J25.3 An Evaluation of Cirrus Cloud Thinning through Improved Integration of Satellite Retrievals and Climate Modeling

Tuesday, 14 January 2020: 3:45 PM
201C (Boston Convention and Exhibition Center)
Yuta Tomii, DRI, Reno, NV; and F. Hosseinpour, J. Mejia, and D. L. Mitchell

Of the three most recognized aerosol approaches to climate engineering, cirrus cloud thinning or CCT is arguably the most difficult to evaluate using a climate model due in part to the sub-grid scale processes and mechanisms relevant to ice nucleation. To avoid this complication, a new satellite retrieval for cirrus clouds has been developed (Mitchell et al., 2018, ACP) that retrieves the cirrus cloud ice particle number concentration N, effective diameter De and ice water content IWC for optical depths between 0.3 and 3.0. Broadly speaking, cirrus clouds form through either homo- or heterogeneous ice nucleation (henceforth hom and het), where hom (het) is characterized by relatively high (low) N. Retrieval results suggest het is dominant during summer over oceans at mid-latitudes. This “het” condition was applied globally to represent cirrus formed via het using the Whole Atmosphere Community Climate Model version 6 (WACCM6), replacing the cirrus De supplied by the Morrison-Gettelman cloud microphysics scheme (MG2) with the De-temperature relationship derived from this het condition. The observed global cirrus cloud condition (based on the CALIPSO satellite), which was related to temperature (T), latitude, season and surface type (land vs. ocean), was represented in WACCM6 by 48 De-T relationships. Two 40-year WACCM6 simulations are being conducted, with one based on the het condition (henceforth the Het simulation) and one based on the observed condition (henceforth the CALIPSO simulation).

The MG2 code was revised so that predicted ice particle size distributions (PSD) in cirrus clouds conform with CALIPSO De values as a function of latitude, season and surface type. New relationships were developed between effective diameter De and PSD mean size Dmean, along with new ice fall speed relationships that are number- and mass-weighted (functions of Dmean and De, respectively). These are used to insure that the MG2 scheme is internally consistent with our CALIPSO De retrievals that partly determine the cirrus PSD in our modified MG2 scheme. This modified MG2 code is freely available to other investigators that have interest in interfacing satellite De retrievals with the cloud components of climate models.

Differences between the cloud radiative effect (CRE) in the CALIPSO and Het simulations will provide an estimate of the CRE contribution from hom-produced cirrus clouds. Conversely, this will also provide an estimate of the CRE from CCT, assuming that all hom cirrus are converted to het cirrus clouds. Since the CALIPSO retrievals of N and De indicate that hom cirrus clouds are most common at high latitudes and during winter at mid-latitudes, the CCT CRE will be strongest at high latitudes and during winter at mid-latitudes.

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