Boundary Layer, Urban Meteorology and Land-Surface Processes

Ascent in a nocturnal low-level jet

Dr. Alan Shapiro

School of Meteorology

09 September 2016, 2:00 PM

National Weather Center, Room 5600
120 David L. Boren Blvd.
University of Oklahoma
Norman, OK

It has long been recognized that nocturnal low-level jets (LLJs) can provide thermodynamic and dynamical support for nocturnal convection over the Great Plains. However, the relation between a LLJ and many occurrences of convective initiation (CI), particularly involving elevated CI in regions removed from obvious frontal boundaries or other mesoscale structures, remains elusive. In such cases CI often happens in association with a LLJ, but it is not clear if the LLJ is a causative agent or if the LLJ and CI arise independently from nocturnal boundary layer processes and simply occur within overlapping domains at similar times.

A theory is presented for the vertical motion induced by a class of spatially variable LLJs. The theory does not address CI per se, but makes a case for how a nocturnal LLJ associated with (i) horizontally varying free-atmosphere geostrophic winds and (ii) horizontally varying buoyancy can force broad zones of mesoscale ascent within and above the atmospheric boundary layer (ABL), thus creating a favorable environment for CI.

We consider the development of a LLJ from the sudden relaxation of the frictional constraint in the ABL around sunset (mechanism proposed by Blackadar, but augmented with a buoyancy effect arising from a differentially heated surface). The governing equations are the linearized Boussinesq equations of motion, thermal energy, and mass conservation equation. Horizontal variations in the free-atmosphere geostrophic wind or ABL buoyancy, as accounted for in the initial (sunset) conditions. The sensitivity of the solution is explored with respect to the free-atmosphere Brunt-Vaisala frequency, Coriolis parameter, ABL depth, and the amplitude and lateral wavelength of the free-atmosphere geostrophic wind or ABL buoyancy variations. A laterally varying buoyancy is much more effective than a laterally varying free-atmosphere geostrophic wind in generating vertical motion, inducing vertical displacements of 500 m to 1 km within 6 h of sunset.

Boundary Layer, Urban Meteorology and Land-Surface Processes Seminar Series website