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Tsing-Chang Chen and James Pfaendtner

Abstract

Recently, HIRS2/MSU data have been used at the Goddard Laboratory for Atmospheres(GLA) to generate global precipitation estimates. A synergistic mix of the GLA precipitation, together with the global wind and moisture fields produced by the Global Data Assimilation System of the European Centre for Medium-Range Weather Forecasts, was employed to delineate the atmospheric branch of the hydrological cycle during the 1978/79 Northern Hemisphere winter and the 1979 Northern Hemisphere summer. The transport of water vapor from source to sink regions was illustrated geographically by a combination of the divergent component of water vapor transport and precipitation.

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David V. Ledvina and James Pfaendtner

Abstract

At this time, most current data assimilation systems use dewpoint depression data, converted to an appropriate moisture variable (relative humidity or mixing ratio), provided by rawinsondes as the lone source of moisture information. Because of the poor spatial and temporal characteristics of this data, additional moisture data are necessary to better resolve the global moisture field. This study investigates the impact of using the Special Sensor Microwave/Imager (SSM/I) total precipitable water (TPW) estimates as an additional source of moisture information.

One forecast and four data assimilation experiments were performed to determine the impact of assimilating SSM/I TPW estimates into the NASA/Goddard Earth Observing System (version 1 ) Data Assimilation System (GEOS-1 DAS). It is shown that assimilation of SSM/I TPW estimates improves the precipitation pattern in the Tropics. In addition, a known dry bias in the GEOS-1 DAS was reduced by over 50% and observation minus first guess (OF) error variance is reduced by nearly 50% after only 3 days of assimilation. Improvements were also noted in monthly and 6-h-averaged precipitation patterns when compared to other independent estimates.

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Meta E. Sienkiewicz and James Pfaendtner

Abstract

Ensembles of assimilation runs were used to assess the sensitivity of the GEOS-1 (Goddard Earth Observing System—Version 1) data assimilation system to data gaps and changes in initial conditions. Perturbations from a “control” assimilation were induced by withholding data for periods ranging from 12 to 96 h. Data assimilation then proceeded with each ensemble member for periods up to one month, and ensemble members (“assimilations”) were examined for convergence to the control assimilation.

Experimental results show that this method is effective in identifying assimilation system weaknesses by determining where assimilations do not converge quickly. The methodology is also useful for determining assimilation “spinup” time. For the GEOS-1 system, convergence of the assimilation ensemble was slow near the poles and in the Southern Hemisphere. This slow convergence was largely due to the sparseness of data in the Southern Hemisphere and to strong polar filtering. The differences between assimilations were primarily differences in the location or intensity of small-scale waves in the larger-scale flow, which tracked eastward with the movement of the small-scale waves. A “fixed” quality control experiment showed that differences in quality control decisions contributed to maintaining differences between the ensemble members. The assimilation convergence was improved when a later version of the GEOS system, with weaker polar filtering, was used.

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Kingtse C. Mo, James Pfaendtner, and Eugenia Kalnay

Abstract

General Circulation Model (GCM) experiments have been performed to determine mechanisms that maintained the blocking episode in the Australian-New Zealand region during the period 8–22 June 1982. A control forecast reproduces the persistent ridge. Several mechanistic experiments lead to the following conclusions. (i) The block was not due to orographic forcing, which has only a small local influence on the winter atmospheric circulation in the Southern Hemisphere. (ii) The block was not produced by the sea surface temperature anomalies (SST). By comparing the relative location of low-level atmospheric vorticity and SST anomalies, we are able to show that during June 1982 the atmospheric blocking was the cause of the SST anomalies in the Pacific. (iii) The block was not a response to tropical heating or the Asian Monsoon. There are only weak effects on the block when the tropical heating or heating in the Pacific region is suppressed. (iv) The most important boundary forcing maintaining this blocking ridge is heating associated with the land-sea contrast. The height fields are more zonally symmetric when the land-sea contrast is suppressed. The local land-sea contrast in the Australian region also contributed to maintain the stationary blocking ridge. The sensible heat release in the subantarctic region is an important mechanism that maintains the block. (v) Finally, the daily spectral energetics of the control experiment suggests that the baroclinic amplification of planetary-scale waves forced by synoptic-scale disturbances played an important role in the evolution of this blocking process.

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Tsing-Chang Chen, Jau-Ming Chen, and James Pfaendtner

Abstract

Systematic prediction errors of the Goddard Laboratory for Atmospheres (GLA) forecast system are reduced when the higher-resolution (2° × 2.5°) model version is used. Based on a budget analysis of the 200-mb eddy streamfunction, the improvement of stationary eddy forecasting is seen to be caused by the following mechanism: By increasing the horizontal spatial resolution of the forecast model, atmospheric diabatic heating over the three tropical continents is changed in a way that intensifies the planetary-scale divergent circulations associated with the three pairs of divergent-convergent centers over these continents. The intensified divergent circulation results in an enhancement of vorticity sources in the Northern Hemisphere. The additional vorticity is advected eastward by a stationary wave train along 30°N, thereby reducing systematic errors in the lower-resolution (4° × 5°) GLA forecast model.

