Search Results

You are looking at 1 - 6 of 6 items for :

  • Author or Editor: David H. Bromwich x
  • Weather and Forecasting x
  • Refine by Access: All Content x
Clear All Modify Search
Ryan L. Fogt
and
David H. Bromwich

Abstract

Antarctic Mesoscale Prediction System (AMPS) forecasts of atmospheric moisture and cloud fraction (CF) are compared with observations at McMurdo and Amundsen–Scott South Pole station (hereafter, South Pole station) in Antarctica. Overall, it is found that the model produces excessive moisture at both sites in the mid- to upper troposphere because of a weaker vertical decrease of moisture in AMPS than observed. Correlations with observations suggest AMPS does a reasonable job of capturing the low-level moisture variability at McMurdo and the upper-level moisture variability at South Pole station. The model underpredicts the cloud cover at both locations, but changes to the AMPS empirical CF algorithm remove this negative bias by more than doubling the weight given to the cloud ice path.

A “pseudosatellite” product based on the microphysical quantities of cloud ice and cloud liquid water within AMPS is preliminarily evaluated against Defense Meteorological Satellite Program (DMSP) imagery during summer to examine the broader performance of cloud variability in AMPS. These comparisons reveal that the model predicts high-level cloud cover and movement with fidelity, which explains the good agreement between the modified CF algorithm and the observed CF. However, this product also demonstrates deficiencies in capturing low-level cloudiness over cold ice surfaces primarily related to insufficient supercooled liquid water produced by the microphysics scheme, which also reduces the CF correlation with observations.

The results suggest that AMPS predicts the overall CF amount and high cloud variability notably well, making it a reliable tool for longer-term climate studies of these fields in Antarctica.

Full access
John Turner
,
David H. Bromwich
, and
Gary M. Carter
Full access
David H. Bromwich
,
Richard I. Cullather
, and
Robert W. Grumbine

Abstract

Analyses and medium-range numerical weather forecasts produced by the National Centers for Environmental Prediction are evaluated poleward of 50°S 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 medium-range 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.

Full access
Keith M. Hines
,
Robert W. Grumbine
,
David H. Bromwich
, and
Richard I. Cullather

Abstract

The surface energy budget in Antarctic latitudes is evaluated for the medium-range numerical weather forecasts produced by the National Centers for Environmental Prediction (NCEP) and for the NCEP–National Center for Atmospheric Research reanalysis project during the winter, spring, and summer special observing periods (SOPs) of the Antarctic First Regional Observing Study of Troposphere project. A significant change in the energy balance resulted from an extensive model update beginning with the forecasts initialized on 11 January 1995 during the summer SOP. Both the forecasts and the reanalysis include significant errors in the surface energy balance over Antarctica. The errors often tend to cancel and thus produce reasonable surface temperature fields. General errors include downward longwave radiation about 30–50 W m−2 too small. Lower than observed cloudiness contributes to this error and to excessive downward shortwave radiation at the surface. The model albedo over Antarctica, about 75%, is lower than that derived from observations, about 81%. During the polar day, errors in net longwave and net shortwave radiation tend to cancel. The energy balance over Antarctica in the reanalysis is, in general, degraded from that of the forecasts.

Seasonal characteristics of the surface energy balance include cooling over East Antarctica and slight warming over West Antarctica during NCEP forecasts for the winter SOP. Wintertime surface warming by downward sensible heat flux is larger than observations by 21–36 W m−2 and tends to balance the excessive longwave cooling at the surface. During the spring SOP, forecast sensible heat flux produces an excessive heating contribution by about 20 W m−2. Latent heat flux during the Antarctic winter for the reanalysis is at least an order of magnitude larger than the very small observed values.

Full access
Melissa A. Nigro
,
John J. Cassano
,
Jonathan Wille
,
David H. Bromwich
, and
Matthew A. Lazzara

Abstract

Accurate representation of the stability of the surface layer in numerical weather prediction models is important because of the impact it has on forecasts of surface energy, moisture, and momentum fluxes. It also impacts boundary layer processes such as the generation of turbulence, the creation of near-surface flows, and fog formation. This paper uses observations from a 30-m automatic weather station on the Ross Ice Shelf, Antarctica, to evaluate the near-surface layer in the Antarctic Mesoscale Prediction System (AMPS), a numerical weather prediction system used for forecasting in Antarctica. The method of self-organizing maps (SOM) is used to identify characteristic potential temperature anomaly profiles observed at the 30-m tower. The SOM-identified profiles are then used to evaluate the performance of AMPS as a function of atmospheric stability.

The results indicate AMPS underpredicts the frequency of near-neutral profiles and instead overpredicts the frequency of weakly unstable and weak to moderately stable profiles. AMPS does not forecast the strongest statically stable patterns observed by Tall Tower, but in the median, the AMPS forecasts are more statically stable across all wind speeds, indicating a possible mechanical mixing error or a negative radiation bias. The SOM analysis identifies a negative radiation bias under near-neutral to weakly stable conditions, causing an overrepresentation of the static stability in AMPS. AMPS has a positive wind speed bias in moderate to strongly stable conditions, which generates too much mechanical mixing and an underrepresentation of the static stability. Model errors increase with increasing atmospheric stability.

Full access
Andrew J. Monaghan
,
David H. Bromwich
,
He-Lin Wei
,
Arthur M. Cayette
,
Jordan G. Powers
,
Ying-Hwa Kuo
, and
Matthew A. Lazzara

Abstract

In late April 2001, an unprecedented late-season flight to Amundsen–Scott South Pole Station was made in the evacuation of Dr. Ronald Shemenski, a medical doctor seriously ill with pancreatitis. This case study analyzes the performance of four of the numerical weather prediction models that aided meteorologists in forecasting weather throughout the operation: 1) the Antarctic Mesoscale Prediction System (AMPS) Polar MM5 (fifth-generation Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model), 2) the National Centers for Environmental Prediction Aviation Model (AVN), 3) the European Centre for Medium-Range Weather Forecasts (ECMWF) global forecast model, and 4) the NCAR Global MM5. To identify specific strengths and weaknesses, key variables for each model are statistically analyzed for all forecasts initialized between 21 and 25 April for several points over West Antarctica at the surface and at 500- and 700-hPa levels. The ECMWF model performs with the highest overall skill, generally having the lowest bias and rms errors and highest correlations for the examined fields. The AMPS Polar MM5 exhibits the next best skill, followed by AVN and Global MM5. For the surface variables, all of the models show high skill in predicting surface pressure but demonstrate modest skill in predicting temperature, wind speed, and wind direction. In the free atmosphere, the models show high skill in forecasting geopotential height, considerable skill in predicting temperature and wind direction, and good skill in predicting wind speed. In general, the models produce very useful forecasts in the free atmosphere, but substantial efforts are still needed to improve the surface prediction. The spatial resolution of each model exerts an important influence on forecast accuracy, especially in the complex topography of the Antarctic coastal regions. The initial and boundary conditions for the AMPS Polar MM5 exert a significant influence on forecasts.

Full access