Spatial and temporal distribution of hailstorms in the Alpine region: a long‐term, high resolution, radar‐based analysis

This article presents a 13-year hail climatology for Switzerland based on volumetric radar reflectivity. Two radar-based hail detection products that are used operationally at MeteoSwiss, namely the Probability of Hail (POH) and the Maximum Expected Severe Hail Size (MESHS), have been reprocessed for the extended convective season (April–September) between 2002 and 2014. The result of these two products is a comprehensive hail distribution map, which highlights regional and local-scale hail characteristics. The map of the annual number of hail days shows a high spatial variability and several maxima over the foothills north and south of the Alps as wells as over the Jura mountains. Directly over the Alps hail frequency exhibits a minimum. Annual hail anomalies show a pronounced variability, which suggests that hail occurrence is strongly controlled by large-scale weather patterns. Furthermore, hail probability exhibits a strong seasonal and diurnal cycle with a maximum in July in the late afternoon. The hail peak over the northern pre-alpine region occurs approximately two hours earlier compared to the south. A possible explanation is the trigger mechanism between the cold pool initiated by early convective cells over the Jura mountains and the development of cells on the northern slope of the Alps. Since radar-based hail signals are only indirect measurements, statistical verification of the hail detection algorithms is crucial. Damage reports from an automobile insurance company are used as an independent dataset. Verification results confirm that radar-based hail algorithms provide valuable information on hail probability. Finally, the synoptic-scale hail-driving weather conditions are investigated using a weather type classification based on upper-air flow direction and mean pressure from a numerical weather prediction model. The results show that six out of nine main synoptic-scale patterns favour the development of hailstorms in Switzerland.

[1]  A. Sterl,et al.  The ERA‐40 re‐analysis , 2005 .

[2]  Harri Hohti,et al.  A Climatological Comparison of Radar and Ground Observations of Hail in Finland , 2010 .

[3]  R. Uijlenhoet,et al.  Radar techniques for identifying precipitation type and estimating quantity of precipitation , 2004 .

[4]  Lans P. Rothfusz,et al.  MPING: Crowd-Sourcing Weather Reports for Research , 2014 .

[5]  Yuqing Wang,et al.  Trends in hail in China during 1960–2005 , 2008 .

[6]  Greg Stumpf,et al.  The S2K Severe weather detection algorithms and their performance , 2004 .

[7]  S. Mecklenburg,et al.  Improving the nowcasting of precipitation in an Alpine region with an enhanced radar echo tracking algorithm. , 2000 .

[8]  José Luis Sánchez,et al.  Synoptic environment, mesoscale configurations and forecast parameters for hailstorms in Southwestern Europe , 2013 .

[9]  Luca Nisi,et al.  Nowcasting severe convection in the Alpine region: the COALITION approach , 2014 .

[10]  V. Pont,et al.  Day-of-the-week variability of hail in southwestern France , 2001 .

[11]  E. García‐Ortega,et al.  Numerical simulation and sensitivity study of a severe hailstorm in northeast Spain , 2007 .

[12]  I. Gladich,et al.  On the diurnal cycle of deep moist convection in the southern side of the Alps analysed through cloud-to-ground lightning activity , 2011 .

[13]  U. Germann,et al.  Can Lagrangian Extrapolation of Radar Fields Be Used for Precipitation Nowcasting over Complex Alpine Orography , 2012 .

[14]  D. Rezácová,et al.  Radar-based hail detection , 2014 .

[15]  Urs Germann,et al.  Diurnal cycle of precipitation over complex Alpine orography: inferences from high‐resolution radar observations , 2013 .

[16]  Iwan Holleman.  Hail detection using single-polarization radar , 2001 .

[17]  H. Huntrieser,et al.  Comparison of Traditional and Newly Developed Thunderstorm Indices for Switzerland , 1997 .

[18]  C. Kottmeier,et al.  Recent trends of thunderstorm and hailstorm frequency and their relation to atmospheric characteristics in southwest Germany , 2009 .

[19]  Amaya Castro,et al.  Crop Damage: The Hail Size Factor , 1996 .

[20]  J. Wyngaard,et al.  Resolution Requirements for the Simulation of Deep Moist Convection , 2003 .

[21]  Jean Dessens.  Hail in Southwestern France. I: Hailfall Characteristics and Hailstrom Environment , 1986 .

[22]  J. Thepaut,et al.  The ERA‐Interim reanalysis: configuration and performance of the data assimilation system , 2011 .

[23]  Radome attenuation — a serious limiting factor for quantitative radar measurements? , 1999 .

[24]  A. V. Delden,et al.  The synoptic setting of thunderstorms in western Europe , 2001 .

[25]  R. Fraile,et al.  Return periods of severe hailfalls computed from hailpad data , 2003 .

[26]  Harold E. Brooks,et al.  An Objective High-Resolution Hail Climatology of the Contiguous United States , 2012 .

[27]  Gregory Thompson,et al.  Changes in hail and flood risk in high-resolution simulations over Colorado's mountains , 2012 .

[28]  Extreme hail day climatology in Southwestern France , 2013 .

[29]  F. Biasioli,et al.  Quantitative hail monitoring in an alpine area: 35‐year climatology and links with atmospheric variables , 2012 .

[30]  Yuqing Wang,et al.  Observed Characteristics of Hail Size in Four Regions in China during 1980-2005 , 2010 .

[31]  Dario B. Giaiotti,et al.  The climatology of hail in the plain of Friuli Venezia Giulia , 2003 .

[32]  M. Kunz,et al.  Recent trends and variabilities of convective parameters relevant for hail events in Germany and Europe , 2013 .

