Satellite Observations of Organizational Regimes in Low-Level Mixed-Phase Clouds over the Southern Ocean

Abstract:

Stratocumulus are the most dominant cloud type by area coverage. Due to their high albedo, they play a key role in the radiative balance of the Earth. Stratocumulus clouds can usually be divided into different self-organized morphological regimes. Two of these regimes are referred to as open and closed mesoscale-cellular convective (MCC) clouds. Closed and open MCC are the most frequent types of MCC regimes in the Southern Ocean (SO) and exert a substantial radiative cooling. The albedo of closed MCCs is higher on average than the albedo of open MCCs for the same cloud fraction (McCoy et al 2017). Their cloud radiative forcing therefore also differs.

In the SO, many of the MCC clouds appear as mixed-phase clouds (MPC). We investigate whether the two cloud phases influence MCC organization and therefore the cloud-radiative effect.

We use the raDAR-liDAR (DARDAR) data product (version 1) from the Cloud-Aerosol-Water-Radiation Interactions (ICARE) Data and Services Center which collocates data from three satellites: Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO), CloudSat and Moderate Resolution Imaging Spectroradiometer (MODIS). From this data set, we use the “Simplified DARMASK Categorization Flag” to recategorize the vertically resolved cloud phase into one cloud phase per data point: liquid, mixed or ice. We collocate the DARDAR product with an MCC classification data set based on a neural network algorithm applied to MODIS (Wood and Hartmann 2006) to determine open and closed MCC regions.

Our preliminary findings suggest that most MPCs are composed of a supercooled liquid top with ice below. This confirms observations from ground-based measurements. We restricted the analysis to a cloud top temperature (CTT) range from −10 °C to 0 °C. In the SO, open MCCs occur more frequently as MPCs (78% DJF, 66% JJA) than liquid clouds (19% DJF, 27% JJA) during both seasons. Closed MCCs also occur more often as MPCs (66%) than as liquid clouds (34%) during summer (DJF). During winter (JJA), however, liquid closed MCCs occur at a similar frequency as MPCs.

At CTT warmer than 8 °C we find a different dependence of the MPC fraction on CTT for open and closed MCC. Further investigations with respect to other environmental parameters on the cloud-phase-MCC relationship will be performed.