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Thomas W. Bettge

Abstract

The 24 systematic forecast errors in the 50 mb geopotential height from the ECMWF and NMC operational prediction models are compared and analyzed. During the winter of 198 the error patterns from the two models over the Northern Hemisphere were remarkably similar. The error signatures within composites of selected dates, which were chosen according to the character of the flow regime over the Rocky Mountains, were also very similar, indicating that improper orographic forcing may represent one deficiency common to both models. The general similarity of the patterns in each composite, however, does not rule out the existence of yet another important common deficiency.

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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.

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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.

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Thomas W. Bettge
,
David P. Baumhefner
, and
Robert M. Chervin

Readily available forecasts of winter season temperature anomalies for the continental United States are analyzed and compared to the observed anomalies for each of the past five winter seasons. Forecast skill is evaluated by different verification methodologies, and it is shown that a judgment of skill can be dependent on the particular verification technique employed. Verification in terms of principal components is shown to be a useful diagnostic aid, in that it allows for the recognition of naturally occurring temperature anomaly patterns in the atmosphere. Other general issues concerning the current state of seasonal climate forecasting also are discussed as they relate to the question of verification strategies.

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Gerald A. Meehl
,
Warren M. Washington
,
Julie M. Arblaster
,
Thomas W. Bettge
, and
Warren G. Strand Jr.

Abstract

A methodology is formulated to evaluate the possible changes in decadal-timescale (10–20-yr period) surface temperature variability and associated low-frequency fluctuations of anthropogenic forcing and changes in climate base state due to the forcing in simulations of twentieth- and twenty-first-century climate in a global coupled climate model without flux adjustment. The two climate change experiments both start in the year 1900. The first uses greenhouse gas radiative forcing (represented by equivalent CO2) observed during the twentieth century, and extends greenhouse gas forcing to the year 2035 by increasing CO2 1% yr−1 compound after 1990 (CO2-only experiment). The second includes the same greenhouse gas forcing as the first, but adds the effects of time-varying geographic distributions of monthly sulfate aerosol radiative forcing represented by a change in surface albedo (CO2 + sulfates experiment). The climate change experiments are compared with a 135-yr control experiment with no change in external forcing. Climate system responses in the CO2-only and CO2 + sulfates experiments in this particular model are marked not only by greater warming at high latitudes in the winter hemisphere, but also by a global El Niño–like pattern in surface temperature, precipitation, and sea level pressure. This pattern is characterized by a relatively greater increase of SST in the central and eastern equatorial Pacific in comparison with the west, a shift of precipitation maxima from the western Pacific to the central Pacific, mostly decreases of Asian–Australian monsoon strength, lower pressure over the eastern tropical Pacific, deeper midlatitude troughs in the North and South Pacific, and higher pressure over Australasia. Time series analysis of globally averaged temperature and an EOF analysis of surface temperature are consistent with previous results in that enhanced low-frequency variability with periods greater than around 20 yr is introduced into the model coupled climate system with a comparable timescale to the forcing. To examine the possible effects of the associated changes in base state on decadal timescale variability (10–20-yr periods), the surface temperature time series are filtered to retain only variability on that timescale. The El Niño–like pattern of decadal variability seen in the observations is present in each of the model experiments (control, CO2 only, and CO2 + sulfates), but the magnitude decreases significantly in the CO2-only experiment. This decrease is associated with changes in the base-state climate that include a reduction in the magnitude (roughly 5%–20% or more) of wind stress and ocean currents in the upper 100 m in most ocean basins and a weakening of meridional overturning (about 50%) in the Atlantic. These weakened circulation features contribute to decreasing the amplitude of global decadal surface temperature variability as seen in a previous sea-ice sensitivity study with this model. Thus the superposition of low-frequency variability patterns in the radiative forcing increases climate variability for periods comparable to those of the forcing (greater than about 20 yr). However, there are decreases in the amplitude of future decadal (10–20-yr period) variability in these experiments due to changes of the base-state climate as a consequence of increases in that forcing.

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