A climatology of high lapse rates and their influence on the occurrence (or non-occurrence) of severe weather over the central United States

Previous work has demonstrated that elevated mixed layers (EMLs) are often important ingredients for the development of deep, moist convection (DMC) over the central United States (US). The EML is typically characterized by a region of high (or, steep) mid-level lapse rates generated over the semi-arid Intermountain West (IW) US that develops an inversion at the base of the mixed layer as they are advected away from their source region.

A 35-year climatology of high 700–500-hPa lapse rates (> 8.0 K km-1), generated from 119 stations within the North American radiosonde network, is compared to a similar climatology of high lapse rates independent of a fixed-pressure depth. The climatologies highlight a warm-season maximum over the IW that expands poleward from March–June and periodically eastward to over the north-central US from June–August as EMLs on the poleward-side of the climatological upper-level anticyclone over the southern-central US.

This study further revisits the mesoscale and synoptic scale influence of the EML as an important ingredient to the development of DMC (occurrence of severe weather) and investigates the noted concurrence of EMLs in association with high surface air and dew point temperature values (> 40°C and > 25°C, respectively) during heat wave events (e.g. non-occurrence) over the central US.

Backward air-parcel trajectories and synoptic-scale composites suggest that both the occurrence and non-occurrence of severe weather are associated with an anomalous extension of high mid-level lapse rates from the IW to over the central US and the development of shallow moist boundary layers (MBLs). These shallow MBLs generally form beneath the EML inversion, either through northward moisture advection from over the Gulf of Mexico and/or by in-situ growth via evapotranspiration processes from underlying vegetation. Analyses suggest that the vertical mixing of the MBL into the mid-troposphere is inhibited during the latter events in the presence of a stronger inversion at the base of the mixed layer and tropospheric-deep subsidence on the anticyclonic shear side of the upper-level jet.