Transcript Towards a hail climatology

Towards a hail climatology
Gennaro Cappelluti, Paul Field & Will Hand
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.
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 1420LT 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.
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.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.5: Variation of
IWP in the slot 1218LT with respect
to 6-12LT. The
main increases are
over land while the
decreases occur
mostly over the
oceans.
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.
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°.
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.14: CDP derived morning hail size
distribution for Mar, Apr, May 2004-2008.
Fig.15: CDP derived afternoon hail size
distribution for Mar, Apr, May 2004-2008.
Fig.3: Time at which the flash density
minimum occurs in Dec, Jan and Feb.
3. Hail algorithm
The retrievals from the radiometer AMSU-B (Advanced Microwave Sounding Unit-B) on board the sunsyncronous 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).
Fig.8
Fig.9
Fig.10
Fig.11
Fig.12
Fig.13
5. Alternative method
6. Conclusions and further work
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.
The hail climatologies here presented have been developed from
satellite observations and through the Met Office global model.
Met Office FitzRoy Road, Exeter, Devon, EX1 3PB United Kingdom
Tel: 01392 886493 Fax: 01392 885681
Email: [email protected]
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.
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