A climatology of high lapse rates and their influence on heat waves east of the Rocky Mountains

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 climatology of high 700–500-hPa lapse rates (> 8.0 K km-1) generated from the North American radiosonde network highlights 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.

The occurrence of EMLs have also been noted in association with high surface air and dew point temperature values (> 40°C and > 25°C, respectively) during heat wave events over the continental US. Backward air-parcel trajectories and synoptic-scale composites suggest these events (e.g., 9–14 July 1995 Chicago heat wave) are associated with an anomalous extension of high mid-level lapse rates from the IW to over the north-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, where the vertical mixing of the MBL into the mid-troposphere is inhibited.