Browsing by Subject "WRF"

Local mountain winds have a diurnal cycle of flowing up the slopes and valleys daytime and down nighttime. It is important to improve the understanding on these thermally driven winds, because they have a major role in pollution transport in mountain areas, which are highly sensitive for air-quality problems. This thesis determines if the slope and valley winds in the Khumbu valley, Himalayas, are driven by the textbook mechanisms. By the textbook mechanisms the slope and valley winds are driven by horizontal temperature differences caused by uneven heating over an area of complex terrain. Slope winds are driven by the horizontal air temperature difference in the slope surface vicinity when the slope surface is heated or cooled. Valley winds are driven by the uneven heating caused by the air volume difference between the valley and above an adjacent plain. If the valley slopes narrow and the floor is elevated towards the head of the valley, both the valley and slope wind mechanisms drive the winds along the valley. The slope and valley winds in the Himalayas are studied using Weather Research and Forecasting model (WRF), that is run for 5 days period in December 2014 with 1 kilometer horizontal grid spacing and 61 vertical levels. Earlier studies have shown that WRF is capable of simulating the thermally driven mountain winds on this resolution with the length scales of the Khumbu valley topography. Horizontal gradient of air temperature and slope wind component at the slope surface have a matching daily cycle in the lower and middle parts of the valley. The boundary layer air volume decreases from the mouth of the valley towards the middle parts of the valley indicating the valley wind mechanism. The daytime potential temperature profiles yield that also the slope wind mechanism drives the winds along the valley. The slope winds have a textbook daily cycle in the lower and middle parts of the valley and the analysis yields that they are driven by the slope wind mechanism. In the upper part of the valley the thermally driven slope winds are dominated by synoptic scale channelling and gravity wave developing into the valley. The daytime up-valley winds are driven by both valley wind mechanism and slope wind mechanism due to the valley narrowing and elevation towards the head of the valley, respectively. Nocturnal along-valley winds are weak less than 0.5 meters per second flowing up or down-valley. The wind patterns are similar to what is shown in earlier studies done in Khumbu valley.