Polar Cloud Microphysics and Surface Energy Budget from AWARE

Principal Investigator(s):
Dan Lubin, The Regents of the University of California-SIO

The ARM West Antarctic Radiation Experiment (AWARE) has successfully deployed and operated the most advanced suite of atmospheric and climate science equipment over sent to Antarctica. This project team has been involved with AWARE since its inception, and has accomplished initial data analysis related to a deep warming and surface-melting event on the West Antarctic Ice Sheet (WAIS).

This project’s objective is to use the full suite of advanced ARM Mobile Facility sensors to test and improve cloud microphysical parameterizations in the Antarctic environment, which has significant contrasts to the Arctic. One partner in this project, the Polar Meteorology Group of the Byrd Polar and Climate Research Center (BPRC) at The Ohio State University, has adapted polar-optimized version of the Weather Research and Forecasting regional model (WRF), based on extensive experience with mesoscale modeling in the Polar Regions. Real-time Antarctic forecasts are run daily with Polar WRF model in support of U.S. Antarctic Program operations.

We will analyze the AMF-2 cloud research radar and lidar data to develop joint probability density functions for cloud-top temperature, cloud depth, ice flux into cloud top and out of cloud base, reflectivity weighted mean ice fall speed and vertical velocity field characteristics to develop statistics that may be used for observational studies of the cloud microphysical processes and cloud model simulation evaluations. We will also use radar simulator output to produce measurements consistent with the model microphysical assumptions.

We are at a fortunate juncture in microphysical modeling where three current and highly relevant schemes can be tested with AWARE data within the timeframe of this project: (1) the two-moment microphysical scheme currently used as an option in WRF with similarities to the microphysics scheme in the NCAR Community Atmosphere Model (CAM), (2) a conceptual improvement on the WRF scheme that is closer to fundamental physics, and (3) an independent aerosol-aware scheme.

Unlike studies of Arctic clouds, for which considerable progress has been made, Antarctic clouds are still terra incognita. The one thing we can say for certain is that their widespread orographic forcing and larger vertical velocities are a significant contrast from most Arctic clouds. So our approach to this microphysical modeling effort will be exploratory: (1) we will compile the ARM data for both input to the models and evaluation of the model simulations; (2) we will run WRF for the specified months and meteorological regimes with all three microphysical schemes; (3) we will vary parameters in a general way as described above, to evaluate sensitivities; (4) we will identify the most significant discrepancies in WRF model output compared with observations; and (5) for the significant discrepancies, we will delve into the specifics of the model to determine what needs to be improved.

Our work will not follow the traditional “case study” approach, where a short time period is chosen containing a cloud with “classical” properties (this is often done in the Arctic). So little is known about Antarctic clouds that we cannot arbitrarily decide what type of mixed-phase cloud is “classical.” Instead, we will perform this modeling work for longer time periods including those identified objectively as pertaining to distinct meteorological regimes. For the WAIS, this work will also yield insights into similarities and contrasts with all-wave cloud radiative forcing over the Greenland Ice Sheet, which has been shown to enhance surface melting.