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

Abstract

Nocturnal low-level jets (NLLJs) are commonly observed over the Southern Great Plains (SGP) and have been linked to occurrence of the nocturnal maximum in mesoscale convective systems over this region during the late spring and summer. Researchers have long proposed that the Blackadar mechanism of an inertial ageostrophic oscillation superimposed on a southerly geostrophic flow is a likely cause of the southerly NLLJ. The NLLJ has also been the subject of recent research, in part driven by the community focus on Plains Elevated Convection at Night (PECAN) field campaign. Findings from this recent research include that the NLLJ is not just composed of a southerly jet, but also includes a maximum in the westerly winds above the southerly jet. The potential importance of baroclinicity on the sloping terrain in contributing to the NLLJ structure has also been a recent focus with a contribution to the NLLJ structure likely to be as or even more important than the Holton mechanism which assumes constant buoyancy along the slope. While temperature and moisture advection associated with the NLLJ has been shown known to create favorable conditions for deep convection, recent investigations have proposed a variety of other mechanisms. These mechanisms include persistent ascent driven by an inertial-gravity associated with a local maximum in the lateral variations in buoyancy, quasi-geostrophic ascent associated with warm advection, and the destabilization and maintenance of deep convection by bores and long-period gravity waves.

This thesis investigates the evolution of the structure of the NLLJ following two cold frontal passages over the SGP region during the International H2O Project field campaign (IHOP_2002). The NLLJs where examined in subsequent days after cold frontal passages on 25 May and 5 June 2002. The data set utilized includes radiosonde observations made at 3-h intervals from five sites maintained by the ARM (Atmospheric Radiation Measurement). These data sets provided insight into processes contributing to the NLLJ structure and to the return of the favorable conditions over the SGP for deep convection including the spatial and temporal variations in Convective Available Potential Energy. The analysis of data from IHOP_2002 was supplemented with the fields from the ERA-5 Reanalysis. The 30-km horizontal grid and 1-h temporal resolution of this data set allowed deeper insight into the temporal and spatial variations of conditions over the sloping terrain.

The findings from this analysis include that the southerly and westerly component of the NLLJ strengthened in both intensity and height following the two cold frontal passages. The intensity of the southerly NLLJ was linked to the ageostrophic enhancement expected from Blackadar mechanism superimposed on a far larger general increase in the background southerly geostrophic wind. The increase in the southerly geostrophic flow extended over a depth of 3 to 4 km apparently in association with heating over the sloped terrain on synoptic time-scales. In order to better understand the evolution of NLLJ in these post-frontal periods, the evolution of conditions following the frontal passage on 5 June was investigated in detail. Specifically, the period was divided into a pre-moistening, moistening and post-moistening phases. A key finding was that the buoyancy gradient on and over the slope became increasingly non-uniform during this recovery period. During the post-moistening period, the buoyancy gradient at the surface became clearly non-linear with an increased gradient evident over a distance of 2 degrees of longitude in response to diurnal heating. Calculations showed that the enhancement to the pressure gradient force that occurred after peak heating exhibited spatial variability on scales of < 150 km, but was smoothed out at scales of > 300 km. The leading edge of this enhanced gradient was associated with a transition in the day-time boundary layer depth changing from ~4 km (above ground level) to the west compared to only ~1 km over the moist air mass. The circulations at this transition in boundary layer height had the characteristics of a dryline, which formed in the late afternoon and moved up the slope after sunset. Thus, during this post-moistening phase the recently proposed concept of a uniformly linear buoyancy gradient on the slope is no longer valid in the vicinity of this enhanced and moving gradient in buoyancy associated with the dryline. In this region, the vertical profile in the southerly geostrophic wind also does not remain constant with height during the night.

Our results suggest that over the western slopes, the NLLJ can be inherently heterogeneous due to the non-uniform and evolving gradients in buoyancy over the slope and that advection by the southerly winds above the NLLJ also impact the variations in NLLJ structure. These results stand in contrast to recent studies that have shown that the NLLJ becomes heterogeneous primarily due to advection by the westerly component of the NLLJ over the sloping terrain in the presence of a linear variation in buoyancy on the slope. Our finding of an inherently non-linear structure during the post-moistening phase also made it difficult to determine the extent to an inertial oscillation was contributing to NLLJs during this period. The impacts of this changing buoyancy gradient was associated with the NLLJs strengthening with intensification of a baroclinic zone as the height of the southerly low-level jet was consistent with being produced from a thermal wind reversal along sloped terrain. Other key findings from our analysis are that the long wave radiative impacts likely impact the thermal gradients and that the diurnal reversals in the buoyancy gradient on the slope varied in magnitude, timing and height. For example, early in the post-frontal period the buoyancy reversal took place near the surface in association with the nocturnal stable layer, while the late in the period, the reversal took place aloft in associated with the growth of the morning boundary layer.

Our analysis also has implications for understanding the mechanisms for the initiation and maintenance of deep convection in the NLLJ environment. Specifically, NLLJs were critical in returning conditions favorable for convection, with moisture transport playing a greater role on earlier after the frontal passage and ascent later in the recovery process after the frontal passage. The nature of the nocturnal boundary layer also varied significantly during the recovery period following the frontal passage changing initially from shallow (< 100 m) and strong inversions initially to deeper (~500 to 800 m) and less stable inversions capped by an inversion. Another relevant finding is that late in the recovery period, the conditions on the higher slopes to the west become less favorable for deep convection during the night as the layer of high CAPE becomes more shallow. This finding that convection will become less likely to occur over higher terrain later in the night is consistent with the concept of a west-to-east progression of nocturnal convection over the Great Plains. Our analysis also provided insight into the vertical motions in the NLLJ environment that could influence the initiation and maintenance of deep convection. Specifically, the analysis appeared consistent with the presence of inertial-gravity wave generated in the deep, warm residual layers to the west. Ascent was also associated with warm advection as has been argued to occur from quasi-geostrophic forcing. Our analysis shows, however, that the warm advection by the ageostrophic motions is similar in magnitude to the geostrophic forcing and occurs over a deeper depth. Thus, a semi-geostrophic framework is likely to be relevant than quasi-geostrophic theory. Other mechanisms for ascent are also discussed.