| Paper: | FR-A2.1 |
| Session: | Atmospheric Applications of Radiometry II |
| Time: | Friday, March 30, 10:40 - 11:00 |
| Presentation: |
Oral
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| Topic: |
Clouds and precipitation: |
| Title: |
Retrieval of liquid water path in drizzling clouds with a combination of ground based active and passive remote sensors |
| Authors: |
Maria Cadeddu; Argonne National Laboratory | | |
| | Virendra Ghate; Argonne National Laboratory | | |
| Abstract: |
Retrieval of liquid water path in drizzling clouds from microwave radiometers is a challenging task. Even when precipitation is light and does not affect the quality of the measurements, the departure from the Rayleigh approximation (due to the increased size of precipitating hydrometeors) requires the inclusion of cloud microphysical properties, such as the drop size distribution (DSD), in the radiative transfer calculations.
It was recently shown [Cadeddu, 2017] that frequencies at and above 90 GHz contain useful information on drop size distribution that can be used to separate to a certain degree cloud from drizzle liquid water in lightly precipitating clouds therefore improving the retrievals. Because precipitation in warm marine clouds affects both the spatial distribution of clouds (cloud fraction) and the cloud liquid water path, improved microwave retrievals can provide better understanding of the mechanism under which precipitation is produced.
In this work we present the case for retrieving liquid water path from unpolarized zenith pointing ground-based radiometers under drizzle and light rain. First we show radiative transfer computations to investigate the effect of the DSD on brightness temperatures and we show that a 3-channel optimal estimation retrieval has 3 degrees of freedom, allowing the retrieval of precipitable water vapor, liquid water path and to some degree the partition between cloud and drizzle liquid water.
In the second part of the work data from the Atmospheric Radiation Measurement (ARM) Program Eastern North Atlantic (ENA) site are used to apply the technique to a drizzling scenario. A ceilometer, cloud radar, and lidar data are first used to retrieve vertical profiles of drizzle microphysical properties such as drizzle effective diameter and drizzle liquid water content below cloud base. The information is then provided to a radiative transfer code that simulates the propagation of microwave radiation accounting for absorption and scattering effects. The optimal estimation technique is then used to retrieve total and cloud liquid path accounting for hydrometeor scattering effects using the retrieved drizzle size distribution parameters and drizzle liquid water content to constrain the retrieval. The retrieved LWP is therefore consistent with the cloud and precipitation microphysical properties. The retrieval results are summarized and an outlook of future developments is presented.
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