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| Titel |
Microphysical properties of mid-latitude frontal clouds |
| VerfasserIn |
Jonathan Crosier, Thomas Choularton, Keith Bower, Paul Connolly, Ian Crawford, Martin Gallagher, James Dorsey, Christopher Westbrook |
| Konferenz |
EGU General Assembly 2011
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 13 (2011) |
| Datensatznummer |
250055252
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| Zusammenfassung |
| Frontal systems associated with mid-latitude cyclones generate large regions of
cloud which can perturb the regional radiation balance, as well as precipitate large
quantities of water to the surface. Predictability of the spatial distribution and temporal
development of the radiative properties and precipitation associated with frontal clouds
requires knowledge of the microphysical processes acting, as well as the dynamical
forcing.
We present in-situ and remote sensing data obtained from frontal clouds in the vicinity of
the Chilbolton Facility for Atmospheric and Radio Research (CFARR), in the southern region
of the United Kingdom. Range Height Indicator (RHI) scans from the 3 GHz dual
polarisation Doppler CAMRa (Chilbolton Advanced Meteorological Radar) were conducted
along a radial where concurrent in-situ measurements of microphysical properties were
obtained. The in-situ measurements were obtained from the UK Facility for Airborne
Atmospheric Research (FAAM) BAe146 aircraft. A vertical pointing 35 GHz Doppler cloud
radar was also operating at the CFARR site. Case studies were simulated using the
Advanced Research WRF (Weather Research and Forecasting) model version 3.1
using the dual moment Morrison microphysics scheme which predicts the mixing
ratios and number concentrations of hydrometeors in 5 categories (droplets, rain,
ice, snow and graupel). The WRF simulations were run on 4 domains nested from
12 km to 1 km horizontal resolution and initialised/driven at boundaries by GFS
data.
Nucleation at cloud top was seen as the primary source of ice at the time of
measurement for the deep frontal cases where cloud top temperatures were approximately
-35oC. However, one case with lower cloud top height/temperature (approximately
-25oC), while still containing ice, was capped with a layer of supercooled liquid
water at cloud top. The ice nucleated at cloud top grew via deposition/aggregation
into snow. Pristine plate-like ice crystals were frequently observed (temperature
approx. -20oC) in regions of high differential reflectivity (identified by the CAMRa).
Embedded convection originating from near the surface was observed frequently and was
associated with the production of regions of supercooled water within the frontal
cloud. High number concentrations (peaking over 100 L-1, typically 40 L-1) of ice
crystals were found in the temperature range -3oC to -8oC. These ice crystals were
relatively small (300 μm in length compared to over 1 mm for the snowflakes) and
appeared to be un-rimed columns based on in-situ imagery. The high ice crystal
concentrations are most likely a result of Secondary Ice Production (SIP) resulting from the
Hallett-Mossop (HM) process of rime splinter ejection. The horizontal extent to which SIP
appeared to influence ice number concentrations was 10’s of km and contained a
significant amount of condensate (peak over 0.5 g m-3). WRF simulations of the
cases do not appear to reproduce the regions of high ice crystal concentration in the
temperature range -3oC to -8oC, despite the inclusion of the HM process in the Morrison
microphysics scheme used. It appears the HM process is important in controlling the
transfer of water into/within frontal systems which contain significant amounts of
supercooled water. Further through the release of latent heat of fusion the HM process can
have an important influence on the dynamical structure of the cloud and hence the
inability of the WRF model to adequately simulate this process may affect the ability
of the model to forecast the evolution and precipitation from the weather system. |
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