Extended Abstract (108K)7B.5
Vertical reflectivity profile classification and correction in radar composites in Finland
Jarmo Koistinen, Finnish Meteorological Institute, Helsinki, Finland; and H. Pohjola and H. Hohti
At the Finnish Meteorological Institute vertical profiles of reflectivity (VPR) have been measured and diagnosed regularly from a network of 7 C-band Doppler radars. Profiles are measured in 200 m thick layers every 15 minutes. The measurements provide useful statistics about VPR characteristics in a cold climate as the radars are located between the latitudes 60 - 67 N. An automatic classification algorithm for the VPRs has been created, which applies time-space interpolated radio sounding data as an independent temperature source. Classification of 240 000 vertical profiles of reflectivity from a one year long period revealed that 40 % of all VPRs originated from clear air echoes reaching the ground (mostly insects and birds in summer, possibly drifting snow and solitary ice crystals in winter), 19 % from overhanging precipitation i.e. ice crystal clouds (typically Altostratus or Cirrostratus) or snowfall layers aloft, and 41 % involved precipitation reaching the ground level. Of all precipitation cases at ground level 56 % was snow, 5 % melting snow, 10 % rain with bright band (i.e. pronounced melting layer aloft) and 39 % rain without bright band. The statistics of the these VPRs include e.g. probability distributions of the reflectivity factor at each height level, echo top heights, vertical reflectivity gradients and bright band properties in each precipitation type. Such information is valuable for the planning of satellite based precipitation measurements and their validation (e.g. GPM measurements) especially in snowfall.
Precipitation in cold climates, like that in Finland, is quite shallow, in snowfall only 2-5 km high. In such cases the vertical profile of reflectivity is the dominating factor in the accuracy of measurements of precipitation at ranges of 50-250 km from a radar. The large bias of 2-20 dB, observed in gauge-radar comparisons, is fully originated from the sampling difference between the actual reflectivity (precipitation) at the ground level and in the contribution volume aloft. We have implemented a real time VPR correction scheme of the measured dBZ to represent reflectivity at ground level. The correction is calculated to all ranges (0-250 km) in a network of 7 C-band Doppler radars. The basic principle is comparison of the beam smoothed reflectivity aloft to the reflectivity at ground level. Vertical profiles are estimated from the measured VPRs close to the radars (0-40 km) and from climatological VPRs adjusted with the actual height of the freezing level. The magnitude of the VPR correction at each composite bin is a quality-weighted time-space average value based on the observed and climatological profiles from all neighboring radars. The correction field is continuous so that steps will not appear at the composite locations where data from two radars meet. We will present validation results of the method. Statistics of the magnitude of the correction as a function of range and vertical structure of precipitation will be shown. For example, wintertime measurement bias (mostly snowfall) at range 140 km from a radar will be reduced typically by 5 dBs.
Session 7B, Radar echo classification and data quality control
Friday, 8 August 2003, 4:00 PM-6:00 PM
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