247 Using Operational LDR Measurements to Improve Radar Quantitative Precipitation Estimates Through Accurate Classification of the Vertical Reflectivity Profile

Thursday, 31 August 2017
Zurich DEFG (Swissotel Chicago)
Caroline Sandford, Met Office, Exeter, U.K.; and A. J. Illingworth, R. J. Thompson, and D. Harrison

Correcting for vertical profile of reflectivity (VPR) is an important step in obtaining accurate quantitative precipitation estimates (QPEs) from meteorological radar. Changes in drop diameter and dielectric factor with height, particularly in the vicinity of the freezing level, lead to order of magnitude changes in the relationship between reflectivity and water content. Adjusting for this relationship is crucial to determining precipitation rates at the surface.

Different types of meteorological situations generate different types of VPR. In cold stratiform conditions, the formation of precipitation at around -15 C and growth by aggregation and deposition cause a smooth increase in reflectivity towards the freezing level. At the 0 C isotherm, the melting of large, low density snowflakes causes a sharp increase in reflectivity, as the dielectric factor increases due to the liquid water coating. A corresponding decrease in fully melted reflectivity due to the decrease in diameter completes the characteristic melting layer "bright band". These conditions are dominant in the UK climate. However, in convective conditions, and in cases where the distribution of frozen droplets comprises mostly smaller, higher density ice, no bright band is present. The use of climatological or average VPRs in such conditions causes underestimation of precipitation at the surface.

The linear depolarisation ratio (LDR) has unique skill in identifying the melting layer behaviour of VPRs. In responding to average canting angle, particularly at high axis ratios, LDR is sensitive to the same large melting snowflakes that cause the reflectivity bright band. Since LDR measurements are dominated by the highest depolarisation values in the radar pulse volume, the LDR bright band is still clearly visible at ranges where the reflectivity bright band is smoothed out by beam broadening. It is therefore possible with LDR to identify which reflectivity pixels are, and are not, affected by bright band at long range.

In this work we present the skill of the linear depolarisation ratio (LDR) in diagnosing the presence or absence of bright band in operational reflectivity measurements. This work builds on earlier investigations into the correlation of peak melting layer LDR with VPR type in high resolution vertical profiles. Here the result is extended to measurements at long range, which are subject to beam broadening. Real time considerations such as data availability, quality control and noisy measurements are addressed to develop an operationally feasible algorithm for VPR classification based on melting layer LDR. The full algorithm is tested in the context of the pixel-scale VPR correction applied by the Met Office operational radar processing software. The use of LDR is shown to have a positive impact on real time QPEs, particularly in high impact convective cases, through application of a local VPR shape suitable to the meteorological situation.

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