 |
| Titel |
Testing remote sensing on artificial observations: impact of drizzle and 3-D cloud structure on effective radius retrievals |
| VerfasserIn |
T. Zinner, G. Wind, S. Platnick, A. S. Ackerman |
| Medientyp |
Artikel
|
| Sprache |
Englisch
|
| ISSN |
1680-7316
|
| Digitales Dokument |
URL |
| Erschienen |
In: Atmospheric Chemistry and Physics ; 10, no. 19 ; Nr. 10, no. 19 (2010-10-08), S.9535-9549 |
| Datensatznummer |
250008817
|
| Publikation (Nr.) |
 copernicus.org/acp-10-9535-2010.pdf |
|
|
|
|
|
| Zusammenfassung |
Remote sensing of cloud effective particle size with passive sensors like
the Moderate Resolution Imaging Spectroradiometer (MODIS) is an important
tool for cloud microphysical studies. As a measure of the radiatively
relevant droplet size, effective radius can be retrieved with different
combinations of visible through shortwave and midwave infrared channels. In
practice, retrieved effective radii from these combinations can be quite
different. This difference is perhaps indicative of different penetration
depths and path lengths for the spectral reflectances used. In addition,
operational liquid water cloud retrievals are based on the assumption of a
relatively narrow distribution of droplet sizes; the role of larger
precipitation particles in these distributions is neglected. Therefore,
possible explanations for the discrepancy in some MODIS spectral size
retrievals could include 3-D radiative transport effects, including sub-pixel
cloud inhomogeneity, and/or the impact of drizzle formation.
For three cloud cases the possible factors of influence are isolated and
investigated in detail by the use of simulated cloud scenes and synthetic
satellite data: marine boundary layer cloud scenes from large eddy
simulations (LES) with detailed microphysics are combined with Monte Carlo
radiative transfer calculations that explicitly account for the detailed
droplet size distributions as well as 3-D radiative transfer to simulate
MODIS observations. The operational MODIS optical thickness and effective
radius retrieval algorithm is applied to these and the results are compared
to the given LES microphysics.
We investigate two types of marine cloud situations each with and without
drizzle from LES simulations: (1) a typical daytime stratocumulus deck at
two times in the diurnal cycle and (2) one scene with scattered cumulus.
Only small impact of drizzle formation on the retrieved domain average and
on the differences between the three effective radius retrievals is noticed
for both cloud scene types for different reasons. For our, presumably
typical, overcast stratocumulus scenes with an optical thickness of 8 to 9
and rain rates at cloud bottom up to 0.05 mm/h clear drizzle impact on the
retrievals can be excluded. The cumulus scene does not show much drizzle
sensitivity either despite extended drizzle areas being directly visible
from above (locally >1 mm/h), which is mainly due to technical
characteristics of the standard retrieval approach. 3-D effects, on the other
hand, produce large discrepancies between the 1.6 and 2.1 μm channel
observations compared to 3.7 μm retrievals in the latter case. A general
sensitivity of MODIS particle size data to drizzle formation is not
corroborated by our case studies. |
| |
|
| Teil von |
|
|
|
|
|
|
|
|