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Ensemble simulations with the Terrestrial Systems Modelling Platform (TSMP) covering northwestern Germany are evaluated for three summertime convective storms using polarimetric X-band radar measurements. Using a forward operator, the simulated microphysical processes have been evaluated in radar observation space. Observed differential reflectivity (ZDR) columns, which are proxies for updrafts, and multi-variate fingerprints for size sorting and aggregation processes are captured by the model, but co-located specific differential phase (KDP) columns in observations are not reproduced in the simulations. Also, the simulated ZDR columns, generated by only small-sized supercooled drops, show smaller absolute ZDR values and a reduced width compared to their observational counterparts, which points to deficiencies in the cloud microphysics scheme as well as the forward operator, which does not have explicit information of water content of ice hydrometeors. Above the melting layer, the simulated polarimetric variables also show weak variability, which can be at least partly explained by the reduced particle diversity in the model and the inability of the T-matrix method to reproduce the polarimetric signatures of snow and graupel; i.e. current forward operators need to be further developed to fully exploit radar data for model evaluation and improvement. Below the melting level, the model captures the observed increase in reflectivity, ZDR and specific differential phase (KDP) towards the ground. The contoured frequency altitude diagrams (CFADs) of the synthetic and observed polarimetric variables were also used to evaluate the model microphysical processes statistically. In general, CFADs of the cross-correlation coefficient (ρhv) were poorly simulated. CFADs of ZDR and KDP were similar but the model exhibits a relatively narrow distribution above the melting layer for both, and a bimodal distribution for ZDR below the melting layer, indicating either differences in the mechanism of precipitation formation or errors in forward operator which uses a functional form of drop size distribution. In general, the model was found to underestimate the convective area fraction, high reflectivities, and the width/magnitude of ZDR columns, all leading to an underestimation of the frequency distribution for high precipitation values.

Columns of differential reflectivity, the difference between the horizontal and vertical reflectivity,hereafter Zdr columns, are vertical columns of enhanced Zdr that extend above the environmental 0°Clevel. These are easily identified when observed by polarimetric radars. Physically, these columns consistof rain dominated by large drops that are being lofted above the freezing level and have been recognizedas a proxy for the location of updrafts. Their potential for nowcasting severe weather has been shown inseveral past studies. We have developed an algorithm that identifies and tracks Zdr columns fromvolumetric radar data along with 3D wind fields from MultiDoppler analysis to spatially correlate Zdrcolumns with updrafts. Since Zdr columns are a manifestation of an updraft, different Zdr columnsproperties for example Zdr column maximum height, volume, and area are expected to be related toupdraft intensity levels. In turn intensification of updrafts, as indicated by changes in the Zdr columnsproperties, should be translated in intensification of observed precipitation at the surface. For theestimation of rain rates, we used a radar-based polarimetric approach, which will allow us to monitor thetemporal evolution for a number of identified convective rain cells. These cells will be identified fromsummer events observed by the C-band polarimetric German network. For each cell, the properties of Zdrcolumns are correlated with rain rate values. Similarly, correlations are also calculated for updraftvolumes, updraft intensity, and other updraft properties. For the nowcasting of observed rain rates, anextrapolation algorithm based on spatial and temporal properties of rain was used. Preliminary resultshave shown that higher precipitation rates are generally associated with Zdr columns, and cells without aZdr column produce lower precipitation rates, as expected. Zdr column height and volume show apositive correlation with precipitation intensity at the surface. The time lag between the intensification ofthe Zdr column and associated increase in precipitation at the surface varies significantly between cells,but it is generally short compared to previous studies, varying mostly between 5 to 15 minutes. An earlyidentification of cells associated with ZDR columns could benefit the skill of the nowcasting of localizedrain cells, which often are smoothed during extrapolation.

Conventionally, the short term prediction of precipitation (or nowcast) observed by a network of weather radars is conducted by means of advection schemes. Such schemes often lead to underestimated areas of predicted precipitation cells and they do not include the vertical structure of the cells, which can carry important information on the evolution of precipitation. To mitigate such limitations on nowcasts, storm cells have been treated as 3D targets, allowing the exploitation of their vertical and dynamic structure. In this work, storm cell targets are identified and tracked from 2D reflectivity nationwide fields collected by the German meteorological service DWD. Then, the vertical structure of the cells are analyzed in terms of the polarimetric-based differential reflectivity Z DR -column, which is a predictor of potential changes in precipitation. Knowing the location and tendency of storm cells and their associated Z DR columns, the nowcast skills can be further improved, mainly in localized regions.

