Synergistic radar and radiometer retrievals of ice hydrometeors

Simon Pfreundschuh,David Duncan,Stefan A Buehler,Patrick Eriksson,Manfred Brath,Richard Larsson,Robin Ekelund

doi:10.5194/amt-13-4219-2020

Abstract

Abstract. Remote sensing observations at sub-millimeter wavelengths provide higher sensitivity to small hydrometeors and low water content than observations at millimeter wavelengths, which are traditionally used to observe clouds and precipitation. They are employed increasingly in field campaigns to study cloud microphysics and will be integrated into the global meteorological observing system to measure the global distribution of ice in the atmosphere with the launch of the Ice Cloud Imager (ICI) radiometer on board the second generation of European operational meteorological satellites (Metop-SG). Observations at these novel wavelengths provide valuable information not only on their own but also in combination with complementary observations at other wavelengths. This study investigates the potential of combining passive sub-millimeter radiometer observations with a hypothetical W-band cloud radar for the retrieval of frozen hydrometeors. An idealized cloud model is used to investigate the information content of the combined observations and establish their capacity to constrain the microphysical properties of ice hydrometeors. A synergistic retrieval algorithm for airborne observations is proposed and applied to simulated observations from a cloud-resolving model. Results from the synergistic retrieval are compared to equivalent radar- and passive-only implementations in order to assess the benefits of the synergistic sensor configuration. The impact of the assumed ice particle shape on the retrieval results is assessed for all retrieval implementations. We find that the combined observations better constrain the microphysical properties of ice hydrometeors, which reduces uncertainties in retrieved ice water content and particle number concentrations for suitable choices of the ice particle model. Analysis of the retrieval information content shows that, although the radar contributes the largest part of the information in the combined retrieval, the radiometer observations provide complementary information over a wide range of atmospheric states. Furthermore, the combined observations yield slightly improved retrievals of liquid cloud water in mixed-phase clouds, pointing towards another potential application of combined radar–radiometer observations.

Highlights

Ice hydrometeors play an important role for both weather and climate
We find that the combined observations better constrain the microphysical properties of ice hydrometeors, which reduces uncertainties in retrieved ice water content and particle number concentrations for suitable choices of the ice particle model
It should be noted that the cloud in this part is a mixed-phase cloud and that both retrievals successfully retrieve ice water content (IWC) and cloud liquid water content (CLWC)

Summary

Introduction

Ice hydrometeors play an important role for both weather and climate. They influence the Earth’s energy budget through their interaction with incoming and outgoing radiation, constitute a part of the global hydrological cycle, and are coupled to the dynamics of the atmosphere in multiple ways (Bony et al, 2015). Observations of ice clouds are required for understanding the role of clouds in a changing climate (Boucher et al, 2013), to provide information on the dynamical state of the atmosphere in numerical weather prediction (NWP) models (Geer et al, 2017) and to validate climate models (Waliser et al, 2009). Pfreundschuh et al.: Synergistic radar and radiometer retrievals of ice hydrometeors and shapes, which can only be partially resolved by available spaceborne sensors

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Discussion

Conclusion