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Richard D. Rosen
and
David A. Salstein

Abstract

General circulation fields and statistics are derived for the months of January and July 1983 from different sets of daily global analyses produced by NMC. The two sets differ only in that one has been archived prior to an initialization step, whereas the other has been archived after initialization. To place our comparisons of these pre- and post-initialized NMC fields in perspective, we have also calculated circulation statistics from analyses produced by the ECMWF for the same months and from analyses of available rawinsonde data.

As suggested by earlier studies, the adiabatic normal mode initialization procedure used until recently at NMC is found to reduce the strength of the Hadley cell. However, the impact of this initialization is not large. In fact, the difference between the pre- and post-initialized NMC Hadley cells is generally smaller than that between the NMC and ECMWF Hadley cells. Other zonal mean circulation fields appear even less affected by NMC's initialization. Locally, the initialization does noticeably dampen the maxima in the vertical motion field, and it modifies the velocity potential field over certain regions of the globe, particularly in the western Pacific. For the most part, however, the post-initialized NMC fields, which have been the only NMC fields normally available to the scientific community, appear suitable for use in diagnostic studies of the circulation. More effort is needed to understand and reduce the differences between the NMC and ECMWF fields, if possible.

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Richard D. Rosen
and
David A. Salstein

Abstract

The sensitivity of various zonal mean general circulation statistics to the choice of the averaging period used to define them is tested with upper-air data for the Northern Hemisphere taken from the NMC global analysis for the winter of 1976–77. We find that averaging periods of less than about 10 days do not permit a clear separation of total eddy momentum and heat fluxes into their transient and standing eddy components. Between 10 and 30 days, the definition of these components is less sensitive to the specific averaging period chosen during the winter. We illustrate one use of monitoring general circulation statistics on short time scales by studying the evolution of 10-day mean eddy fluxes of sensible heat and their relation to changes in the meridional temperature gradient during this winter. It appears, for this one season at least, that the standing waves regulated the temperature structure of midlatitudes, whereas the transient waves merely responded to the temperature gradient that was imposed.

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Richard D. Rosen
and
David A. Salstein

Abstract

Statistics concerning the budgets of angular momentum heat and water vapor over the Northern Hemisphere are computed by two different methods for the winter of 1976–77. The first method employs an objective analysis scheme applied to the set of conventional upper air sounding obtained from the hemispheric network of rawinsonde stations. The second method uses grid-point values produced daily by the NMC global Hough analysis based on data from several sources. Our results show that the gridded Hough data do not contain mean meridional circulations, thus seriously limiting their usefulness for studies in which these cells play a major role. In addition, the gridded data appear to yield unreasonably large values of water vapor. On the other hand, they produce a realistic temperature structure and seem quite adequate for use in studies of midlatitude waves and their transports. They have also proven much easier to work with than the conventional station data. We find, too, that these station data have their own deficiencies caused largely by gaps in the rawinsonde network, such as those resulting from the loss of several ocean weather ship stations since 1973.

Our study also provides an added appreciation for the highly amplified nature of atmospheric waves during the 1976–77 winter. A strong conversion of kinetic energy from its eddy to zonal mean state and a large standing eddy heat flux are both evident. Additionally, transient eddy momentum fluxes were found to peak at 230 mb, a level not usually included in previous general circulation statistics.

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Richard D. Rosen
,
David A. Salstein
, and
Thomas Nehrkorn

Abstract

Prompted by the need for forecasts of changes in the length of day on 1-10 day time scales, this paper examines the quality of predictions of a proxy variable, namely the atmosphere's relative angular momentum (M) made by the medium-range forecast model (MRF)of the Nationa Meteorological Center during December 1985-November 1989. Skillful forecasts of M relative to persistence am produced by the MRF over its entire 1-10 day range, as found previously. Errors in the MRF are smaller than those of a damped persistence of anomaly empirical model only out to 8 dan however. Moreover, beyond about 3-4 dam MRF tommts of M for day N + 1 show no more skill than forecasts made by simply persisting the MRF prediction for day N, suggesting that significant room for improvement in dynamical forecasts of M still exists.

Errors in the MRF foments of M are separated into their bias and nonsystematic components. Bias errors became especially prominent with the introduction of the most recent version of the MRF examined here, MRF88, whereas random errors in the M forecasts appear not to have been affected by model changes. Both types of errors in the M forecasts can be traced to problems with forecasts of the zonal mean zonal wind, [u], in the tropics. Bias errors in MRF88 forecasts of the globally integrated quantity M are large despite notable reductions in biases in [u] forecasts locally since the MRF was fim introduced. Evidence is offered that the pattern of bias in 10-day forecasts of [u] develops much earlier in the forecast cycle.

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Richard D. Rosen
,
David A. Salstein
, and
JoséP. Peixoto

Abstract

Maps of the streamfunction field for the vertically integrated flux of water vapor over the Northern Hemisphere are presented for each of six annual periods. This approach reveals the existence of a broad-scale cellular pattern in the flow that is not so readily apparent from the more conventional analyses of its zonal and meridional components. In addition, changes in the moisture flow from one year to the next are clearly pictured, a particular good example of which is provided by the behavior of the streamfunction over the Pacific during the period considered.

