Analysis of the Characteristics of Inertia-Gravity Waves during an Orographic Precipitation Event

A numerical experiment was performed using the Weather Research and Forecasting（WRF） model to analyze the generation and propagation of inertia-gravity waves during an orographic rainstorm that occurred in the Sichuan area on 17 August 2014. To examine the spatial and temporal structures of the inertia-gravity waves and identify the wave types, three wavenumber-frequency spectral analysis methods（Fourier analysis, cross-spectral analysis, and wavelet cross-spectrum analysis）were applied. During the storm, inertia-gravity waves appeared at heights of 10-14 km, with periods of 80-100 min and wavelengths of 40-50 km. These waves were generated over a mountain and propagated eastward at an average speed of 15-20 m s^（-1）. Meanwhile, comparison between the reconstructed inertia-gravity waves and accumulated precipitation showed there was a mutual promotion process between them. The Richardson number and Scorer parameter were used to demonstrate that the eastward-moving inertia-gravity waves were trapped in an effective atmospheric ducting zone with favorable reflector and critical level conditions, which were the primary causes of the long lives of the waves. Finally, numerical experiments to test the sensitivity to terrain and diabatic heating were conducted, and the results suggested a cooperative effect of terrain and diabatic heating contributed to the propagation and enhancement of the waves.

Antecedent snowmelt and orographic precipitation contributions to water supply of Pakistan disastrous floods,2022

In 2022,the Pakistan witnessed the hottest spring and wettest summer in history.And devastating floods inundated a large portion of Pakistan and caused enormous damages.However,the primary water source and its contributions to these unprecedented floods remain unclear.Based on the reservoir inflow measurements,Multi-Source Weighted-Ensemble Precipitation(MSWEP),the fifth generation ECMWF atmospheric reanalysis(ERA5)products,this study quantified the contributions of monsoon precipitation,antecedent snow-melts,and orographic precipitation enhancement to floods in Pakistan.We found that the Indus experienced at least four inflow up-rushes,which was mainly supplied by precipitation and snowmelt;In upper Indus,abnormally high temperature continued to influence the whole summer and lead to large amounts of snowmelts which not only was a key water supply to the flood but also provided favorable soil moisture conditions for the latter precipitation.Before July,the snowmelt has higher contributions than the precipitation to the streamflow of Indus River,with contribution value of more than 60%.Moreover,the snowmelt could still supply 20%-40%water to the lower Indus in July and August;The leading driver of 2022 mega-floods over the southern Pakistan in July and August was dominated by the precipitation,where terrain disturbance induced precipitation account to approximately 33%over the southern Pakistan.The results help to understand the mechanisms of flood formation,and to better predict future flood risks over complex terrain regions.

A Study on a Snowband Associated with a Coastal Front and Cold-Air Damming Event of 3-4 February 1998 along the Eastern Coast of the Korean Peninsula

A 24-h simulation with the Advanced Regional Prediction System （ARPS） nonhydrostatic model is performed for the heavy snowfall event of 3-4 February 1998 along the eastern coast of Korean Peninsula; the results are used to understand the snowfall process, including why the precipitation maxima formed along the Yeongdong coastal region rather than over the mountain slope and ridge top during. The numerical simulation with a 4-kin horizontal grid spacing and 43 levels reproduces very well the narrow snowband located off the eastern Korean coast, away from, instead of over, the Yeongdong coastal mountain range. The general evolution of the snowband agrees quite well with radar observations, while the water-equivalent precipitation amount agrees reasonably well with radar precipitation estimate. The simulation results clearly show that the snow band developed due to the lifting by a coastal front that developed because of the damming of cold air against the eastern slope of the coastal mountain range. The damming was enhanced by the advection of cold air by a tow-level mountain-parallel jet from the north, formed due to geostrophic adjustment as the on-shore upslope air was decelerated by the mountain blocking. As the onshore flow weakened later due to synoptic-scale flow pattern change, the cold front propagated off shore and the precipitation dissipated.

Satellite Observations of Reflectivity Maxima above the Freezing Level Induced by Terrain

Previous studies have recognized reflectivity maxima above the freezing level(RMAF) within stratiform precipitation over mountain slopes, however, quantitative studies are limited due to the lack of adequate identification criteria. Here, we establish an identification method for RMAF precipitation and apply it to the Tropical Rainfall Measuring Mission(TRMM) Precipitation Radar(PR) observations. Using the TRMM 2A25 product from 1998 to 2013, we show that the RMAF structure in reflectivity profiles can be effectively identified. RMAF exists not only in stratiform precipitation but also in convective precipitation. RMAF frequency is positively correlated with elevation, which is thought to be caused by enhanced updrafts in the middle layers of stratiform precipitation, or in the low to middle layers of convective precipitation over mountains. The average RMAF heights in stratiform and convective precipitation were 1.35 and 2.01 km above the freezing level, respectively, which is lower than previous results. In addition, our results indicate that the RMAF structure increased the echo top height and enhanced precipitation processes above the RMAF height, but it suppressed the downward propagation of ice particles and the near-surface rain rate. Future studies of orographic precipitation should take into account the impact of the RMAF structure and its relevant dynamic triggers.