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| Titel |
In-situ statistical validation of airborne reflectivity measurements during AMMA2006 and Megha-Tropiques 2010 experiments: importance of hydrometeor growth processes and particle size distribution variability. |
| VerfasserIn |
Régis Dupuy, Christophe Duroure, Etienne Fontaine, Christophe Gourbeyre, Wolfram Wobrock, Alfons Schwarzenboeck, Dominique Bouniol, Alain Protat |
| 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 |
250055366
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| Zusammenfassung |
| During the AMMA 2006 experiment, collocated remote sensing and in-situ estimation of
radar reflectivities of MCS systems have been performed by the French Falcon research
aircraft using airborne radar (RASTA) and in-situ measurements of particle size distributions
(PSD) obtained with hydrometeors imaging instrumentation (2DC, 2DP and 1DP
Knollenberg probes). In addition, the Megha-Tropiques experiment in August 2010, located
in the same region, allowed us to extend the observed MCS cases using modern imaging
probes (2DS, CIP and PIP) as compared to AMMA.
In-situ estimation of reflectivity is very sensible to the assumed diameter to mass function
which resumes the main mode of hydrometeor growth (vapour diffusion at low
supersaturation, dendritic growth, agglomeration or riming). This function could be estimated
using the observed hydrometeor shapes (condensation pristine crystals, dendrites, snowflakes
or graupels). A rather good statistical estimation of this function can be obtained for the more
intense part of the cloud system (the core convective regions) using a power law function of
the diameter with a pre-factor and a characteristic exponent. In case we use the same diameter
to mass relationship also for the regions outside of the convective core, we may
observe both under and over estimation of reflectivity due to the variability of the
local main growth processes (transition between riming to agglomeration) and the
variability of the shape of the PSD (e.g. bimodal spectra on the edges of convective
cores).
We have developed tools to separate the main hydrometeor growth processes along the
flight. On the one hand, we estimate a mean mass density of the hydrometeors using an
iterative comparison of the radar reflectivity observed near the aircraft with the radar
reflectivity computed with the PSD. On the other hand, we derive a “rugosity exponent” from
the bi-dimensional histograms of surface and perimeter of a local population of
hydrometeor images. The large values of rugosity exponent correspond to low densities
dendrites and snowflakes while dense objects such as graupels or hails have low
rugosity exponent. Therefore, this exponent can represent the differences in growth
processes.
Using the mean mass density, the rugosity exponent and a classical shape recognition
technique, we observe that the variability of the growth processes is smaller in MCS than in
mid-latitude convective systems.
Moreover, these two experiments allow statistical comparison of reflectivities
measured with an airborne radar and derived from in-situ measurements on a large data
set (few thousands km with a 250 meters resolution) with various microphysical
properties. We also compare these results with simulated reflectivities obtained with the
DESCAM detailed microphysical cloud model utilised for large convective systems. |
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