Sensitivity of simulated summer monsoonal precipitation in Langtang Valley, Himalaya, to cloud microphysics schemes in WRF

Abstract

A better understanding of regional‐scale precipitation patterns in the Himalayan region is required to increase our knowledge of the impacts of climate change on downstream water availability. This study examines the impact of four cloud microphysical schemes (Thompson, Morrison, Weather Research and Forecasting (WRF) single‐moment 5‐class, and WRF double‐moment 6‐class) on summer monsoon precipitation in the Langtang Valley in the central Nepalese Himalayas, as simulated by the WRF model at 1 km grid spacing for a 10 day period in July 2012. The model results are evaluated through a comparison with surface precipitation and radiation measurements made at two observation sites. Additional understanding is gained from a detailed examination of the microphysical characteristics simulated by each scheme, which are compared with measurements using a spaceborne radar/lidar cloud product. Also examined are the roles of large‐ and small‐scale forcings. In general, the schemes are able to capture the timing of surface precipitation better than the actual amounts in the Langtang Valley, which are predominately underestimated, with the Morrison scheme showing the best agreement with the measured values. The schemes all show a large positive bias in incoming radiation. Analysis of the radar/lidar cloud product and hydrometeors from each of the schemes suggests that “cold‐rain” processes are a key precipitation formation mechanism, which is also well represented by the Morrison scheme. As well as microphysical structure, both large‐scale and localized forcings are also important for determining surface precipitation.

Bias Correction of High‐Resolution Regional Climate Model Precipitation Output Gives the Best Estimates of Precipitation in Himalayan Catchments

“…It is speculated that the cause of the dry bias over the Sutlej basin in the raw WRF model output is related to a failure to represent an early-morning maximum in precipitation during the monsoon period. Such a peak is apparent in the simulated output for the Beas basin and also reported over other locations focused on south facing slopes of the HKKH (e.g., Bhatt & Nakamura, 2005;Orr et al, 2017;Prasad, 1974). However, confirmation of its existence was hampered by the unavailability of subdaily station data.…”

“…It is also likely that the bias is related to the representation of localized diurnal mountain-valley winds, which play a key role in inducing convection. Adequate representation of these winds in regional climate models is a long-standing issue, with a number of previous studies highlighting the difficulty of representing them in the HKKH region (e.g., Bhatt & Nakamura, 2006;Norris et al, 2017;Orr et al, 2017;Sato, 2013;Shrestha & Deshar, 2014).…”

“…This suggests that a failure to simulate this morning peak in precipitation is the explanation for the dry bias in the Sutlej basin during the monsoon. Enhanced precipitation in the morning would be expected in theory due to enhanced near-surface convergence of katabatic flow, resulting in upward motion that can cause convection, as has been reported over other locations focused on south-facing slopes of the Himalaya [see, e.g., Bhatt & Nakamura, 2005;Orr et al, 2017;Prasad, 1974).…”

“…However, despite the fine spatiotemporal resolution and improved representation of key dynamical and physical processes of a regional climate model, their representation of precipitation (and the subsequent impact of bias correction) over the HKKH remains uncertain. For example, previous studies examining the performance of the Weather Research and Forecasting (WRF) model at high resolution in the HKKH region have typically focused on either one small river catchment or over larger regions that considered only a limited number of in situ measurements for relatively short periods of time of 1 year or less (Bonekamp et al, ; Collier & Immerzeel, ; Li et al, ; Maussion et al, ; Norris et al, ; Orr et al, ), therefore failing to fully assess the model's ability to reproduce the actual detailed patterns of precipitation. Furthermore, though multiple studies have shown the importance of bias correction of precipitation over topographically complex regions (e.g., Bordoy & Burlando, ; Lafon et al, ; Teutschbein & Seibert, ), the usefulness of such methods over the HKKH has yet to be conclusively proven (Shrestha et al, ).…”

Dynamical Drivers of the Local Wind Regime in a Himalayan Valley

“…The summer monsoon is the predominant large‐scale driver of precipitation in the eastern areas of the HKKH (Shea, Wagnon, et al, ; Wagnon et al, ; Yang et al, ), while in the western areas it is the winter westerly disturbances (Ueno et al, ). In addition to the synoptic scale systems, the local valley wind regimes also affect precipitation by transporting moisture and clouds up the slopes and valleys during the day, especially at high altitudes (Bollasina et al, ; Egger et al, ; Karki et al, ; Orr et al, ; Shea, Wagnon, et al, ; Tartari et al, ). They also affect snow redistribution (Wagnon et al, ).…”