Dual-Doppler techniques for 3D wind retrievals have been widely used for some time, but very little effort has been dedicated to their validation. In particular, the vertical component of the wind field w is not well sampled by the radars due to the scanning strategies used for weather radars, and relies heavily on imposed conditions such as the mass continuity equation and the vorticity equation. Therefore, validation of the retrieved w with independent observations remains important before using the retrievals for the study of weather systems dynamics. The polarimetric Doppler radars JuXPol and BoXPol perform volume scans followed by a Range Height Indicator (RHI) scan and a vertically pointing scan. In this study we use the latter to derive w above the BoXPol radar and to compare it to the vertical component of the wind field retrieved by the Dual-Doppler technique. Both profiles of w show similar overall shapes with minima and maxima situated at about the same heights. The values of w obtained from the vertical scan are within the range of the profiles obtained with the Dual-Doppler method, except above an altitude of 4km where the vertical scan shows higher upward velocities than the Dual-Doppler. This can be caused by a tilted updraft which can only be partly sampled by the vertical scan, and results from different sampling times. Bias and root mean square error indicate a relative good agreement between both estimates especially for the median and high values of w, but an underestimation of the Dual-Doppler w at the lowest values.

Ensemble simulations were conducted for three summertime convective storms over a temperate region in northwestern Germany using the Terrestrial Systems Modeling Platform (TSMP). The simulated microphysical processes were evaluated with polarimetric observations from two X-band radars, with the help of a forward operator applied to the model data. TSMP was found to generally underestimate the convective area fraction, high reflectivities, and the width/magnitude of so-called differential reflectivity (ZDR) columns indicative of updrafts, all leading to an underestimation of the frequency distribution for high precipitation values. The statistical distributions of ZDR and specific differential phase (KDP) were however similar, while the cross-correlation coefficient (phv) was poorly simulated, probably due to little variability of assumed hydrometeor shapes and orientations in the forward operator. The observed model bias in the ZDR columns could be associated with small size of supercooled raindrops and poorly resolved three dimensional flow at km-scale simulations, besides the treatment of freezing process in the model, which warrants further research.

The large amount of data collected by weather radars creates a challenge for interpretation of radar observations. Typically, weather radars scan about 10 different elevations with a radius of 100 to 400 km around the radar, with a few hundred meters of radial resolution in approximately 5 minutes. To overcome this problem, different techniques have been used in the past to reduce and display radar data in a way that is easy to comprehend and interpret. In this paper we describe 3 different techniques: Contour Frequency by Altitude Diagrams (CFAD), Quasi-Vertical Profiles (QVP) and range-defined QVP. For each method we show examples and make a brief analysis of the visible features. We then discuss the advantages and disadvantages of each method, as well as the best types of application depending on the particular use.

This study analyzes radar observations of evaporation in rain and investigates its impact on surface rainfall and atmospheric cooling rates. A 1D model is used to examine the impact of raindrop evaporation on the evolution of the initial raindrop size distribution (DSD), the resulting reflectivity (Z), and differential reflectivity (ZDR) and surface rain rates. Raindrop evaporation leads to a decrease of Z and an increase of ZDR toward the surface because of the depletion of small raindrops that evaporate first and thus enhance the mean raindrop size. The latter effect, however, can be reduced because of the increasing temperature toward the surface and may even lead to a decrease of ZDR toward the surface. Two events with significant rain evaporation, observed simultaneously by a polarimetric X-band radar and a K-band Micro Rain Radar (MRR), offer quite detailed insight into the evaporation process. During the first event, which exhibits an initial ZDR > 1.5 dB in the upper rain column, raindrops undergo relatively weak evaporation as deduced from the decrease of the small raindrop fraction observed by the MRR. The second event is characterized by a lower initial ZDR < 0.5 dB with all raindrops evaporating before reaching the ground. A retrieval scheme for estimating the evaporation-related cooling rate and surface precipitation from polarimetric radar observations below the bright band is derived based on MRR observations. The algorithm is then used to simulate polarimetric X-band radar observations, which might mitigate uncertainties in the surface rainfall retrievals due to evaporation at far distances from the radars and in the case of beam blocking.

Understanding the life cycle of precipitating systems is important for better nowcasting of significant weather and for improving weather prediction models. Doppler and polarimetric weather radars have the ability to look simultaneously into the microphysics and dynamics of precipitation systems. In this work we make use of these powerful tools to perform a combined analysis of the dynamics and microphysics of precipitation systems, in order to better understand storm development. By using data from multiple Doppler radars, the 3D wind field may be retrieved. Simultaneously, the polarimetric capability of the radars allows the retrieval of hydrometeor types and microphysical processes. Therefore, we are able to identify the processes through which precipitation systems evolve by revealing the interdependence between storm dynamics and microphysics. Prominent examples of such interactions are the development of socalled Zdr columns, that are associated with storm updrafts, and consequent freezing and potential hail formation; or regions of riming that are difficult to spot from polarimetry alone, but which require an updraft for supply of supercooled water, which could be seen in the wind field retrievals; or the development of lightning through lifting of graupel in convective updrafts that cause vertical electric charge separation. In this work we apply the Dual-Doppler technique DDA to analyze two large hail events that occurred near the Bonn and Julich radars: the 5th July 2015 and 22 June 2017. The life cycle of these 2 systems will be examined to better understand the formation and growth of hail, from the perspective of dynamics and microphysics.

