HR: 17:30h
AN: A34B-07    [Abstracts]
TI: Simulating Arctic mixed-phase clouds: Sensitivity to environmental conditions and cloud microphysical processes
AU: * Sednev, I
EM: isednev@lbl.gov
AF: LBNL, 1 Cyclotron Road, Berkeley, CA 94720 United States
AU: Menon, S
EM: smenon@lbl.gov
AF: LBNL, 1 Cyclotron Road, Berkeley, CA 94720 United States
AB: The importance of Arctic mixed-phase clouds on radiation and the Arctic climate are evaluated using the NASA GISS single column model (SCM) and ARM observations. The observations were conducted during the DOE ARM campaign (MPACE) in October 2004 at the North Slope of Alaska. Of interest is the ability of the SCM cloud microphysics scheme to reproduce the development and persistence of mixed-phase stratiform clouds observed during MPACE. Improvements to the GISS SCM include the redesign of numerical algorithms used in the turbulence scheme and reformulation of the numerical surface boundary conditions to better represent the environmental conditions in which these cloud systems persist. The modified GISS SCM simulates the observed temperature and humidity fields more realistically during the two week observational period (October 5 to 20) compared to the older version, within the uncertainties of the forcing data used to initialise/drive the model. Although we simulate the entire two week episode, we mainly focus on two periods: October 5th and 6th, where multiple level clouds were observed; and Oct 8-12, where a single level stratus was observed. Both these cases had mixed-phase clouds. Our simulations show that despite its simplicity, the SCM cloud scheme is able to reproduce the qualitatively typical behavior of mixed-phase stratiform Arctic clouds for both cases. However, some modifications are required to improve the description of microphysical processes such as the ice nucleation mechanisms and processes related to the formation of precipitation. Sensitivity tests are conducted to understand the impact of the addition of various ice nucleation mechanisms (contact, immersion, condensational, etc.) to the cloud field and radiative fluxes. Observations of ice/water drop size distributions, cloud phase, etc., allow us to constrain model microphysical modifications more realistically. Additional observations of the temporal behaviour of properties such as cloud cover, liquid/ice water paths and radiative fluxes simulated by the model will also be compared with observations. This will help in evaluating the importance of the environment and the representation of cloud microphysics in simulating the Arctic mixed-phase cloud structure and lifetime, and finally to evaluate the impact of mixed-phase clouds on radiation.
DE: 0320 Cloud physics and chemistry
DE: 0321 Cloud/radiation interaction
SC: Atmospheric Sciences [A]
MN: Fall Meeting 2005