18th Conference on Weather and Forecasting, 14th Conference on Numerical Weather Prediction, and Ninth Conference on Mesoscale Processes

Monday, 30 July 2001
Low-level jet dynamics and parameterization of surface fluxes in the stable boundary layer
Robert M. Banta, NOAA/ERL/ETL, Boulder, CO; and R. K. Newsom and J. K. Lundquist
The representation of turbulent surface fluxes between the stable boundary layer (SBL) and the earth's surface in NWP models is crude, because the processes responsible for the fluxes are poorly understood. Even over relatively flat terrain, many different processes contribute to subgrid vertical transports in the SBL, including the generation of gravity waves and turbulence, the formation of the nocturnal low-level jet (LLJ), and the activity of local drainage flows, which occur even over relatively flat terrain. Waves and turbulence are produced mostly in the shear layers above and below the LLJ maximum, and those that most affect fluxes between the surface and the atmosphere are below the jet max. Thus, the behavior of the LLJ is seen as important to understanding these fluxes. The acceleration of the jet determines the shear between the surface and the jet ma; increased shear promotes mixing. On the other hand, surface cooling increases the stability in the surface layer, which suppresses vertical mixing. This promotes decoupling of the LLJ layer from surface friction and further acceleration of the jet. The magnitude of the vertical exchange between surface and atmosphere depends on where the balance between these 2 opposing processes is struck.

Unique data from 2 field projects have allowed new insight into these processes. The unique datasets are from 2 scanning, boundary-layer Doppler lidars. The field projects are the CASES-99 experiment, designed to study the nocturnal SBL over the Kansas prairie, and the Southern Oxidants Study (SOS) 1999 intensive field campaign in Nashville TN. In the former the lidar was embedded in an array of instrumentation that included a 60-m tower, and the latter (SOS-99) included extensive atmospheric chemistry measurements that allowed the effects of the vertical mixing to be assessed.

Jetlike structure in the wind-speed profiles was seen on all clear nights of both projects. During CASES the nose of the jet was often between 50 and 100 m, below the minimum range of even UHF radar wind profilers. These very low jets thus have not been included in previous systematic studies of the SBL. We have analyzed data from all experimental nights of both experiments, to develop plots of the time-dependent behavior of the speed, height, and direction of the jets from before sunset to after sunrise. The scans are also analyzed to determine the amplitude of the turbulent fluctuations as a function of time and height. An advantage of the lidar scan data is that individual scans, for example vertical-slice (RHI) scans, can be inspected during periods of high interest to see the structure of the flow–e.g., waves or turbulence and the length scales of the features observed. We also produced animations of sequences of repeated scans to further enhance our ability to interpret the data.

Examples of jet behavior and the structure of the resulting waves or turbulence will be presented, and implications toward a new parameterization of turbulent fluxes in the SBL for mesoscale NWP models will be discussed.

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