Search Results

You are looking at 1 - 10 of 21 items for :

  • Author or Editor: David P. Baumhefner x
  • Monthly Weather Review x
  • Refine by Access: All Content x
Clear All Modify Search
DAVID P. BAUMHEFNER

Abstract

A number of global real-data numerical forecasts have been calculated using the two-layer NCAR (National Center for Atmospheric Research) general circulation model. The purpose of these experiments was threefold: 1) to evaluate the model's ability to predict the real atmosphere, 2) to develop a global forecasting model which will make use of the data obtained by the proposed GARP (Global Atmospheric Research Program), and 3) to help determine some of the internal, empirical constants of the model. In order to evaluate the accuracy of the predictions, several “skill scores” were calculated from the forecasted and observed variables. A by-product of this research was the testing of five different types of data-initialization schemes. Over 50, 4-day forecasts have been run, in which the initialization schemes and internal constants were varied.

The results from these experiments indicate that the present two-layer model is capable of forecasting the real atmosphere with reasonable skill out to 2 days at the surface and 4 days in the middle troposphere. The best initialization scheme for this particular model, thus far, appears to be the complete balance equation. However, several of the simplified initialization techniques are very close in terms of forecasting skill.

Full access
David P. Baumhefner

Abstract

No abstract available.

Full access
DAVID P. BAUMHEFNER

Abstract

A diagnostic, nonlinear balanced model is applied in order to describe numerically the three dimensional structure of the tropical atmosphere. Several comparisons and experiments are made to gain insight into the physical processes and reliability of the model. These include different types of stream functions and temperature analyses, and the addition of surface friction and latent heat. A comparison between the kinematic vertical motion and the final numerical result is performed.

Obtained by using the complete form of the balance model, the derived vertical motion for August 12–14, 1961, in the Caribbean is presented in the form of cross sections. The vertical velocity fields, which are displayed in partitioned form, are compared with the analyzed moisture distribution. The validity of the computed vertical motion is discussed along with its possible influence on the tropical weather.

Full access
David P. Baumhefner

Abstract

A pilot study that evaluates the potential forecast skill of winter 10–30-day time-mean flow from a low-resolution (R15) climate simulation model is presented. The hypothesis tested is that low-resolution climate model forecasts might be as skillful as high-resolution numerical weather prediction model forecasts at extended-range timescales, if the low-frequency evolution is primarily a large-scale process and if the systematic error of the climate model is less detrimental than high-resolution forecast model error.

Eight forecast cases, each containing four ensemble members, are examined and compared to high-resolution forecasts discussed by Miyakoda et al. The systematic error of the climate model is examined and then used to reduce the forecast error in an a posteriors fashion. The operational utility of these climate model forecasts is also assessed.

The low-resolution climate model is quite successful in duplicating the skill of the high-resolution forecast model. If the forecast systematic component of error evaluated from the same eight cases is removed, the climate model forecasts improve in a comparable fashion to the high-resolution results. When information from the low-resolution climate simulation is used to estimate the forecast systematic error, the improvement in skill is less successful. These results show that a low-resolution climate model can be a viable tool for numerical extended-range forecasting and imply that large ensembles can be integrated for the same cost as higher-resolution model integrations.

Full access
Thomas W. Bettge and David P. Baumhefner

Abstract

The total and systematic errors in the 500 mb geopotential height forecasts from the NMC grid-point and spectral operational models are compared and contrasted for two recent winters. The spectral model is shown to be an improvement in the forecasts through a more skillful prediction of the planetary-scale (zonal wavenumbers 1-2) quasi-stationary wave amplitudes, and through the elimination of the grid-point model's large systematic error at low latitudes.

In agreement with estimates from related studies, the systematic error in the NMC spectral model accounts for 15-20% of the total error variance. Approximately one-half of the total systematic error resides in the planetary scales.

Full access
Thomas W. Bettge and David P. Baumhefner

Abstract

The design of a digital filter is outlined and its application as a band-pass filter to separate various scales from an atmospheric field in a limited domain is discussed. The accuracy of the filter is demonstrated by decomposing both a function with specified wave components and a 500 mb geopotential field within a 90°longitudinal area of the globe. The boundary effects of the non-periodic domain are not negligible, but tests using various boundary conditions show that little contamination exists inside 7–10 grid points from the boundaries. The suitability of the technique to examine the spatial wavenumber characteristics of a geopotential field within a limited domain is demonstrated.

