Date of this Version
During the morning hours of 8 March 1999, freezing precipitation fell over portions of western Nebraska, including the communities of McCook, North Platte and Valentine between midnight and 10:00 A.M. CST. The freezing precipitation resulted in dangerous travel conditions, causing several automobile accidents, along with the delay and temporary closing of area public schools 8 March 1999. The freezing rain changed over to snow later the same morning.
A forecast for snow, rather than freezing precipitation, was issued by the local forecast office. The forecast issued was based predominately on numerical weather prediction guidance, and the observed temperature profile of the atmosphere. The observed temperature profile of the atmosphere was below 0°C throughout its entire vertical extent. The purpose of this study is to present important components of cloud microphysics that will be useful in an operational forecasting environment. It has been shown in numerous studies that clouds consist predominately of water vapor until the cloud reaches a critical temperature of -15°(Rogers and Yau, 1989.) In general, for the lower 48 conterminous states, ice crystal growth by deposition, aided by the presence of ice nuclei (predominately vermiculite), is maximized at -15°C (Houghton, 1950.)
In this particular situation, an operational forecaster must possess a basic understanding of cloud microphysics in order to accurately assess the atmospheric potential for liquid versus freezing or frozen precipitation. Numerical weather prediction guidance, nomograms, and thickness schemes did not accurately predict precipitation type in this particular atmospheric environment, which included a low level stratus cloud layer at a temperature between 0°C and -10°C. For this case, the synoptic and sub-synoptic-scale environment, as well as vertical atmospheric structure will be examined.
By substituting the latent heat of sublimation for the latent heat of evaporation in the Clapeyron-Clausius equation, where Hvap = 597.3 cal/gm, and Hsub= 677.0 cal/gm, then solving for temperature, it shows that the saturation vapor pressure over ice is less than over water (Smith, 1995.) Therefore, the saturation vapor pressure of super-cooled water droplets is more than that of ice nuclei. Because of this difference in saturation vapor pressures, the ice crystals will grow at the expense of the water vapor droplets in the cloud (the Bergeron Process, Ahrens, 1991.) At temperatures between -10°C and -15°C, ice crystal growth is maximized, as the difference in saturation vapor pressure is greatest (Neuberger, 1967) Once the ice crystal growth process begins, the threat of liquid precipitation falling from the cloud and freezing on surface features becomes minimal. It follows logically then, that at temperatures between 0°C and -10°C, there is a greater threat of super-cooled water droplets falling from the cloud, freezing as they deposit on surfaces.