Integral Radar Volume Descriptors for Quantitative Areal Precipitation
Zusammenfassung der Projektergebnisse
The central objective of the whole project cluster AQUARadar was to significantly enhance the quality of radar derived precipitation estimates by fully exploiting the spatial and temporal variability of 3D radar volume data. Different approaches have been considered within AQUARadar to achieve the common purpose. Within this subproject mainly events are investigated to estimate the total rainfall of an individual storm over its lifetime using so-called Integral Radar Volume Descriptors (IRVD). The assumption is that a small set of descriptors provides sufficient information of the underlying precipitation system to set an enhanced rainfall estimator, at least integrated over time and space. In a first step analyses are based on pseudo-radar data and rain rates of a regional weatherforecast model. We investigate and extend two approaches: The first approach estimates total rainfall from an individual storm over its lifetime while the second approach assesses the areawide instantaneous rainfall from a multiplicity of such storms by the use of measurements of the areal coverage of the storms exceeding a threshold radar reflectivity. The concept is extended by adding more predictors to significantly enhance the rainfall estimates. The horizontal expected value and the horizontal standard deviation of enclosed reflectivities at the ground, the mean brightband fraction and its trend, the fractional area with reflectivities exceeding a threshold τ and an orographic rainfall amplifier provide relative errors smaller than 10% in approximately 75 (depending on the input set) of 100 considered rain events in the first approach. In the second approach we achieve a relative error below 10% in approximately 63% elements of the test set. In a second step, we apply and further extend the methodology to real radar observations but confine ourselves to the first approach. A series of 65 rain events with lifetimes between 1.25 and 4.75 hours over Germany are investigated using the IRVD concept. However, estimation models based on IRVDs alone remain within the range of accuracy achievable with the Marshall-Palmer Z-R-relationship. We see the range-dependent resolution of the vertical reflectivity pofile as one reason for the moderate results. Based on the assumption that IRVDs account for microphysical differences among rain events and thus also for the variability in drop size distributions, we consider to use the IRVDs to correct a standard Z-R-relationship. Including the Marshall-Palmer estimator in the model provides considerable enhancements compared to the Marshall-Palmer alone estimates. In the combined model the Marshall-Palmer estimator, the standard deviation of the horizontally enclosed reflectivities near the ground, the mean echo top height and the mean effective efficiency are the most robust descriptors. These descriptors explain 92% of the variance in integral volumetric rainfall. If the weights of relevant descriptors are redetermined by minimizing the relative errors, in 23 (28) out of 65 cases the relative error is below 10% (20%), while the Marshall-Palmer relationship alone provides only in 13 (22) cases relative errors smaller then 10% (20%). A comparison of the IRVD-models for pseudo-rain events from the output of COSMO-DE and observed rain events also showed the potential of the object-based IRVD-method for a validation of atmospheric models. Application of the method to upcoming COSMO versions with a two-moment microphysical scheme or an even more complete spectral scheme for cloud processes might hint at remaining shortcomings of the models. Therefore, we plan to composit radar volume data of collocated radars to enhance the resolution of the vertical reflectivity profile for a larger area which should increase the accuracy of several IRVDs.
Projektbezogene Publikationen (Auswahl)
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(2007): AQUARadar- Verbesserung der quantitativen Niederschlagsschätzung mit Regenradaren. In: DMG, Mitteilungen DMG 03/2007, Offenbach am Main: 5-7
Trömel, S.
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(2007): Radarvolumendeskriptoren von Niederschlagszellen zur quantitativen Niederschlagsschätzung. Proceedings of DACH (2007)
Trömel, S., C. Simmer
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(2009): Towards the use of Integral Radar Volume Descriptors for quantitative areal precipitation estimation - results from pseudoradar observations. J. Atmos. Oceanic Technol. 26(9): 17981813
Trömel, S., C. Simmer, J. Braun, T. Gerstner, M. Griebel