[33]  José Luis Sánchez,et al.  Nowcasting of kinetic energy of hail precipitation using radar , 2013 .

[34]  J. L. Sánchez,et al.  Regional and yearly variations of hail frequency and intensity in France , 2011 .

[35]  S. A. Changnon,et al.  A Method for Estimating Crop Losses from Hail in Uninsured Periods and Regions , 2001 .

[36]  R. Donaldson.  RADAR REFLECTIVITY PROFILES IN THUNDERSTORMS , 1961 .

[37]  M. Kunz,et al.  Detection of hail signatures from single-polarization C-band radar reflectivity , 2015 .

[38]  M. Dixon,et al.  TITAN: Thunderstorm Identification, Tracking, Analysis, and Nowcasting—A Radar-based Methodology , 1993 .

[39]  Alexei Pozdnoukhov,et al.  Data-driven exploration of orographic enhancement of precipitation , 2010 .

[40]  V. Barros,et al.  Climatology of hail in Argentina , 2012 .

[41]  Roberto Fraile,et al.  Hailstone size distributions in southwestern France , 1994 .

[42]  A. Witt,et al.  An Enhanced Hail Detection Algorithm for the WSR-88D , 1998 .

[43]  Urs Germann,et al.  The relation between airflow and orographic precipitation on the southern side of the Alps as revealed by weather radar , 2010 .

[44]  J. Webb,et al.  Climatology of severe hailstorms in Great Britain , 2001 .

[45]  Arthur Witt,et al.  The Storm Cell Identification and Tracking Algorithm: An Enhanced WSR-88D Algorithm , 1998 .

[46]  Silvio Davolio,et al.  Orographic triggering of long lived convection in three dimensions , 2009 .

[47]  Tomeu Rigo,et al.  Pilot project for intensive surveillance of hail events in Terres de Ponent (Lleida) , 2007 .

[48]  S. A. Changnon,et al.  Long-Term Fluctuations in Hail Incidences in the United States , 2000 .

[49]  Steven A. Amburn,et al.  VIL Density as a Hail Indicator , 1997 .

[50]  H. McMaster.  The Potential Impact of Global Warming on Hail Losses to Winter Cereal Crops in New South Wales , 1999 .

[51]  G. Held.  The Probability of Hail in Relation to Radar Echo Heights on the South African Highveld , 1978 .

[52]  Harri Hohti,et al.  Climatology of Severe Hail in Finland: 1930–2006 , 2009 .

[53]  Hans-Heinrich Schiesser,et al.  The use of weather radars to estimate hail damage to automobiles: an exploratory study in Switzerland , 2002 .

[54]  Iwan Holleman,et al.  If cannons cannot fight hail, what else? , 2006 .

[55]  R. Blong,et al.  A hail climatology of the greater Sydney area and New South Wales, Australia , 2005 .

[56]  D. Roos,et al.  A comparative study of hailstorms in Switzerland, Canada and South Africa , 1985 .

[57]  Witold F. Krajewski,et al.  Review of the Different Sources of Uncertainty in Single Polarization Radar-Based Estimates of Rainfall , 2010 .

[58]  G. K. Mather,et al.  An Observed Relationship between the Height of the 45 dBZ Contours in Storm Profiles and Surface Hail Reports , 1976 .

[59]  R. Blong,et al.  Relationship between radar-derived hail kinetic energy and damage to insured buildings for severe hailstorms in Eastern Australia , 2006 .

[60]  A. Waldvogel,et al.  Criteria for the Detection of Hail Cells , 1979 .

[61]  G. Foote,et al.  Results of a Randomized Hail Suppression Experiment in Northeast Colorado. Part I: Design and Conduct of the Experiment , 1979 .

[62]  H. Schiesser,et al.  Cloud-to-ground lightning activity in relation to the radar-derived hail kinetic energy in Switzerland , 2001 .

[63]  Yuqing Wang,et al.  Climatology of Hail in China: 1961-2005 , 2008 .

[64]  W. Skamarock,et al.  The resolution dependence of explicitly modeled convective systems , 1997 .

[65]  U. Germann,et al.  Radar precipitation measurement in a mountainous region , 2006 .

[66]  Bradley G. Illston,et al.  An Analysis of Severe Hail Swaths in the Southern Plains of the United States , 2004, Trans. GIS.

[67]  M. Kunz,et al.  High-resolution assessment of the hail hazard over complex terrain from radar and insurance data , 2010 .

[68]  M. Löffler-Mang,et al.  Characteristics of a new automatic hail recorder , 2011 .

[69]  G. Morgan.  A General Description of the Hail Problem in the Po Valley of Northern Italy , 1973 .

[70]  J. Štalec,et al.  Hail characteristics of different regions in continental part of Croatia based on influence of orography , 2009 .

[71]  Richard L. Thompson,et al.  Nationwide comparisons of hail size with WSR-88D vertically integrated liquid water and derived thermodynamic sounding data , 1998 .

[72]  Volker Wulfmeyer,et al.  Mechanisms initiating deep convection over complex terrain during COPS , 2008 .

[73]  U. Germann,et al.  MAP D-PHASE: Real-Time Demonstration of Weather Forecast Quality in the Alpine region , 2009 .

[74]  Iwan Holleman,et al.  Uncertainties in radar echo top heights used for hail detection , 2006 .

[75]  F. Vinet.  Climatology of hail in France , 2001 .

[76]  Ulrich Corsmeier,et al.  The influence of mesoscale circulation systems on triggering convective cells over complex terrain , 2006 .

[77]  W. Schmid,et al.  Hailstorms in Switzerland: Left Movers, Right Movers, and False Hooks , 1993 .