S m YA new method of correcting data from the United Kingdom operationd weather radar network is described. The physically based scheme is designed to produce estimates of instantaneous precipitation rate at the surface by compensating for the effects of the bright band, range and low-level orographic growth. In a preliminary study, the characteristic shape of the vertical profile of reflectivity factor was examined using a climatological dataset derived from rangeheight indication scans recorded by a high-resolution radar. The results were then used to construct an idealized reflectivity factor profile. In the correction procedure, the heights of significant turning points in the profile are diagnosed from relevant meteorological data at each radar pixel. The parametrized profile is weighted by the radar-beam power profile and the surface precipitation rate found by an iterative method in real-time. The scheme has some important advantages over the alternative correction strategies. Detailed quantitative evaluation in typical cases of mainly frontal rainfall over southern England suggested that root-mean-square errors in estimates of instantaneous precipitation rates over 5 km pixels were reduced to less than half compared to the same radar data subject to only a fixed range correction.
Studies of a numerical model of fog are presented. These indicate that: (i) For given initial conditions, a reduction in turbulent diffusion leads to earlier and thicker fog formation.(ii) The formation and growth of fog on a realistic time scale is simulated by the inclusion of radiative cooling due to water vapour and carbon dioxide and (when formed) fog droplets.(iii) The inclusion of gravitational settling is necessary to simulate realistic liquid water contents.(iv) The nature of the underlying surface (through its thermal properties) influences the time of onset of fog.The effect of cloud cover and of solar insolation on dispersing fog was assessed. These results are broadly consistent with, and help to account for, the results of a parallel field study by Roach, Brown, Caughey, Garland and Readings (1976).
SUMMARYThe results of a field study of the evolution of radiation fog on three nights are presented. A parallel numerical model (Brown and Roach 1976 -referred to hereafter as 11) was also developed to test ideas suggested by the principal features observed during the field study. These were:(i) Periods of significant fog development appeared to occur when wind speeds dropped below 05-1 ms-l .(ii) The liquid water content of the fog was a small fraction of the total condensed out by cooling. The balance of water appears to have been deposited on the ground.(iii) Radiative cooling (deduced from radiative flux divergence measurements) was in general greater than the actual cooling.(iv) As the surface became radiatively shielded by developing fog, the radiation inversion migrated to the fog top, accompanied by the establishment of a convective regime with a slight superadiabatic lapse rate in the lower part of the fog.(v) Ammonium sulphate was a principal constituent of condensation nuclei. It is shown that these features are consistent with the suggestion that the development of radiation fog is primarily controlled by a balance between radiative cooling, which encourages fog, and turbulence, which inhibits it. Gravitational settling of fog droplets and soil heat flux also emerge as important factors. The role of cloud microphysics is not passive, but is less clearly defined as yet. The numerical model (11) reproduces most of the observed features, but not the stepwise growth or sharpness of the fog top.Some of the practical implications of this work for forecasting and for fog modification are briefly discussed.
SUMMARYHigh resolution observations from a tethered balloon of nocturnal stratocumulus on three occasions are presented. Measurements of the microphysical properties of the clouds (drop-size distributions, concentrations and liquid water contents) were obtained with an Axially Scattering Spectrometer Probe (ASSP). A ground based 95 GHz radiometer was used to infer the integrated liquid water path through the cloud. In the two thick clouds studied the drop-size data show an almost monotonic increase of mean radius from cloud base to top. The liquid water content increases with height above cloud base at slightly less than the adiabatic rate and shows considerable variability towards cloud top. The observations support the concept of the inhomogeneous mixing of the cloudy air with the dry air entrained at the cloud top. Measurements were also made of infrared net radiative fluxes, and very good agreement is found with the theoretical fluxes from a high resolution radiative transfer scheme, using the ASSP drop spectra and liquid water contents, scaled by the 95 GHz radiometer data. . INTRODUCTIONThis paper presents detailed observations of the radiative and microphysical structure of nocturnal stratocumulus clouds on three occasions. It forms part of a study of stratocumulus, using the tethered balloon facility at Cardington, Bedfordshire. The use of a tethered balloon as opposed to an aircraft results in data with high vertical resolution. The first case study (19-20 November 1976) has been the subject of concentrated research and the two accompanying papers describe in detail the design of the experiment and the derivation of the energy balance (Roach et a/. 1982; Paper 1) and the turbulence measurements (Caughey ei a/. 1982; Paper 11). In addition, results are presented here from two further case studies (26-27 October 1977 and 15 January 1978) for which the microphysical and radiation data have been analysed.Apart from the contribution of this work to the analysis of the 1976 case study described in Paper I, the observations throw light on two important aspects of cloud physics. The first is the study of the transfer of radiation through clouds and the application of this work to models of the cloudy boundary layer. There is an extensive literature dealing with the numerous mathematical techniques which have been devised for treating radiative transfer through clouds and many theoretical studies of ideal clouds have been made (for reviews see Paltridge and Platt 1976, Bolle 1977, Lenoble 1977, Feigelson 1978 clouds and the evolution of the drop-size distribution function. The considerable effort which has been put into improving the performance of an Axially Scattering Spectrometer Probe (ASSP) (Knollenberg 1976) has resulted in an instrument which gives accurate dropsize distributions, although absolute calibration of the estimated liquid water content relies on other instruments (Ryder 1976, Roach 1977. High quality profiles of drop-size distribution were obtained in all three case studies. Measurements were...
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