Towards a hail climatologyGennaro Cappelluti, Paul Field & Will Hand

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1. Introduction

There is no recognised global method of detecting hail. The goal of this work in progress is to produce a global hail climatology.Hail forms in strong thunderstorms, particularly those with intense updraughts, high liquid water content, great vertical extent, large water droplets and where a good portion of the cloud layer is below freezing. The growth rate is maximized at about -12°C and becomes vanishingly small below -30°C as at those temperatures supercooled water droplets become rare. For this reason, in mid-latitudes hail is most common during Spring and early Summer where surface temperatures are high enough to promote the instability associated with strong thunderstorms, but the upper atmosphere is still cool enough to support ice. Hail is also much more common along and near mountain ranges because mountains force horizontal winds upwards (orographic lifting), thereby intensifying the updraughts within thunderstorms and making hail more likely. Generally hail falls during the afternoon and evening periods with peaks of intensity between 14 and 20 hours local time.

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2. Lightning

The presence of graupel/hail is fundamental for cloud charging leading to lightning discharges. The most important charging process is thought the non-inductive mechanism whereby ice crystals growing by diffusion rebound from collisions with graupel/hail growing by accretion of water droplets.The global lightning climatology derived from the photodiode detector (PDD) and the world wide lightning location network (WWLLN) indicates a strong diurnal signal over the tropics and mid-latitude land areas. The peak in the diurnal signal is around 17LT but is earlier in Europe and later in North America (Fig.1). Over the sea the diurnal cycle is much weaker and peaks around 8LT. NASA global lightning climatology based on 5 years of OTD (optical transient detector) and 8 years of LIS (lightning imaging sensor) satellite data shows similar patterns and points out that the flash density peaks at 14-20LT over land and around 8LT over sea (Fig.2) while the minimum occurs at 6-12LT over land and 12-18LT over sea, worldwide and in all seasons (Fig.3). A similar diurnal behaviour is seen in ground based local hail climatologies.

3. Hail algorithm

The retrievals from the radiometer AMSU-B (Advanced Microwave Sounding Unit-B) on board the sun-syncronous satellites NOAA15, 16 and 17 (Fig.4-5) provide a good estimate of ice water path but as the bands used by the instrument (89, 150 and 183 GHz) detect graupel, hail, snow and anvil ice, these data cannot be used in isolation to determine just the hail/graupel content. The hail climatology here presented has been generated by an algorithm that combines AMSU-B IWP retrievals with the OTD/LIS NASA lightning climatology (Fig.6). For each grid box 1°×1°, the algorithm selects from the lightning climatology the maximum 2 hourly flash density that occurs in that area over the year (FDmax) and uses this value to partition the IWP between hail and the remaining forms of ice in the grid box (Fig.7). The technique assumes that the percentage of hail in each 1°×1° region (s) and 2 hourly time slot (t) is proportional to a factor H (the fraction of hail out of ice in the time slot in which the flash density peaks along the year) and the ratio between flash density FD(s,t) and maximum flash density FDmax(s):

HWP(s,t) = IWP(s,t) · H · FD(s,t) / FDmax(s)The factor H has been set initially to 0.7 and may depend on parameters such as location, season and hour (ongoing investigation).

4. Hail climatology

This technique has been employed to process more than four years of AMSU-B data (Feb 2004 - Mar 2008) and some aspects of the resulting hail climatology have been reported in Fig.8-10 (Dec, Jan and Feb) and Fig.11-13 (Jun, Jul and Aug). Fig.8 and Fig.11 point out the average hail water path between midnight and midday local time while Fig.9 and Fig.12 the HWP mean between 12 and 24LT. The hail water path is generally greater over land and in the afternoon, in good agreement with the observed climatologies. Also, the HWP is significantly less widespread over the oceans than the IWP. The maps in Fig.10 and Fig.13 show the hour at which the retrieved hail water path peaks, in Dec-Feb and Jun-Aug respectively. The plots point out that in Europe and Asia the HWP peaks earlier than elsewhere, in agreement with Fig.1. Also over the oceans the hail timing matches quite well the lightning observations.

Fig.3: Time at which the flash density minimum occurs in Dec, Jan and Feb.

Fig.2: Time at which the OTD/LIS flash density peaks in Dec, Jan and Feb.

Fig.1: WWLLN flash density data separated into six regions. Similar diurnal patterns are seen in each region but the land and ocean peaks do not occur at the same local time. The average diurnal amplitude variation of land events is about three times larger than the oceanic.

Fig.7: HWP mean at 12-18LT in Sep, Oct and Nov. The generated hail climatology details hail content with a resolution of 2 hours and 1°×1°.

Fig.5: Variation of IWP in the slot 12-18LT with respect to 6-12LT. The main increases are over land while the decreases occur mostly over the oceans.

6. Conclusions and further work

The hail climatologies here presented have been developed from satellite observations and through the Met Office global model.

These hail climatologies will be validated against the available local ground based hail climatologies and other satellite datasets (e.g. NASA/JAXA TRMM).

The hail algorithm will be tested and refined in order to improve the partitioning of ice species.

Fig.15: CDP derived afternoon hail size distribution for Mar, Apr, May 2004-2008.

Fig.14: CDP derived morning hail size distribution for Mar, Apr, May 2004-2008.

Fig.4: AMSU-B IWP daily mean for Sep, Oct and Nov. AMSU-B soundings are sensitive not only to hail/graupel but also to snow and anvil ice.

Fig.6: OTD/LIS average flash density between 16 and 18LT in autumn. The signal over the oceans is much lower than AMSU-B IWP.

5. Alternative method

Another way to estimate the hail content is through the convective diagnosis procedure (CDP), a semi-empirical technique used by the Met Office to obtain information about quantities such as probability of lightning and maximum hail size at the ground (Fig.14-15). The CDP outcomes exhibit a diurnal variation consistent with the data above but also a stronger correlation with high topography.