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Quantifying the Ratio of Non-Synoptically Forced Precipitation Events Over CONUS Using the Quasigeostrophic Omega Equation
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DOI：10.1029/2026JD047250.png)
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Abstract
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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.
Keywords:
large-scale vertical motion
synoptically forced precipitation
non-synoptic precipitation
precipitation classification
satellite sounder
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