Wind and temperature in a glacierised Himalyan valley, and their controlling mechanisms
The Hindu-Kush Karakoram Himalaya (HKKH) contains the third largest quantity of snow and ice in the world, after the Polar Regions. Meltwater from this snow and ice feeds many of the major rivers in Asia, which ultimately provide water for 1.9 billion people. Due to its complex and rugged terrain, as well as a scarcity of in-situ measurements and fine-scale numerical modelling studies, important factors influencing melt and precipitation, such as the local valley wind regimes and the lapse rates of near-surface temperatures, are poorly understood in the HKKH. This thesis aims to improve understanding of the valley wind regime and temperature lapse rate for the glacierised Dudh Koshi River Basin in the Nepalese Himalaya, which includes the Khumbu Glacier, by utilising results from a high-resolution atmospheric model and measurements from a field campaign. First, the mechanisms controlling the local wind regime in the Dudh Koshi Valley are investigated, by running the Weather Research and Forecasting (WRF) model at 1 km horizontal resolution for one month in the summer and one month in the winter. The WRF model output is found to well represent the diurnal cycle of the wind when compared to existing in-situ observations, which is characterised by strong up-valley near-surface winds during the day and weak (predominantly up-valley) winds at night in both months. A momentum budget analysis reveals that the predominant physical drivers of the near-surface wind acceleration are the pressure gradient, advection and turbulent vertical mixing, which are extremely spatially variable over the valley. The results also show that the local wind regime and its drivers are strongly affected by the presence of glaciers, which act to weaken the up-valley flow. Second, as the Khumbu Glacier is largely debris-covered (along with many glaciers in the HKKH), a new debris-cover category is added into the WRF model. This enhancement is found to improve the model representation of near-surface temperature, relative humidity, wind speed and radiation, in comparison to the default category of clean-ice glaciers. The addition of the new debris-cover category, and the resulting change in near-surface temperature and wind speed are found to have consequent effects on water vapour, hydrometeors and ultimately snow cover. Third, to investigate the temperature lapse rate, a series of temperature sensors was installed throughout the Dudh Koshi Valley and over the Khumbu Glacier for 18 days during the pre-monsoon season in 2017, and the entire monsoon season. Lapse rates are found to vary considerably, both diurnally and over the pre-monsoon and monsoon periods. Temperature budget analysis based on output from the WRF model reveals that (both off-glacier and on the debris-covered glacier) the near-surface temperature during the day is warmed by turbulent vertical mixing and cooled by advection. Furthermore, at night a relationship is identified between strong downslope winds on the glacier and shallow lapse rates, due to warming from advection and cooling from turbulent vertical mixing. In addition, in the monsoon season there is a substantial contribution from latent cooling during the day. This is the first work to provide a full momentum and temperature budget analysis for a valley in the HKKH region. It is hoped that the advances made in this thesis may ultimately help inform developments to weather and climate models over the region, including highlighting the need for debris-covered glaciers to be represented in atmospheric models.