Low Level Jet Making the Atmosphere Unstable Again

This study investigates the spatial and temporal variation of the nocturnal low level jet (LLJ) over the Southern Great Plains and its relationship to stability of the atmosphere. Radiosonde measurements from 4 sites in Oklahoma and 1 in Kansas are utilized. As part of the field project International H₂O Project, radiosondes were deployed every 3 hours from 25 May – 15 June 2002. The LLJ occurred in cycles, with a synoptic scale disruption to the southerly low level flow marking the end of consecutive days with a LLJ. One particular cycle began when a cold front swept through Kansas and Oklahoma on 5 June 2002. In the following days the LLJ ramped up, with southerly flow absent on the 6^th to 20 m s^-1 by the 8^th across the region. A westerly LLJ was also observed to occur ~1500 m above ground level (AGL). Height and strength varied spatial and temporally with a tendency to have stronger and lower altitude LLJ’s at the western Oklahoma site and peaking between midnight and 3am CST. Layers aloft (0.5-2.0 km AGL) became increasingly unstable overnight where differential advection took place. HYSPLIT back trajectory analysis suggests mesoscale vertical motions occurred on at least one night leading to the cooling of layers aloft. In some cases the atmosphere underwent rapid destabilization overnight with convective available potential energy increasing ~1000 J kg^-1 over a 3 hour period. High spatial variability in stability over the domain resulted from mesoscale features such as a retrograde dryline over western Oklahoma.