Comparison of mechanical and thermal effects of lake urmia: a case study

Simulation of the lakes’ impact on precipitation and the quantification of the effects resulting from the contribution of different processes have proved challenging in numerical weather prediction. Lakes have differences with their surrounding lands in roughness, surface temperature and humidity content, which are the root causes of the mechanical and thermal mechanisms that lead to changes in the amount of precipitation in the downwind of a lake. In this study, the effect of Lake Urmia on precipitation in the form of snowfall is simulated for a case of heavy precipitation associated with severe weather across a significant part of the Southwest Asia on 11 Dec. 2013 using the Weather Research and Forecasting (WRF) model coupled with the Community Land Model (CLM) in simulations with and without the lake. The results illustrate that omitting the lake reduces the humidity content and precipitable water in the vertical column above the lake. In this case, however, the role of mechanical effect due to differences in roughness between the lake and its surrounding lands is noticeably larger than the thermal effect due to differences in the moisture and heat supplied by the lower surface to the air masses above. Associated with the roughness differences, a distinct change in the location of the convergence line of the antithetical currents is identified over the lake. The case highlights the importance of the proper representation of surface characteristics in mesoscale models to gain improvements on the fine-scale details of precipitation.