During overnight cooling of the planetary boundary layer overland, near-surface wind speeds have been found to decrease approximately linearly with screen temperature. However numerical models incorporating only near-surface form drag or increased roughness length suggest that inertial oscillations in the stable layer limit the decrease in wind speed. A simple gravity-wave parametrization imposes a strong drag force that has been found to give reasonable quantitative agreement with surface observations and to agree qualitatively with vertical wind-profile changes measured on several evenings of observation. The vertical wind profiles clearly show the effect of an increasingly strong drag force in the lower atmospheric levels as the surface cools.

COLPEX: FIELD AND NUMERICAL STUDIES OVER A REGION OF SMALL HILLS. During stable nighttime periods, large variations in temperature and visibility often occur over short distances in regions of only moderate topography. These are of great practical significance and yet pose major forecasting challenges because of a lack of detailed understanding of the processes involved and because crucial topographic variations are often not resolved in current forecast models. This paper describes a field and numerical modeling campaign, Cold-Air Pooling Experiment (COLPEX), which addresses many of the issues.The observational campaign was run for 15 months in Shropshire, United Kingdom, in a region of small hills and valleys with typical ridge valley heights of 75-150 m and valley widths of 1-3 km. The instrumentation consisted of three sites with instrumented flux towers, a Doppler lidar, and a network of 30 simpler meteorological stations. Further instrumentation was deployed during intensive observation periods including radiosonde launches from two sites, a cloud droplet probe, aerosol monitoring equipment, and an instrumented car. Some initial results from the observations are presented illustrating the range of conditions encountered.The modeling phase of COLPEX includes use of the Met Office Unified Model at 100-m resolution, and some brief results for a simulation of an intensive observation period are presented showing the model capturing a cold-pool event. As well as aiding interpretation of the observations, results from this study are expected to inform the design of future generations of operational forecasting systems (Page 1636)

Plots of near‐surface wind speed against screen temperature during the evening transition, originally observed at the Cardington (UK) site, have been compiled for seven other sites in Britain. It appears that the form of the plot depends significantly on the height of the surrounding terrain. Attempts to simulate the plots with a simple numerical model were initially unsuccessful due to the presence of inertial oscillations in the model. However the inclusion of terms to emulate blocking pressure fields due to neighbouring terrain and obstructions gave better agreement with the observations. © Crown Copyright 2010. Published by JohnWiley & Sons, Ltd.

Within the Met Office's Unified Model the visibility is diagnosed from a set of the model's prognostic variables using a parametrization. This parametrization has been optimized to minimize the error in the model visibility, relative to observed visibility. The performance of the parametrization is dependant on two aspects; (1) the quality of the input meteorological variables and (2) the structure of the parametrization itself. This paper describes a technique for obtaining a quantitative assessment of how much improvement is possible in the structure of the visibility parametrization. This is achieved by constructing an alternative visibility diagnostic scheme using a neural network. This statistical model provides a benchmark against which the performance of the current visibility parametrization can be judged, irrespective of the input error. It was found that the neural network achieved significant improvements over the current diagnostic parametrization: a 22% improvement in the geometric mean of the fractional error, and a 15% improvement in the geometric variance of the fractional error. (C) Crown Copyright 2008. Reproduced with the permission of Her Majesty's Stationery Office. Published by John Wiley & Sons, Ltd.

Results are presented from a comparison of the Met Office Surface Exchange Scheme (MOSES) with observational data. A stand‐alone version of MOSES is used which can be forced directly with observations rather than model output. The forcing data consist of near‐surface measurements of incident radiation, wind speed, temperature, pressure, humidity and precipitation, and the model output is compared with measured surface fluxes of sensible and latent heat and with soil moisture at several levels below the surface. The dataset spans four years and comes from a grassland site with a shallow water table, the Met Office Research Unit at Cardington in Bedfordshire, UK.