Simulation of the role of Caspian Sea surface temperature and air temperature on precipitation intensity in lake-effect snow

Abstract

Heavy snowfall is one of the perilous atmospheric phenomena which occurs occasionally in south-west coasts of the Caspian Sea. In this paper, we have simulated the role of sea surface temperature (SST) and lake-air temperature difference on the lake-effect snow in south-west coasts of the Caspian Sea using Weather Research and Forecasting (WRF) model. SST was increased by 2 °C–4 °C and air temperature decrease by 2 °C–4 °C in model simulations and the resulting precipitation was compared to observed values. The results show that the precipitation is significantly sensitive to the SST and lake-air temperature difference. The results indicated that the regions with heavy precipitation correspond with 4 °C increase in SST and 2 °C decrease in air temperature, add about 60 mm to the total precipitation. Furthermore, the increase in SST enhances snowfall along the coastlines. In overall, an increase between 2 °C and 4 °C in SST over 5 days, increases the precipitation between 8.43% and 19.41%, respectively, and decrease air temperature by 2 °C and 4 °C increase precipitation about 24.78% and 0.72%, respectively. Overall, the highest increase in precipitation in the whole region is related to the 2 °C decrease in air temperature and the 4 °C increase in SST. Since other atmospheric parameters such as pressure, wind field, and humidity flux are affected by temperature change, so excessive decrease in air temperature tends to weaken the low pressure and vertical motion and less precipitation occurs. Therefore, the amount of the lake-air temperature difference is a key factor in strengthening the lake-effect precipitation.Additionally, SST increases the latent heat flux and injects additional humidity to the region which further strengthens orographic uplifts and intensifies unstable condition. In the meantime, stronger wind speed caused by unstable boundary layer over the lake and sufficient fetch, accumulate additional humidity in the convergence zone along the south-west coastlines.