Laurent Delobbe

Bird migration flight altitudes studied by a network of operational weather radars

A fully automated method for the detection and quantification of bird migration was developed for operational C-band weather radar, measuring bird density, speed and direction as a function of altitude. These weather radar bird observations have been validated with data from a high-accuracy dedicated bird radar, which was stationed in the measurement volume of weather radar sites in The Netherlands, Belgium and France for a full migration season during autumn 2007 and spring 2008. We show that weather radar can extract near real-time bird density altitude profiles that closely correspond to the density profiles measured by dedicated bird radar. Doppler weather radar can thus be used as a reliable sensor for quantifying bird densities aloft in an operational setting, which—when extended to multiple radars—enables the mapping and continuous monitoring of bird migration flyways. By applying the automated method to a network of weather radars, we observed how mesoscale variability in weather conditions structured the timing and altitude profile of bird migration within single nights. Bird density altitude profiles were observed that consisted of multiple layers, which could be explained from the distinct wind conditions at different take-off sites. Consistently lower bird densities are recorded in The Netherlands compared with sites in France and eastern Belgium, which reveals some of the spatial extent of the dominant Scandinavian flyway over continental Europe.

Uncertainties in radar echo top heights used for hail detection

Most operational hail detection algorithms for single-polarisation radars are based on the analysis of the vertical profiles of radar reflectivity. At KNMI (Royal Netherlands Meteorological Institute) and RMI (Royal Meteorological Institute of Belgium) the probability of hail is derived from the height of the freezing level and the 45-dBZ radar echo top height (maximum height of the 45-dBZ echo). Echo tops are affected by errors in the measured reflectivity itself and by errors in the height assigned to these reflectivities. This study investigates the quality of radar echo top heights as a function of range and explores the implications for hail detection. The method is based on the comparison between reflectivity measurements from two radars on the vertical cross-section extending between these radars. In a first step, sampling errors related to the radar Volume Coverage Patterns are analysed using idealised storm profiles. Subsequently, real reflectivity data for 25 thunderstorm episodes are compared. It is found that the quality of the maximum reflectivity measurements strongly deteriorates with range and that about half of this degradation can be attributed to overshooting effects. Height assignment differences between the two radars are limited to about 0.5 km. Errors on the reflectivity measurements strongly affect the frequency of 45-dBZ threshold exceedances. However, once the threshold is exceeded, errors in measuring the 45-dBZ echo top heights generally affect the derived probability of hail by less than 20%.

Statistical Characteristics of Convective Storms in Belgium Derived from Volumetric Weather Radar Observations

AbstractHigh-resolution volumetric reflectivity measurements from a C-band weather radar are used to study the characteristics of convective storms in Belgium. After clutter filtering, the data are processed by the storm-tracking system Thunderstorm Identification, Tracking, Analysis, and Nowcasting (TITAN) using a 40-dBZ reflectivity threshold. The 10-yr period of 5-min data includes more than 1 million identified storms, mostly organized in clusters. A storm is observed at a given point 6 h yr−1 on average. Regions of slightly higher probability are generally correlated with orographic variations. The probability of at least one storm in the study area is 15%, with a maximum of 35% for July and August. The number of storms, their coverage, and their water mass are limited most of the time. The probability to observe a high number of storms reaches a maximum in June and in the early afternoon in phase with solar heating. The probability of large storm coverage and large water mass is highest in July and ...

The Impact of Size Distribution Assumptions in a Bulk One-Moment Microphysics Scheme on Simulated Surface Precipitation and Storm Dynamics during a Low-Topped Supercell Case in Belgium

In this researchthe impact of modifying the size distribution assumptionsof the precipitatinghydrometeors in a bulk one-moment microphysics scheme on simulated surface precipitation and storm dynamics has been explored for long-lived low-topped supercells in Belgium. It was shown that weighting the largest precipitatingicespeciesofthemicrophysicsschemetosmallgraupelresultsinanincreaseofsurfaceprecipitation becauseofcounteractingeffects.Ontheonehand,theprecipitation formationprocesssloweddown,resulting in lower precipitation efficiency. On the other hand, latent heat release associated with freezing favored more intense storms. In contrast to previous studies finding decreased surface precipitation when graupel was presentin the microphysics parameterization,stormswererather shallowin the authors’simulations.This left little time for graupel sublimation. The impact of size distribution assumptions of snow was found to be small, but more realistic size distribution assumptions of rain led to the strongest effect on surface precipitation. Cold pools shrunk because of weaker rain evaporation at the cold pool boundaries, leading to a decreased surface rain area.

Generation and Verification of Rainfall Estimates from 10-Yr Volumetric Weather Radar Measurements

AbstractVolumetric measurements from a C-band weather radar in Belgium are reprocessed over the years 2005–14 to improve the quantitative precipitation estimation (QPE). The data quality is controlled using static clutter and beam blockage maps and clutter identification based on vertical gradients, horizontal texture, and satellite observations. A new QPE is obtained using stratiform–convective classification, a 40-min averaged vertical profile of reflectivity (VPR), a brightband identification, and a specific transformation to rain rates for each precipitation regime. The rain rates are interpolated on a 500-m Cartesian grid, linearly accumulated, and combined with hourly rain gauge measurements using mean field bias or kriging with external drift (KED). The algorithms have been fine-tuned on 13 cases with various meteorological situations. A detailed validation against independent daily rain gauge measurements reveals the importance of VPR correction. A 10-yr verification shows a significant improvemen...

Evaluation of microphysical assumptions of the COSMO model using radar and rain gauge observations

The exact forecast of precipitation is a challenge. New microphysics formulations were introduced recently into the COSMO model in order to improve the precipitation forecast. An important modification was the change from the autoconversion and accretion scheme following the Kessler (1969) formulation to the Seifert and Beheng (2001) scheme. The other main modification was implemented in the snow parameterisation by replacing the constant intercept parameter to a temperature dependent intercept parameter. These micro-physics modifications are evaluated in detail in three case studies (one stratiform and two convective cases) by comparing the modelled and observed reflectivity and precipitation data. Comparisons to weather radar reflectivity data show that especially light to moderate precipitation forecast (< 20 dB) is improved. For the evaluation of the modelled precipitation, weather radar and rain gauge data are combined in order to get spatially high resolution data of high accuracy. For the quality analysis, the new error measure SAL (analysis of structure, amplitude and location) is used. The results show that the new microphysics formulations improve the precipitation amplitude forecast of up to 50% for the convective cases while the forecast for the stratiform case is not improved.