Full access
David P. Baumhefner and Richard C. J. Somerville

Abstract

No abstract available.

Full access
David P. Baumhefner and Paul R. Julian

Abstract

The error produced by an observational system of remotely sensed temperature profiles is partially simulated with aid of an operational retrieval scheme. A given temperature distribution is converted to radiances and then back to temperatures. A comparison is made between the retrieved temperatures and the original values.

The sensitivity of the retrieval scheme to various input parameters, such as guess profiles, the statistical coefficients used in the retrieval scheme, and cloudiness, is examined. Experiments with the placement of a reference level from which to integrate the geopotential field are performed.

The relative growth rate of the simulated initial error is examined by forecasting with two initial states, one case with the observational error and one central case. Error growths are calculated for different reference levels, clear, and cloudy cases.

The results show that the generated error fields do not seriously contaminate forecasts of the large-scale baroclinic waves for periods up to one week, providing the retrievals are free of the effects of clouds and an accurate guess is used. In general, the results support the need for reference-level information, and if forecast error in the low troposphere is to be a minimum, placing of the reference level at sea level.

Full access
Steven L. Mullen and David P. Baumhefner

Abstract

The relative importance of different parameterized physical process and baroclinic dynamics in numerical simulations of explosive oceanic cyclogenesis is examined. The numerical simulations are derived from a global spectral model having nine vertical levels and a rhomboidal 31 truncation. Eleven cases of rapid cyclogenesis over the North Pacific Ocean that occurred in a 150-day simulation of perpetual January conditions are used as initial conditions for model sensitivity experiments. Statistical techniques based on predictability theory are employed to estimate the relative importance of the sensitivity experiments.

Results from the simulation comparisons indicate that the total diabatic heating accounts for about one-half of the cyclone's deepening rate, with baroclinic dynamics accounting for the remaining part. The absence of diabatic heating also leads to a systematic error in the position of the cyclone. Surface fluxes of sensible heat are responsible for about one-half of the deepening rate due to diabatic processes, while latent heating due to grid-scale resolvable precipitation in conjunction with surface latent heat flux accounts for most of the remaining half. An increase in the surface drag over the ocean to its larger land value was found to be of comparable importance to both surface sensible heat flux and latent beat release, but was only half as important as the total diabatic heating. These changes in the model were judged to produce highly significant response especially at low levels. The addition of radiative heating, the substitution of a Kuo-type cumulus parameterization scheme for the model's default moist convective adjustment scheme, and a factor of 4 increase in the horizontal diffusion did not produce significant responses.

The case-to-case variability exhibited by the 11-member ensemble is examined. The potential danger in attempting to generalize results from a single case of explosive cyclogenesis as being representative of those for the ensemble average is illustrated.

Full access
Byron A. Boville and David P. Baumhefner

Abstract

The error growth associated with the usual upper boundary formulation (and location) in numerical weather prediction (NWP) and general circulation models (GCMs) is studied. The experimental strategy is to look at the growth of differences between the equilibrated climate simulation of a control simulation and three ensembles of 30 day simulations branching off of the control. The control simulation is a seasonal integration of a medium horizontal-resolution GCM with 30 levels extending from the surface to the upper mesosphere. Each ensemble consists of nine cases with initial conditions taken at 10-day intervals from the control. The main experiment uses a model identical to that of the control except that only the bottom 15 levels (below 10 mb) are retained. The additional experiments either perturb the initial conditions or alter the physical parameterizations (horizontal diffusion) to obtain information on the significance of the results of the main experiment.

It is found that random error growth rate in the troposphere for the case of the altered upper boundary is slightly faster than that for initial-condition uncertainty alone. However, this is not likely to make a significant impact in operational forecast models at present because the uncertainty in the initial conditions is so large.

Systematic errors in the troposphere due to the upper boundary are relatively small prior to day 20. However, the ten day mean errors from days 20–30 are about the same magnitude as the anomalies that extended range forecasts are attempting to predict. The upper boundary treatment is likely to cause significant systematic errors in such forecasts. In the lower stratosphere, the errors are substantial within the first few days, particularly in the winter hemisphere.

Full access