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Richard D. Rosen
,
David A. Salstein
, and
JoséP. Peixoto

Abstract

Annual mean holds of the zonal and meridional components of the vertically integrated water vapor flux in the atmosphere have been computed objectively from six years of Northern Hemispheric upper air data. Emphasis is placed on the longitudinal structure of these fields in order to identify regional contributions to the annual variations of the zonally averaged moisture fluxes. These temporal changes, particularly those associated with the meridional transport of latent heat. appear large enough to impact significantly on the maintenance of the atmospheric energy balance. While the distribution and quality of some upper air data make it difficult to estimate the degree to which the analyses precisely duplicate atmospheric behavior everywhere, this study provides documentation of a wide range of interannual variability. In this regard, the behavior of one of the years chosen for study contrasts sharply with that of the rest of the sample, as was similarly noted for other quantities evaluated by Rosen et at. (1976).

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Richard D. Rosen
,
Dayid A. Salstein
,
Alvin J. Miller
, and
Klaus Arpe

Abstract

Modern, high resolution atmospheric and geodetic data are finding increasing use in geophysical studies of the angular momentum balance of the earth-atmosphere system. Such studies require knowledge of the level of uncertainty associated with each data set. Here we attempt to assess the accuracy of global atmospheric angular momentum determinations by comparing time series of this quantity derived from both the NMC and ECMWF zonal wind analyses for 1981–85. We find that over this period the rms difference between the two atmospheric series is comparable to the difference between an atmospheric momentum series and the series that is consistent with observed (nontidal) changes in earth rotation.

Most of the difference between the NMC and ECMWF global momentum values comes from the region 33°S–16°N. Differences between NMC and ECMWF regional momentum values become vanishingly small in the high latitudes of the Northern Hemisphere, but they remain large throughout the entire Southern Hemisphere. An annual signal of up to 0.5 m s−1 exists in the difference between the NMC and ECMWF tropospheric zonal wind fields. Although seemingly small, this signal is vertically coherent and leads to a marked annual component in the NMC-ECMWF difference momentum values in the tropics and also for the Northern Hemisphere as a whole.

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Richard D. Rosen
,
David A. Salstein
,
JoséP. Peixoto
,
Abraham H. Oort
, and
Ngar-Cheung Lau

Abstract

A number of Northern Hemisphere circulation fields and statistics are derived for the months of January and June 1979 from level III-b analyses produced by GFDL using a 4-dimensional data assimilation scheme which incorporates measurements from a wide variety of sources. In particular, hemispheric maps and zonal cross sections of the wind, specific humidity, and the eddy fluxes of momentum, heat and moisture are examined. Certain quantities related to the atmosphere's energy cycle are also considered. These fields and statistics are compared with those derived from analyses that rely solely on the conventional rawinsonde station data taken during the same months. In the case of the monthly mean zonal and meridional winds, we also present results based on the level III-b analyses of the ECMWF.

The station-based analyses yield zonal mean statistics and hemispheric integrals that are generally comparable to those from the level III-b analyses. For example, the intensity of the Northern Hemisphere Hadley cell in January produced by the station analyses lies between those of the III-b analyses, which differ by as much as 35%. On regional scales, however, there are some large differences in the circulation fields between the station-based and level III-b analyses over areas of sparse station coverage. For example, the station-based analysis of the 200 mb field of transient eddy momentum flux in January does not include a significant region of northward flux over the northeast Pacific that is contained in the GFDL analysis. It is not yet clear, though, to what extent model biases may be affecting the GFDL analysis in this or in other station-sparse areas. In the case of the subtropical Pacific jet in January, the station-based analysis appears to underestimate its extent, but there are also considerable differences between the two level III-b analyses in this region. In addition, the GFDL analyses often appear to be noisy. Improvements in the level III-b analyses need to be made before full confidence can be placed in results based on modern data assimilation techniques.

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Richard D. Rosen
,
David A. Salstein
,
Thomas Nehrkorn
,
Margaret R.P. McCalla
,
Alvin J. Miller
,
Jean O. Dickey
,
T. Marshall Eubanks
, and
J. Alan Steppe

Abstract

Forecasts of zonal wind fields produced by the medium-range forecast (MRF) model of the National Meteorological Center are used to create predictions of the atmosphere's angular momentum at lead times of 1–10 days. Forecasts of this globally integrated quantity are of interest to geodesists and others concerned with monitoring changes in the earth's orientation for navigational purposes. Based on momentum forecasts archived for the period December 1985–November 1986, we find that, on average, the MRF exhibits positive skill relative to persistence-based forecasts at all lead times. Over our entire one-year study period, the improvement over persistence exceeds 20% for 2–6-day forecasts and remains as large as 10% even for 10-day forecasts. On the other hand, skill scores for the MRF momentum predictions vary considerably from month to month, and for a sizeable fraction of our study period the MRF is less skillful than persistence. Thus, although our initial impression of the overall quality of the MRF momentum forecasts is favorable, further improvement is certainly desirable.

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