Sleet often occurs in the transition from freezing rain to snow. There are a couple of reasons for this. One reason is a CAA pattern and the other is evaporative cooling. When temperatures are below freezing in the lower PBL, evaporational cooling or CAA will increase the depth of freezing temperatures. This increase in depth of freezing temperatures will freeze raindrops before they reach the surface. As evaporational cooling or CAA continue the elevated warm layer (EWL) becomes increasingly smaller. Once the EWL is less than 25 mb, then the precipitation will fall as snow. This change over from freezing rain (or rain) to snow is often marked with a wintry mix transition, where several forms of wintry precipitation will fall at the same time. Sleet can also occur in the transition from snow to rain. This could be due to a WAA pattern or a warming of the PBL. Sleet tends to be brief when it is involved with changing precipitation types, but this essay is concerned with the "sleet storm". In the sleet storm, the majority of the precipitation is sleet and the sleet falls for several hours. The typical sleet storm starts off as freezing rain. Evaporative cooling and CAA increase the depth of sub-freezing temperatures in the PBL to allow raindrops to freeze before reaching the surface. However, the depth of freezing temperatures does not significantly erode the EWL. The reason the EWL is not eroded is due to the shallow nature of the polar air in the PBL. Also WAA could be occurring within the EWL. Sleet storms are often preceded by a strong shallow cold front passage. An upper level trough overrides the cold PBL temperatures. The temperatures above the PBL can be quite warm. In many cases for lower elevation regions, there will be rain (partially melted snow flakes) at 800 millibars and sleet at the surface. This temperature structure of the troposphere becomes locked in place for several hours. Again, this temperature structure is shallow polar subfreezing air in the lower PBL and warmer above freezing air somewhere between 850 and 700 millibars. After a polar front passage, the shallow depth of the cold air can persist for many hours or even days, especially if the middle and upper level wind flow opposes the air flow associated with the polar air. These sleet storms can produce over an inch of sleet. |