An Assessment of the NCEP Operational Global Spectral Model Forecasts and Analyses for Antarctica during FROST*

Abstract

Analyses and medium-range numerical weather forecasts produced by the National Centers for Environmental Prediction are evaluated poleward of 50ЊS during the July 1994 special observing period of the Antarctic First Regional Observing Study of the Troposphere project. Over the Antarctic plateau, the poor representation of the continent's terrain creates ambiguity in assessing the quality of surface variables. An examination of the vertical temperature profile, however, finds the near-surface temperature inversion strength to be substantially smaller than the observed climatology at the zero forecast hour. This arises from surface temperatures that are warmer than expected. Significant adjustment occurs in a variety of fields over the first few days of the mediumrange forecast, which likely results from the initial hour's suspect temperature profile. A spatially oscillating series of forecast anomalies in the zonally averaged temperature cross section stretches to middle latitudes by day 3. Near-surface and upper-troposphere values are found actually to improve at the South Pole with forecast time, although some fields continue to adjust through day 7. Although the examination presented here does not give a complete diagnosis, differences between observations and analyses suggest deficiencies with the model initial fields have a major role in producing the substantial model drift found. Atmospheric moisture over the continental interior does not change significantly with forecast hour, although the distinct contrast between nearshore and interior conditions lessens with forecast time. A spurious high-latitude wave pattern is found for a variety of variables. The pattern of this distortion remains constant with forecast hour. Over the ocean, large forecast pressure and height differences with analyses are associated with blocking conditions. However, it is unclear whether this results from deficiencies in the forecast model or the meager observational network over the Southern Ocean.

Atmospheric Moisture and Cloud Cover Characteristics Forecast by AMPS*

“…2 and 6) is that relative humidity observations are assimilated into the initial conditions in AMPS, while clouds are generated by the model physics. Bromwich et al (1999) note that the distinction between coastal and interior moisture conditions lessens with forecast time in older versions of the National Centers for Environmental Prediction (NCEP) operational Global Spectral Model. Notably, NCEP's Aviation Model (AVN) supplies the initial and boundary conditions for AMPS, so the problems observed in Bromwich et al (1999) may still affect the current AMPS forecasts.…”

Validation of ECMWF and NCEP-NCAR reanalysis data in Antarctica

“…For NCEP, Casey and Mawson are outperformed by other stations. These four stations are all located in East Antarctica and their inferior performance may be due to the influence of katabatic wind induced by steep topography (Bromwich et al, 1999). Similar to Fig.…”

“…Cullather et al (1997) compared the two reanalysis products for a 10-yr (1985-1995) period and found that surface pressure in the two reanalysis datasets is in better agreement with observations than surface air temperature and wind, and that the errors in both reanalysis at the standard pressure levels decrease with time. Bromwich et al (1995) found that the ECMWF reanalysis data are superior to those of the NCEP-NCAR in reproducing the atmospheric moisture budget in high southern latitudes for the period 1985-1992. Bromwich et al (1999 verified NCEP reanalysis data against FROST data and showed that the reanalysis produces warmer surface temperatures and weaker near-surface temperature inversions.…”

Forecasting for a Remote Island: A Class Exercise

“…For comparison, Bromwich et al (1999) discussed the performance of NCEP forecasts south of 30°S and found that near the latitude of Bouvet Island the 12-and 24-h forecasts of surface pressure were generally within 1 mb of the NCEP analysis for the month of July 1994. Although this is a very limited sample, the result is encouraging.…”

“…Such katabatic winds arise when dense cold air formed above the cold ice sheet drains downhill. Katabatic flows are notorious for causing some of the strongest winds on earth, as for example along the Antarctic coast (Parish 1981;Bromwich and Parish 1998). In the case of Bouvet Island, a cold drainage flow along the east-west ridge of ice cliffs likely helps deflect storm winds downward along the windward slopes and across the camp site (Fig.…”