Chris Rattray - July 29

School of Meteorology MS Thesis Defense

Evolution of the Structure and Dynamics of the Nocturnal Low-Level Jet after the Passage of a Cold Front and Insights into the Initiation and Maintenance of Nocturnal Convection

Chris Rattray

Thursday, July 29^th

^2:00pm

Google Meet joining info

Video call link: https://meet.google.com/ecw-juwq-cqw

Or dial: ‪(US) +1 337-350-3366 PIN: ‪712 218 561#

2:00pm The nocturnal low-level jet (NLLJ) over the United States Great Plains (USGP) is characterized by a southerly wind-maximum several hundred meters above ground during the warm season. The southerly wind enhancement associated with the NLLJ accounts for as much as one third of the water vapor into the continental United States from the Gulf of Mexico. The NLLJ has been proposed as an explanation for the nocturnal maximum in convection over the region and implicated severe weather events, flooding in the Midwest and amplifying drought conditions over the USGP. Studies have suggested that future trends in precipitation over the USGP associated with climate change may depend on changes in intensity of the NLLJ. Several mechanisms have been proposed to explain the occurrence of the NLLJ including an inertial oscillation, a thermal wind oscillation along sloped terrain, uneven heating along sloped terrain and mean seasonal on the slope.  However, despite the important role of the NLLJ to weather and climate, there is a lack of consensus on the mechanism(s) responsible for the NLLJ over the USGP,

This study focuses on the examination of the evolution of the NLLJ and return of convectively unstable conditions following a passage of cold front over the Southern Great Plains (SGP)  during the International H₂0 Project (IHOP_2002). The data sets utilized included rawinsondes deployed from 5 Department of Energy Atmospheric Radiation Measurement Program (ARM) at 3-h intervals supplemented with ERA5 reanalysis data.

Specifically, this study will address the questions:

1) What is the day-to-day evolution of the NLLJ after a cold frontal passage has diminished the thermal gradients along the slope?

2) How does this evolution facilitate convection?

Confirming recent studies, our results show that the NLLJ had both a maximum in the southerly component and an elevated westerly maximum. These maximums, however, increased in both magnitude and height in the days following the frontal passage. A key finding from this study is that this increase in the intensity of the southerly NLLJ was primarily linked to the strengthening of the southerly geostrophic wind (Vg) with a secondary ageostrophic enhancement as expected from an inertial oscillation. However, the horizontal pressure gradient force (PGF) also varied significantly both along the slope and with the diurnal cycle resulting in large changes in Vg(z). These variations in the PGF were much more pronounced at scales of ~150 km compared to ~300 km. Causes of these diurnal variations in the PGF were explored and it is speculated that this was related to decoupling of heterogeneous boundary layers. These characteristics of the PGF make it difficult to quantify the role of the inertial oscillation. This study also addresses the extent to which conditions that favor nocturnal convection evolved following the frontal passage. Findings include spatial variations in CAPE that would favor deep convection earlier in the night on the western slopes, and ascent associated with warm advection. In contrast to recent studies, this study found that the warm advection was due to both the geostrophic and ageostrophic flow bringing the quasi-geostrophic assumption into question.