John J. Cassano

Mesoscale Modeling of Katabatic Winds over Greenland with the Polar MM5

Abstract Verification of two months, April and May 1997, of 48-h mesoscale model simulations of the atmospheric state around Greenland are presented. The simulations are performed with a modified version of The Pennsylvania State University–National Center for Atmospheric Research fifth-generation Mesoscale Model (MM5), referred to as the Polar MM5. Global atmospheric analyses as well as automatic weather station and instrumented aircraft observations from Greenland are used to verify the forecast atmospheric state. The model is found to reproduce the observed atmospheric state with a high degree of realism. Monthly mean values of the near-surface temperature and wind speed predicted by the Polar MM5 differ from the observations by less than 1 K and 1 m s−1, respectively, at most sites considered. In addition, the model is able to simulate a realistic diurnal cycle for the surface variables, as well as capturing the large-scale, synoptically forced changes in these variables. Comparisons of modeled profil...

Evaluation of Polar MM5 simulations of Greenland's atmospheric circulation

A complete annual cycle over the Greenland ice sheet is simulated with the Polar MM5, a mesoscale model optimized for use over extensive ice sheets. These simulations are compiled from a series of short duration (48 hour), forecast mode, simulations. The model output is compared to observations primarily from the Greenland Climate Network automatic weather station (AWS) array. The model simulations show a high degree of skill for all variables evaluated with the AWS data (pressure, temperature, water vapor mixing ratio, wind speed and direction, downwelling shortwave radiation, and net radiation) for all seasons, although the use of a fixed albedo in the Polar MM5 leads to large errors in the simulated net radiation budget over melting ice surfaces during the summer months. The modeled precipitation distribution agrees with available observations in the interior of the ice sheet but is excessive along the steep margins of the island. A discussion of possible future applications of the Polar MM5 is presented.

Evaluation of Polar MM5 Simulations of Antarctic Atmospheric Circulation

Abstract Evaluation of a complete annual cycle of nonhydrostatic mesoscale model simulations of the Antarctic atmospheric circulation is presented. The year-long time series are compiled from a series of overlapping short-duration (72 h) simulations of the atmospheric state with the first 24 h being discarded for spinup reasons, and the 24–72-h periods used for model evaluation. The simulations are generated with the fifth-generation Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model (MM5), which is modified for polar applications, and is referred to as the Polar MM5. With a horizontal resolution of 60 km, the Polar MM5 has been run for the period of January 1993–December 1993, creating short-term simulations from initial and boundary conditions provided by the European Centre for Medium-Range Weather Forecasts (ECMWF) Tropical Ocean Global Atmosphere (TOGA) operational analyses. The model output is compared with observations from automatic weather stations, upper-air d...

Satellite-based model detection of recent climate-driven changes in northern high-latitude vegetation productivity

[1] We applied a satellite remote sensing based production efficiency model (PEM) using an integrated AVHRR and MODIS FPAR/LAI time series with a regionally corrected NCEP/NCAR reanalysis daily surface meteorology and NASA/GEWEX Surface Radiation Budget shortwave solar radiation inputs to assess annual terrestrial net primary productivity (NPP) for the pan-Arctic basin and Alaska from 1983 to 2005. Our results show that low temperature constraints on Boreal-Arctic NPP are decreasing by 0.43% per year (P < 0.001), whereas a positive trend in vegetation moisture constraints of 0.49% per year (P = 0.04) are offsetting the potential benefits of longer growing seasons and contributing to recent disturbances in NPP. The PEM simulations of NPP seasonality, annual anomalies and trends are similar to stand inventory network measurements of boreal aspen stem growth (r = 0.56; P = 0.007) and atmospheric CO2 measurement based estimates of the timing of growing season onset (r = 0.78; P < 0.001). Our results indicate that summer drought led to marked NPP decreases in much of the boreal forest region after the late-1990s. However, seasonal low temperatures are still a dominant limitation on regional NPP. Despite recent drought events, mean annual NPP for the pan-Arctic region showed a positive growth trend of 0.34% per year (20.27 TgC/a; P = 0.002) from 1983 to 2005. Drought induced NPP decreases may become more frequent and widespread as regional ecosystems adjust to a warmer, drier atmosphere, though the occurrence and severity of drought events will depend on future patterns of plant-available moisture.

