April 25, 2022 - 3:00 pm
April 25, 2022 - 4:00 pm
Boundary Layer, Urban Meteorology, and Land-Surface Processes
Nocturnal ascent and low-level jet over a baroclinic area
Monday, April 25
A remarkable feature of the regional climatology of the central United States and the Great Plains is the existence of a nocturnal maximum in convective rainfall during the warm season. This study follows up the work of Shapiro et al. (2018; hereafter S18), in which a linear theory predicts weak but persistent ascent in the nocturnal boundary layer in baroclinic environments such as the United States Great Plains. In the S18 theory, the sudden decay of dry-convective mixing in the convective boundary layer at sunset triggers inertia-gravity waves as well as a Blackadar (1957)-like nocturnal low-level jet. The theory predicts that air parcels in the ascent phase of the wave can rise 500 m – 1 km in 6 hours, which may promote convective initiation and play a role in that region’s well-known nocturnal maximum in convective rainfall.Â In the present study, the CM1 numerical model (Cloud Model 1, developed at NCAR by George Bryan) is used toÂ examine whetherÂ the main predictions from the S18 ascent/jet theory arise in a more realistic setting.Â Specifically, in these simulations, theÂ non-linear terms in the governing equations are retained, and the sudden and total shutdown of turbulence at sunset is replaced by a more realistic evening transition.
After confirming that a Blackadar-like nocturnal low-level jet is generated by the CM1 when run in a barotropic mode [no horizontal temperature gradient; free-atmosphere geostrophic wind is constant and southerly at 10 m/s], experiments focus on the impact of baroclinicity. A baroclinic zone arises gradually in the CM1-simulated boundary layer over the course of a 4-day integrationÂ periodÂ mostly throughÂ a temporally constant prescribed deficit in the moisture flux at the surfaceÂ (a Gaussian function of x, with user-specified amplitude and length scale) which causes extra daytime heating in the center of the domain. Aspects of the inertia-gravity wave response predicted in S18 are present in all the simulations. In particular, the main zone of ascent that develops after the evening transition is found to slowly descend with time.Â The peak vertical velocity and duration of the ascent are qualitatively similar toÂ what is predicted by the S18 theory, however theÂ onset of the ascent is generally earlier than predicted in S18.