Common Transient Drainage flows and Cold Pools

Turbulence in stable weak-wind conditions is generated partly by shear associated propagating transient modes and shear related to local terrain-induced circulations. Here we analyze data from 21 turbulence stations and a 20 m tower that includes 6 sonic anemometers in the lowest 5 m during the Shallow Cold Pool Experiment. The network was deployed in a typical shallow valley about 270 m wide and 12 m deep with a downvalley slope of 2-3%. Three-dimensional structure includes smaller-scale gullies. Because the valley is shallow and less organized than those normally examined in the literature, a more flexible concept of the cold pool and downvalley cold air drainage is required. The top of the cold pool is diffuse. The cold pool and drainage flows are intermittent and cold pools on different scales are superimposed. At the same time, even weak intermittent cold pools substantially reduce the near surface turbulence compared to that over nearby flatter terrain.

The computation of the vertical profile of the turbulence in the cold pool requires extra attention to reduce the impact of non-turbulence modes on the estimated turbulence variables. The turbulence is fine scale and responds to the vertical structure of the drainage wind maximum, even when it is as low as 1 m above the ground. The interaction between the turbulence structure and the downvalley cold air drainage is examined. This analysis is extended to the valley slopes and upland regions.