The organized behavior of differential radar reflectivity (ZDR) is documented in the cold regions of a wide variety of stratiform precipitation types occurring in both winter and summer. The radar targets and attendant cloud microphysical conditions are interpreted within the context of measurements of ice crystal types in laboratory diffusion chambers in which humidity and temperature are both stringently controlled. The overriding operational interest here is in the identification of regions prone to icing hazards with long horizontal paths. Two predominant regimes are identified: category A, which is typified by moderate reflectivity (from 10 to 30 dBZ) and modest +ZDR values (from 0 to +3 dB) in which both supercooled water and dendritic ice crystals (and oriented aggregates of ice crystals) are present at a mean temperature of −13°C, and category B, which is typified by small reflectivity (from −10 to +10 dBZ) and the largest +ZDR values (from +3 to +7 dB), in which supercooled water is dilute or absent and both flat-plate and dendritic crystals are likely. The predominant positive values for ZDR in many case studies suggest that the role of an electric field on ice particle orientation is small in comparison with gravity. The absence of robust +ZDR signatures in the trailing stratiform regions of vigorous summer squall lines may be due both to the infusion of noncrystalline ice particles (i.e., graupel and rimed aggregates) from the leading deep convection and to the effects of the stronger electric fields expected in these situations. These polarimetric measurements and their interpretations underscore the need for the accurate calibration of ZDR.

This study presents a first analysis of precipitation and related microphysical processes observed by three polarimetric X-band Doppler radars (BoXPol, JuXPol and KiXPol) in conjunction with a ground-based network of disdrometers, rain gauges and vertically pointing micro rain radars (MRR) during the High Definition Clouds and Precipitation for advancing Climate Prediction (HD(CP)2) Observational Prototype Experiment (HOPE) during April and May 2013 in Germany. While JuXPol and KiXPol were continuously observing the central HOPE area near Forschungszentrum Juelich at a close distance, BoXPol observed the area from a distance of about 48.5 km. MRRs were deployed in the central HOPE area and one MRR close to BoXPol in Bonn, Germany. Seven disdrometers and three rain gauges providing point precipitation observations were deployed at five locations within a 5×5 km2 region, while another three disdrometers were collocated with the MRR in Bonn. The daily rainfall accumulation at each rain gauge/disdrometer location estimated from the three X-band polarimetric radar observations showed a very good agreement. Accompanying microphysical processes during the evolution of precipitation systems were well captured by the polarimetric X-band radars and corroborated by independent observations from the other ground-based instruments.

This study presents a first analysis of precipitation and related microphysical processes observed by three polarimetric X-band Doppler radars (BoXPol, JuXPol and KiXPol) in conjunction with a ground-based network of disdrometers, rain gauges and vertically pointing micro rain radars (MRRs) during the High Definition Clouds and Precipitation for advancing Climate Prediction (HD(CP)2) Observational Prototype Experiment (HOPE) during April and May 2013 in Germany. While JuXPol and KiXPol were continuously observing the central HOPE area near Forschungszentrum Jülich at a close distance, BoXPol observed the area from a distance of about 48.5 km. MRRs were deployed in the central HOPE area and one MRR close to BoXPol in Bonn, Germany. Seven disdrometers and three rain gauges providing point precipitation observations were deployed at five locations within a 5 km × 5 km region, while three other disdrometers were collocated with the MRR in Bonn. The daily rainfall accumulation at each rain gauge/disdrometer location estimated from the three X-band polarimetric radar observations showed very good agreement. Accompanying microphysical processes during the evolution of precipitation systems were well captured by the polarimetric X-band radars and corroborated by independent observations from the other ground-based instruments.

During the AMMA campaign, the Doppler and polarimetric radar Ronsard was deployed in Kopargo, Benin. In the present paper, three squall lines sampled during that period are described. It was the first time this type of system had been observed with a polarimetric radar in the West African region. The strongest case occurred on 28 July 2006, and the others on 12 September and 30 June. The polarimetric characteristics of these systems are described and a hydrometeor classification algorithm is applied in order to identify the microphysics of the systems. Finally, the 3D wind field is retrieved for the 28 July event. The convective part of the squall lines is generally characterized by maxima of reflectivity, differential reflectivity (ZDR), and specific differential phase (KDP). The results of the hydrometeor classification show that the convective region is composed of strong (>30 mm h-1) to moderate rain (5-30 mm h-1) below the melting level, and graupel above. This zone seems to be characterized by relatively strong updraughts (5-10 m s-1), responsible both for the formation of graupel by riming and for the growth of drops due to coalescence. To the rear the systems are characterized by light rain (