The impact of Guizhou topography on the distribution of freezing rain in early January 2011

Abstract

Among different winter precipitation types, freezing rain is the most dangerous and the most difficult to forecast. Nearly 84% of total freezing rain days in China occur over Guizhou province. Statistical analysis indicates that the frequency distribution of freezing rain over Guizhou is correlated to its topography. In this article, a typical freezing rain case from 2011 is selected to explore the impact of Guizhou topography on the distribution of freezing rain.Synoptic analysis shows that this event is dominated by two cold currents below 800 hPa and a warm, moist southwesterly at 700 hPa, which contribute to an overlaying surface cold layer and a warm layer aloft in central Guizhou. Freezing rain is developed through a ‘warm rain process’ under the mid–lower tropospheric warm layer. Contrasting numerical terrain experiments demonstrate that Guizhou terrain plays an essential role in the formation of freezing rain by influencing the set‐up of the surface cold layer, warm layer and precipitation. Except for the effect of altitude, the vertical distance between the cold centre in the cold flow and the surface height also impacts on the formation of the surface cold layer. The intensities of the warm layer and inversion change after altering terrain height. The number of rainwater particles reaching the surface layer decreases as the surface temperature increases in the lowered terrain experiment, whereas the block of steeper windward slope results in an increase of surface precipitation in the heightened terrain experiment. The regions experiencing freezing rain shrink after reducing the terrain but spread southeastwards after raising the terrain. The unique topography of Guizhou including its elevation and distribution contributes to the highest frequency of freezing rain in China.