Evaluating the feasibility of using spaceborne-radar to estimate CCN scavenging by drizzle in marine stratocumulus

Observations collected during several recent field campaigns targeting marine stratus/stratocumulus (DYCOMS-II, EPIC, and ASTEX) suggest that drizzle is both stronger and more widespread than previously realized. Despite extensive efforts to understand and quantify CCN production processes in the marine boundary layer (via sea spray injection, new particle formation, condensation of gases, and entrainment from the free troposphere) the removal of CCN due to drizzle has been largely neglected. Simple calculations show that the production of a single 0.1mm diameter drizzle droplet requires the collection of more than 100 20?m cloud droplets and that, given the range of cloud-base drizzle rates encountered during DYCOMS-II (0.05 to 1.65 mm day-1), estimated CCN scavenging rates ranged from ~0.3×1010 to ~12×1010 m-2 day-1. In the absence of new CCN production processes these removal rates are sufficient to completely deplete the boundary layer CCN population in < 1 day for cases with strong drizzle. These simple estimates of the removal rates make it clear that drizzle-scavenging plays a significant role in the CCN budget within the boundary-layer. Due to the combined effects of the vast extent of the stratocumulus decks, the albedo difference between the stratocumulus layer and the underlying ocean, and the susceptibility of the albedo to CCN concentration, processes which affect the marine stratocumulus CCN budget are extremely important to global climate.

The rate of CCN scavenging depends linearly on the drizzle rate and on the Volume-Weighted Mean Diameter (CVWMD) of the collected cloud droplets to the –3rd power. It is important to note that the CCN scavenging rate depends on the cloud-base drizzle rate, (which is 4 or more times greater than the drizzle rate at the ocean surface) and that the CCN scavenging rate is effected only negligibly by the final disposition of the drizzle drop (whether the drizzle drop reaches the ocean surface or evaporates completely). Intrinsic factors governing CVWMD are: the cloud-droplet size spectra, the drizzle drop versus cloud droplet collection efficiency and the trajectory of drizzle drops through the cloud-layer.

Since CVWMD depends on factors that cannot be easily measured it is necessary to formulate an empirical relationship between drizzle rate and the CCN scavenging rate in terms of measurable quantities -- namely the droplet effective radius at cloud-top. This relationship will be developed using aircraft in situ measurements of aerosol/CCN size spectra (below cloud), cloud-droplet size spectra, and high-rate air motion estimates collected during the DYCOMS-II project. Two distinct approaches will be employed to estimate the CCN scavenging rate: (i) computation of cloud droplet flux by applying eddy-correlation techniques to the FSSP cloud-droplet size spectra. Essentially all cloud droplets form within tens of meters above cloud base – thus, the cloud droplet flux represents the combined effect of the processes occurring above the flight level (parcels moving downward through the flight level have been modified by entrainment and drizzle). Cloud droplets are lost through both evaporation and collection by drizzle -- thus, the upward flux of cloud-droplets represents an upper-limit on the CCN scavenging rate and must be corrected for the loss due to evaporation (using ozone fluxes and other indications of entrainment/evaporation). (ii) eddy-covariance and relaxed eddy accumulation estimates of aerosol fluxes using PCASP and other aerosol measurements from the below-cloud flight legs (these estimates will be difficult to make due to the limited sample volumes and time resolutions of the aerosol measurements and their susceptibility to contamination by drizzle).

Since opportunities for direct measurement of CCN scavenging in marine stratocumulus using instrumented aircraft are inherently very few and far between, CCN scavenging estimates derived from the DYCOMS-II dataset will be examined in terms of satellite-measurable quantities namely: radar reflectivity (from the Cloud-Profiling Radar onboard CloudSat), Cloud-top heights (from the CALIPSO lidar), and cloud-droplet effective radius estimates (from MODIS). Data from the DYCOMS-II field campaign will be used to simulate what would be seen by the aforementioned satellite sensors: radar reflectivities from the Wyoming Cloud Radar (WCR) will be resampled to approximate the 500m vertical resolution of CloudSat, smoothed cloud-top heights from the SABL lidar and the WCR will be used as a surrogate for CALIPSO cloud-top heights, and FSSP data will be used to estimate cloud-droplet effective radii near cloud top (MODIS measurements which were available during DYCOMS-II will also be used). The simulated satellite measurements will be compared to the direct estimates of the CCN scavenging in order to assess the feasibility of using a suite of satellite observations (most notably radar reflectivities from CloudSat) in order to estimate the effect of drizzle scavenging in marine stratocumulus on a global scale.