Mechanisms for Springtime Onset of Isolated Precipitation across the Southeastern United States

Rosana Nieto Ferreira,Thomas M Rickenbach

doi:10.3390/atmos12020213

Abstract

This study uses four-year radar-based precipitation organization and reanalysis datasets to study the mechanisms that lead to the abrupt springtime onset of precipitation associated with isolated storms in the Southeast United States (SE US). Although the SE US receives relatively constant precipitation year-round, previous work demonstrated a “hidden” summertime maximum in isolated precipitation features (IPF) whose annual cycle resembles that of monsoon climates in the subtropics. In the SE US, IPF rain abruptly ramps up in May and lasts until sometime between late August and early October. This study suggests that the onset of the IPF season in the SE US is brought about by a combination of slow thermodynamic processes and fast dynamic triggers, as follows. First, in the weeks prior to IPF onset, a gradual seasonal build-up of convective available potential energy (CAPE) occurs in the Gulf of Mexico. Then, in one-to-two pentads prior to onset, the upper-tropospheric jet stream shifts northward, favoring the presence of slow-moving frontal systems in the SE US. This poleward shift in the jet stream location in turn allows the establishment of the North Atlantic subtropical high western ridge over the SE US which, with associated poleward transport of high CAPE air from the Gulf of Mexico, leads to the establishment of the warm-season regime of IPF precipitation in the SE US.

Highlights

The Southeast United States (SE US) does not have well-defined wet and dry rainy seasons [1], it displays a strong seasonality of the subset of precipitation associated with sub-mesoscale isolated storms—hereafter referred to as isolated precipitation features, or IPF [2]
A study of high-resolution surface-radar precipitation for the SE US over four years [3] showed that the annual cycle of precipitation in the SE US has a 6:1 ratio of summer-to-winter IPF rain, while the mesoscale precipitation features (MPF) rain rate is on average the same year-round
The chain of events that lead to the onset of the IPF season in the SE US includes (1) a slow seasonal increase in convective available potential energy (CAPE) over the Gulf of Mexico, combined with (2) a fast northward shift of the upper-tropospheric jet stream and associated shift slow moving frontal systems at the surface, and (3) the arrival of the North Atlantic subtropical high (NASH) western ridge, which transports high CAPE

Summary

Introduction

The Southeast United States (SE US) does not have well-defined wet and dry rainy seasons [1], it displays a strong seasonality of the subset of precipitation associated with sub-mesoscale isolated storms—hereafter referred to as isolated precipitation features, or IPF [2]. A study of high-resolution surface-radar precipitation for the SE US over four years [3] showed that the annual cycle of precipitation in the SE US has a 6:1 ratio of summer-to-winter IPF rain, while the MPF rain rate is on average the same year-round. The onset of the IPF rainy season was shown to begin in south Florida in mid-April and occur nearly simultaneously across the southeastern coastal plain during early-to-mid May. Subsequently, onset progressed northward and inland from mid-June through early July. Onset progressed northward and inland from mid-June through early July They found that the timing of the IPF season onset in spring was more consistent from year to year in the southern portion of the domain, suggesting a mechanism of onset related to the annual cycle of solar radiation.

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