Evaluation of the NASA GMAO Modern Era Retrospective-Analysis for Research and Applications (MERRA) in Polar Latitudes
R. I. Cullather
October 26, 2012 15:25:59
Description of Problem
The work performed under this task relates to high latitude process modeling and analysis. MERRA is a state-of-the-art global numerical reanalyses that has recently been produced by the NASA GMAO, and covers the period from 1979 until the present. The work associated with MERRA seeks to characterize its performance over polar regions including the Arctic Ocean and continental ice sheets and, where appropriate, apply MERRA for the purpose of regional climate study. Additionally, work is to be performed with experts in ice sheet modeling along with the GEOS-5 model development team to integrate and evaluate a community ice sheet model within the GEOS-5 modeling framework. Analysis of GEOS-5 climate simulations and GEOS-5 products is also conducted to evaluate the representation of the water and energy cycle in the model and assimilation systems at high latitudes, and to contribute to better understanding the Earth system, models and reanalyses. To that end, both in situ and remote sensing observations will be considered along with MERRA and other existing reanalyses and the climate simulations. Topics of particular interest are ice sheet surface mass balance, precipitation processes and high latitude climate processes in general. The position is part of a team, and as such, the incumbent’s expertise may be useful to other activities, generally related to atmospheric reanalysis.
Scientific Objectives and Approach
In the first stage of this work with MERRA, the focus has been on assessing components of the atmospheric energy and moisture balances with prior studies. To this end a combined approach has been used to compare fixed sub-regions with prior studies, and to make concurrent comparisons with in situ observations at point locations. The second stage is the application of MERRA for regional climate study. The approach used here is to take advantage of lessons learned from the budget assessments in order to understand the strengths and limitations of reanalyses. This work also makes use of other data products including those from the European Centre for Medium-Range Weather Forecasts (ECMWF) and the U.S. National Centers for Environmental Prediction (NCEP) in order to improve confidence in the conditions described by MERRA.
The goals of the ice sheet integration project are to fully couple a dynamical ice sheet model (ISM) to GEOS-5, improve the representation of critical processes in ISMs with initial focus on grounding line dynamics and melt-freeze parameterizations beneath ice shelves, and investigate the sensitivity and feedbacks of the atmosphere-ocean-cryosphere coupled system. The project seeks to improve the representation of the surface climate over polar ice sheets, adequately simulate energy and moisture fluxes between ice sheets and other model components, and support a prognostic capability for eustatic response to dynamically- and thermodynamically-induced ice sheet variations. ISMs may be thought of as self-contained dynamical representations of ice sheets, with surface mass balance (accumulation) and surface temperature as primary inputs. In two-way coupling, the exchange of fluxes may impact the GCM. As GEOS-5 currently does not run a snow hydrology model over ice sheets, the work plan of the first year has focused on establishing a snow model for the land ice surface category. Additionally the project has sought to identify strengths and weaknesses in prospective ISMs, develop downscaling methods for surface mass balance, and use the developed snow model to estimate surface mass balance from MERRA. This will lead to one-way coupling of an ice sheet model in early 2012.
Cullather and Bosilovich (2011, 2012) evaluates the polar atmospheric moisture and energy budgets in MERRA and have been published in special collections of the Journal of Climate on MERRA and on the U.S. CLIVAR/SeaFlux Workshop on Surface Fluxes: Challenges for High Latitudes. An analysis of variations in baroclinic instability in the summertime Arctic and its relation to sea ice cover was presented at the American Meteorological Society Polar Meteorology and Oceanography conference and at the World Climate Research Programme Open Science Conference, and results are being assembled for publication.
For the ice sheet model development, a pro-type snow hydrology model suitable glacial ice has been developed. The model utilizes the Stieglitz snow model, which currently runs on non-glacial land surfaces in GEOS-5. Over ice sheets, the primary difficulties in simulating snow hydrology are associated with initialization and a never-ending supply of snow accumulation which may destabilize the model. These issues have been finessed by applying a fixed snow depth, which is replenished at each time step at the bottom interface. Melt water is allowed to percolate through the snowpack to a critical snow density, and then refreezes or becomes runoff. The snowpack is initialized to the annual mean surface temperature. This configuration, which corresponds to a state-of-the-art surface mass balance model that is typically not found in a GCM, is undergoing testing. Cullather met with participants of the SeaRISE modeling workshop held at Goddard on 26-27 September 2011, and participated in the Ice Sheet System Model (ISSM) workshop, 12-13 December 2011 at NASA Jet Propulsion Laboratory. The ISSM and the Los Alamos/NCAR Community Ice Sheet Model (CISM) are prospective ISMs for coupling with GEOS-5. Methods for transitioning between the coarse-resolution surface mass balance of GEOS-5 to a higher spatial resolution suitable for ISMs have been investigated using synthetic downscaling of a high resolution data set. These results are being evaluated for publication. The ice sheet modeling investigation is in collaboration with NASA PI Sophie M.J. Nowicki. . Additional evaluation of the GEOS-5 model performance in high latitudes is associated with decadal simulations produced for the IPCC. A preliminary assessment was presented at the WCRP Open Science Conference.
Refereed Journal Publications
Cullather, R. I., and M. G. Bosilovich, 2011: The moisture budget of the polar atmosphere in MERRA. J. Climate, 24(11), 2861-2879.
Cullather, R. I., and M. G. Bosilovich, 2012: The energy budget of the polar atmosphere in MERRA. J. Climate, 25(1), 5-24.
Other Publications and Conferences
Cullather, R.I., B. Zhao, and M.G. Bosilovich, 2011: The Twenty-First Century Arctic atmospheric energy budget from coupled model simulations and numerical reanalyses. World Climate Research Programme Open Science Conference, 24-28 October, Denver, CO.
Cullather, R.I.,, M.G. Bosilovich and S.M.A. Häkkinen, 2011: Arctic storm tracks and their relation to sea ice cover. World Climate Research Programme Open Science Conference, 24-28 October, Denver, CO.
Cullather, R.I. and M.G. Bosilovich, 2011: Synoptic variability in the Arctic summer. 11th AMS Conference on Polar Meteorology and Oceanography, 2-5 May, Boston, MA.