The elevated warm layer is a vertical layer of air aloft in which the temperature is above freezing. Below and above this layer of air the temperature is below freezing. In the diagram below there is a layer of below freezing layer from the earth’s surface to the base of the elevated warm layer and there is also a layer of below freezing air above the elevated warm layer. Within the elevated warm layer the temperature is above freezing and this has significant impacts on precipitation type that reaches the surface. This writing focuses on the erosion of the elevated warm layer. By erosion, it is meant that the layer shrink in vertical depth, gets colder or is eliminated entirely. Processes that cause an erosion of the elevated warm layer are listed on the diagram below and they are cold air advection, evaporative cooling/melting, and lifting of air. Cold air advection is the horizontal transport of colder air into a location. This can happen behind a cold front, when deeper and colder air penetrates into the forecast region. Especially if the air within the elevated warm layer is fairly dry (low relative humidity), precipitation falling through the elevated warm layer will cool it due to evaporative cooling and melting which are processes that absorb latent heat and cool the air. Precipitation can first start out as rain and then transition to sleet and snow as the elevated warm layer is eroded by cooling temperatures. When air rises, it cools. Thus lifting mechanisms from low level convergence and upper level divergence can erode the elevated warm layer. For example, precipitation can start as light rain and then transition to heavy snow as a strong lifting mechanism moves over the forecast region. Once the lifting mechanism moves on then the precipitation can turn back to light rain again. These are several processes that a forecaster pays attention to during a winter weather event.  |