Quantifying the Ratio of Non‐Synoptically Forced Precipitation Events Over CONUS Using the Quasigeostrophic Omega Equation

The unpredictability of precipitation over land is a major forecasting challenge, particularly when local processes dominate. This study introduces a classification scheme for precipitation events based on synoptic‐scale forcing for ascent, quantified via the quasigeostrophic (QG) omega equation, motivated by a broader goal of studying the role of land‐atmosphere coupling in regimes of weak synoptic forcing. The scheme classifies 3‐hourly precipitation as synoptically or non‐synoptically forced based on threshold values of QG omega. It is first developed using NASA's MERRA‐2 reanalysis, and QG omega fields computed from MERRA‐2 are subsequently used for classification of the IMERG precipitation data set over the continental United States (CONUS). The mean ratio of non‐synoptic to total precipitation events exhibits a strong south‐north gradient from 0.7 at 25°N to 0.35 at 50°N in the zonal average. There is strong seasonality, with synoptically driven precipitation dominant in winter and a non‐synoptic precipitation ratio exceeding 0.8 in the southern half of CONUS in summer. Interannual variability in the non‐synoptic precipitation frequency is largest in the southern CONUS, but secular trends are weak and largely insignificant except for a decreasing trend over the northern Plains and Midwest in both DJF and MAM. Classification of precipitation using QG omega computed directly from satellite sounder data is explored. The sounder‐based non‐synoptic precipitation ratio is significantly overestimated due to a clear‐sky coverage bias in the sounder retrievals.