Polar research: Six priorities for Antarctic science

Antarctica. The word conjures up images of mountains draped with glaciers, ferocious seas dotted with icebergs and iconic species found nowhere else. The continent includes about one-tenth of the planets land surface, nearly 90% of Earths ice and about 70% of its fresh water. Its encircling ocean supports Patagonian toothfish and krill fisheries, and is crucial for regulating climate and the uptake of carbon dioxide by sea water.

The Role of Katabatic Winds on the Antarctic Surface Wind Regime

Abstract Antarctica is known for its strong and persistent surface winds that are directed along topographic pathways. Surface winds are especially strong during the winter period. The high directional constancy of the wind and the close relationship of the wind direction to the underlying terrain can be interpreted as evidence of katabatic wind activity. Observations show that the directional constancy of the Antarctic surface wind displays little seasonal variation. Summertime winds cannot be expected to contain a significant katabatic component, owing to enhanced solar heating of the ice slopes. Observations also show that the coastal environs are subjected to wide variation in atmospheric pressure associated with frequent cyclone activity. The robust unidirectional nature of the Antarctic surface wind throughout the year implies that significant topographic influences other than those from katabatic forcing must be acting. Idealized numerical simulations have been performed to illustrate the potential...

Antarctic Automatic Weather Station Program: 30 Years of Polar Observation

Antarctica boasts one of the worlds harshest environments. Since the earliest expeditions, a major challenge has been to characterize the surface meteorology around the continent. In 1980, the University of Wisconsin—Madison (UW-Madison) took over the U.S. Antarctic Program (USAP) Automatic Weather Station (AWS) program. Since then, the UW-Madison AWS network has aided in the understanding of unique Antarctic weather and climate. This paper summarizes the development of the UW-Madison AWS network, issues related to instrumentation and data quality, and some of the ways these observations have and continue to benefit scientific investigations and operational meteorology.

Synoptic Forcing of Precipitation over Greenland: Climatology for 1961–99

Abstract Analysis of the synoptic climatology and precipitation patterns over the North Atlantic region allows for a better understanding of the atmospheric input to the mass balance of the Greenland ice sheet. The self-organizing map (SOM) technique was applied to the 40-yr European Centre for Medium-Range Weather Forecasts Re-Analysis (ERA-40) daily sea level pressure (SLP) data from 1961 to 1999 to objectively identify synoptic SLP patterns over the North Atlantic region. A total of 35 different SLP patterns were identified. Patterns common to the winter season are characterized by deep low pressure systems that approach Greenland through an active North Atlantic storm track, whereas patterns most common to the summer months are generally weaker and approach the ice sheet from the west through Baffin Bay. The blocking, splitting, and intensification of cyclones by the high elevations of the Greenland ice sheet were identified in this analysis. Analysis of ERA-40 precipitation associated with each SLP p...

A Decade of Antarctic Science Support Through Amps

The Antarctic Mesoscale Prediction System (AMPS) is a real-time numerical weather prediction (NWP) system covering Antarctica that has served a remarkable range of groups and activities for a decade. It employs the Weather Research and Forecasting model (WRF) on varying-resolution grids to generate numerical guidance in a variety of tailored products. While its priority mission has been to support the forecasters of the U.S. Antarctic Program, AMPS has evolved to assist a host of scientific and logistical needs for an international user base. The AMPS effort has advanced polar NWP and Antarctic science and looks to continue this into another decade. To inform those with Antarctic scientific and logistical interests and needs, the history, applications, and capabilities of AMPS are discussed.

Performance of the Weather Research and Forecasting Model for Month-Long Pan-Arctic Simulations

AbstractThe performance of the Weather Research and Forecasting (WRF) model was evaluated for month-long simulations over a large pan-Arctic model domain. The evaluation of seven different WRF (version 3.1) configurations for four months (January, April, July, and October 2007) indicated that WRF produces reasonable simulations of the Arctic atmosphere. Ranking of the model error statistics, calculated relative to the NCEP/Department of Energy Global Reanalysis 2 (NCEP-2), for sea level pressure, 500- and 300-hPa geopotential height, 2-m air temperature, and precipitation identified the model configurations that consistently produced the best pan-Arctic simulations. For all WRF configurations considered, large errors in circulation are evident in the North Pacific. The errors in the North Pacific are manifested as an overly weak and westward-shifted Aleutian low and overly strong subtropical Pacific high simulated by WRF. These circulation errors are nearly barotropic, with a slight increase in magnitude ...