropical Storm Mario. It was also observed that the presence of the Sierra Madre Mountain Range slows down the movement of a tropical cyclones, and as such allowing more time for precipitation to form over the country. Wind profiles also suggest that the windward and leeward sides of mountain ranges during Tropical Cyclones changes depending on the storm path. It has been suggested that in predicting the distribution of rainfall, the direction of movement of a tropical cyclones as well as its adjacent areas be taken into great consideration. While the study shows high amounts of variation in the characteristics of different tropical cyclones with respect of the Sierra Madre Mountain Range, the results of this study can provide insights to pre-disaster operations before tropical cyclones approaches land. The decrease in tropical cyclones speed introduced by the Sierra Madre Mountain Range can be used to identify the possible areas that can experience prolonged rains due to the mountain range. Disaster management authorities can also prepare in advance by identifying which locations can experience orographic enhanced precipitation. However, due to the lack of available data and resources, further studies are recommended due to the study presenting limited cases.

Rain gauge station data from 1987 to 1996 were used to investigate spatial and temporal variability in monthly precipitation and annual and seasonal precipitation patterns over Nepal. Maximum annual precipitation increased with altitude for elevations below 2000 m but decreased for elevations of 2000-3500 m. The data revealed a negative relationship between annual precipitation and elevation only in western Nepal. Annual precipitation averaged on a 0.25°grid exceeded 3000 mm/yr in central Nepal but was less than 1000 mm/yr over Nepal's northwestern mountains. Only winter precipitation over western Nepal was heavier than precipitation over central and eastern Nepal. A time series of standardized precipitation anomalies averaged over Nepal revealed no significant long-term trends. Further, almost no stations exhibited significant long-term trends by Kendall's rank correlation analysis. A correlation analysis between summer monsoon precipitation and the All Indian Rainfall (AIR) index revealed positive and negative correlations in western and eastern Nepal, respectively. This analysis also revealed a positive correlation, but no negative correlation, between summer monsoon precipitation and the Southern Oscillation Index (SOI) in western and eastern Nepal. Composite differences in temperature, 850-hPa winds, outgoing longwave radiation (OLR), and precipitation rates between low and high AIR phases revealed that moist air from the Arabian Sea supported precipitation over western Nepal, whereas cold dry air from the Tibetan Plateau suppressed precipitation over eastern Nepal. However, composite differences in precipitation between low and high SOI phases revealed no anomalies for Nepal.

Rain gauge station data from 1987 to 1996 were used to investigate spatial and temporal variability in monthly precipitation and annual and seasonal precipitation patterns over Nepal. Maximum annual precipitation increased with altitude for elevations below 2000 m but decreased for elevations of 2000-3500 m. The data revealed a negative relationship between annual precipitation and elevation only in western Nepal. Annual precipitation averaged on a 0.25°grid exceeded 3000 mm/yr in central Nepal but was less than 1000 mm/yr over Nepal's northwestern mountains. Only winter precipitation over western Nepal was heavier than precipitation over central and eastern Nepal. A time series of standardized precipitation anomalies averaged over Nepal revealed no significant long-term trends. Further, almost no stations exhibited significant long-term trends by Kendall's rank correlation analysis. A correlation analysis between summer monsoon precipitation and the All Indian Rainfall (AIR) index revealed positive and negative correlations in western and eastern Nepal, respectively. This analysis also revealed a positive correlation, but no negative correlation, between summer monsoon precipitation and the Southern Oscillation Index (SOI) in western and eastern Nepal. Composite differences in temperature, 850-hPa winds, outgoing longwave radiation (OLR), and precipitation rates between low and high AIR phases revealed that moist air from the Arabian Sea supported precipitation over western Nepal, whereas cold dry air from the Tibetan Plateau suppressed precipitation over eastern Nepal. However, composite differences in precipitation between low and high SOI phases revealed no anomalies for Nepal.