Microfronts and Other Nocturnal Submeso Motions over Microtopography

Northwest Research Associates, Incorporated, Seattle WA

Investigators

Abstract

This award examines the boundary layer of air adjacent to the ground surface that cools at night. Strongest cooling occurs with clear skies and low wind speeds. This situation is poorly understood compared to other boundary layers with more significant winds or daytime heating. The strong cooling leads to practical issues including freezing street surfaces and frost damage to sensitive crops. The strong surface cooling also reduces vertical mixing and thus increases the probability of high concentrations of pollutants. The weak winds correspond to large variability of wind direction that spreads the pollutants over a variety of wind directions. This process is currently hard to predict. Formation of dense fog is another outcome of the strong cooling and weak vertical mixing. Local spatial variation of the surface cooling is an important feature requiring more attention. This project will increase understanding through analysis of intense measurements from different sites. Each site contains a network of clustered stations. The above analyses will improve the boundary-layer part of forecast computer models. The general analysis of different subclasses of very stable nocturnal boundary layers, and the relation of submeso motions to these subclasses, will be investigated. Very stable nocturnal boundary layers most often occur with strong stability associated with low wind speeds and clear skies. Submeso motions occur on scales just larger than the largest turbulent eddies but smaller than the mesoscale motions. Submeso motions include microfronts, wave motions, meandering motions, and more complex structures. These motions densely populate the very stable boundary layer. The analyses will be based on time series, spatial information across networks, and combined time-space domains from different datasets. The analysis will first concentrate on microfronts. The first stage focusses on increasing horizontal gradients of temperature. The second stage includes horizontal convergence that concentrates horizontal gradients into microfronts. Analysis of other submeso motions will follow with both individual analyses of each type of submeso motion and collective analysis of all submeso motions. Examination of the large wind-direction variability caused by submeso motions will be examined in detail. Additional data sets will be evaluated with possible recommendation of a new tailored field program. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.