[MZZ39-P05] HUMAN-ASSOCIATED EXTREME EVENTS: FREEZING PRECIPITATION
Keywords:freezing rain, freezing drizzle, weather conditions conducive to freezing precipitation, northern extratropics
We used supplementary synoptic information to evaluate the weather conditions during the freezing events observed at more than 1,500 long-term (i.e., 40+ years) stations in North America and Northern For these stations, we estimated the near-surface temperature (T) and humidity (H) intervals, within which the freezing events do occur. It appears that within these T and H intervals the other precipitation events occurred also and they are not necessary characterized by freezing. Therefore, we used Integrated Global Radiosonde Archive to blend our synoptic data with collocated upper air soundings and selected those that corresponded to freezing events at the ground. For freezing events, we found and quantified (a) unusually warm air as compared to long-term climatology values of corresponding Julian day; (b) much warmer low tropospheric air temperature than in the "nearby" days without freezing event at the ground; (c) frequent near surface temperature inversions, when the air at the 850 hPa is warmer than at the surface and the troposphere temperatures are warm enough to let the snowflakes to be condensed into rain drops, and (d) the lowest troposphere and near surface temperatures are cold enough to cool the temperature of these rain drops below freezing.
Combination of these meteorological variables (near-surface temperature, humidity, and the low troposphere temperature anomalies and gradients) allows us to build a set of the weather conditions that are conducive to freezing precipitation occurrence when it rains.