An ERA‐Interim HAILCAST hail climatology for southern Africa

• Published in: International journal of climatology Vol. 41; no. 1; pp. 262 - 277
• Main Authors: Dyson, Liesl L., Pienaar, Nita, Smit, Ansie, Kijko, Andrzej
• Format: Journal Article
• Language: English
• Published: Chichester, UK John Wiley & Sons, Ltd 01-01-2021; Wiley Subscription Services, Inc
• Subjects: Climate; Climate models; Climatology; ERA‐Interim; Hail; HAILCAST; Numerical weather forecasting; Prediction models; Rain; Rainfall; Rainfall area; Resolution; Seasonal distribution; Soundings; southern Africa; Temporal distribution; Temporal resolution; Weather forecasting; South Africa; Africa
• ISSN: 0899-8418; 1097-0088
• DOI: 10.1002/joc.6619

Understanding the character of hail is an important part of mitigating its impact on life and property. Climatologies of hail over South Africa are rare and based on observed data with a coarse spatial and temporal resolution. In this article, a hail climatology is created for southern Africa from reanalysis data. Pseudo‐soundings are produced from ERA‐Interim data and the HAILCAST model, which predicts the diameter of hail, is run using these soundings from 1979 to 2017 at six‐hourly intervals. The hail day frequency of the ERA‐Interim HAILCAST (EIH) climatology compares well to historical climatologies, although the horizontal resolution of ERA‐Interim makes it difficult to pinpoint the exact location of hail in areas of steep topography. Furthermore, the EIH climatology tends to over‐predict hail with diameters greater than 3 cm. Detail that was previously unavailable, such as the seasonal and temporal distribution of hail over the entire South Africa, is now available from the EIH climatology. The frequency of hail over the winter rainfall area and over countries bordering South Africa is estimated for the first time. It is recommended that the procedure is repeated with numerical weather prediction models with a higher resolution to determine whether the increased resolution will more accurately identify areas of hail.
Large damaging hail occurs from time to time over the Highveld of South Africa. The image is an example of egg‐sized hail which occurred in November 2013 in Pretoria, South Africa. Two hail days in November 2013 over Gauteng cost the South African insurance industry more than US$ 150 million. No up to date hail climatology exists over South Africa and understanding the risk of the impact of large hail is difficult. We create a hail climatology for 1979–2017 from HAILCAST run using ERA‐Interim data to help understand the distribution of, and to mitigate the impact of large hail.