An Observational Study of Convection Initiation under Quiescent Conditions in the Southeastern United States

Predicting convection initiation (CI) is a challenging forecasting problem during the summertime in the southeast US because of the absence of mesoscale to synoptic-scale forcings. Detecting the triggers for CI under these quiescent conditions requires high temporal resolution meteorological measurements combined with field campaigns. To this end, we use meteorological observations from summer, 2014 and summer, 2015 near Belle Mina, Alabama, approximately 27 km west of Huntsville. The summer 2014 observations included measurements from four 10 m micrometeorological towers installed at two grassland sites, a corn crop, and soybean crop approximately 500 m apart at which 1 min means of a suite of meteorological variables were obtained. Temperature and moisture profiles were obtained from an on-site Radiometrics MP-3000A microwave radiometer (MWR). During the 2015 experiment, one 10 m micrometeorological tower was installed at a grassland site, and a Leosphere Windcube 100S lidar and MWR were deployed. Days with CI occurring under quiescent conditions were identified using synoptic analyses, radar observations from the Huntsville radar, and on-site web camera images. Days on which CI occur are characterized by morning Td temperatures >21°C, wind speeds <2 m s-1, and median CAPE values, obtained from the nearest sounding station, of 548 J kg-1. Measurements from the micrometeorological towers indicate that half hour variances of the 1 min specific humidity and pressure show marked increases during the 0-1 h prior to CI. Additional measurements from field campaigns during both summers, which included multiple rawinsonde launches per day, were then used to investigate in more detail the triggers for CI. Measurements from the field campaigns indicate that increases in boundary layer moisture, as well as larger variability in sensible heat flux during the morning, help trigger CI under quiescent conditions in the southeastern US.