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Tsing-Chang Chen, Jau-Ming Chen, James Pfaendtner, and Joel Susskind

Abstract

Global precipitation estimates derived from satellite data at the Goddard Laboratory for Atmospheres for 1979–80 were used to explore time variations in global precipitation. Time series of the area-averaged precipitation [P] over the Asian-Australian (AA) monsoon (60°E–12°W), and the extra-AA monsoon (120°W–60°E) hemispheres were used in describing the variations. A distinct seesawlike intraseasonal variation of precipitation between these two hemispheres emerges from the two time series. Two intraseasonal (30–60 and 12–24 day) modes stand our in die spectral analysis of the two [P] time series. The 30–60 day mode is well known, while the 12–24-day mode is identified here for the first time. Using data generated by the Global Data Assimilation System of the National Meteorological Center, an effort was made to investigate the characteristics of the 12–24-day mode of global precipitation via potential functions for the 200-mb wind, water vapour transport, and precipitation. It is found that the 12–24-day mode exhibits a wavenumber 1 structure and propagates eastward. The seesaw intraseasonal variation of precipitation between the AA and extra-AA monsoon hemispheres is caused not only by the 30–60-day mode but also by the 12–24-day mode.

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Tsing-Chang Chen, Jau-Ming Chen, and James Pfaendtner

Abstract

According to the atmospheric water balance equation, the divergence of the water vapor flux is responsible for the exchange of water vapor between its source and sink regions. Because the water vapor flux divergence is primarily determined by the divergent circulation, time variations of the global hydrological cycle reflect the pronounced low-frequency modes of the global divergent circulation, that is, the annual and intraseasonal (30–60 day) modes. The annual variation of the hydrological cycle is illustrated in terms of hemispheric-mean hydrological variables for the Northern and Southern Hemispheres, while the intraseasonal variations of the global hydrological cycle are illustrated with mean values over two hemispheres that form an east-west partition of the globe. This partition is defined by the 60°E–120°W great circle and was chosen so that the mean precipitation difference and the divergent water vapor transport between the two hemispheres was maximized.

Two years (1979–80) of daily precipitation estimates from the Goddard Laboratory for Atmospheres and 14 years (1979–92) of upper-air data generated by the Global Data Assimilation System at the National Meteorological Center are used in making quantitative estimates of the annual and intraseasonal variations in the global hydrological cycle. The annual variations in hemispheric-mean precipitation [] and water vapor flux divergence [∇ · Q̂ ] for the Northern and Southern Hemispheres are comparable with amplitudes of about 0.5 ∼ 0.7 mm day. Both [] and [∇ · Q̂] vary annually in a coherent way in each hemisphere so that water vapor diverges from the winter hemisphere, where [] reaches its minimum, to the summer hemisphere, where [P^] attains its maximum. In fact, the hemispheric-mean divergence of water vapor flux changes sign during the annual cycle. Intraseasonal variations of hemispheric-mean precipitation 〈〉, evaporation 〈〉, and water vapor flux divergence 〈∇ · Q̃〉 in the two hemispheres in the cast-west direction are comparable with amplitudes of about 0.1 ∼ 0.2 mm day−1, although amplitudes in some cases exceed 0.3 mm day−1. Hemispheric-mean precipitation 〈〉 varies coherently in opposite phase for the two hemispheres, while 〈∇ · Q̃ 〉 varies so that water vapor diverges from the hemisphere of maximum 〈〉 to the hemisphere of minimum 〈P〉. Intraseasonal variations of 〈〉, 〈〉 and 〈∇ · 〉 are in accord with the eastward propagation of the intra seasonal global divergent circulation.

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Tsing-Chang Chen, James Pfaendtner, and Su-Ping Weng

Abstract

The balance equations for atmospheric water vapor and freshwater in the oceans are used to illustrate that evaporation and precipitation represent the major linkage between the atmospheric and oceanic branches of the global hydrological cycle. Attempts are also made to establish the hydrological cycle of the coupled ocean-atmosphere system using water vapor flux divergence, precipitation, and interoceanic freshwater transport estimates from previous studies as well as computational results with more recent data. This hydrological cycle works as follows: the atmospheric water vapor converged to the Pacific results in an excess of precipitation and a freshwater transport across the Bering Strait and the Arctic Sea to the Atlantic. The atmospheric water vapor is then diverged out of the latter ocean due to an excess of evaporation there.

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Siegfried D. Schubert, Richard B. Rood, and James Pfaendtner

The Data Assimilation Office at NASA's Goddard Space Flight Center is currently producing a multiyear gridded global atmospheric dataset for use in climate research, including tropospheric chemistry applications. The data, which are being made available to the scientific community, are well suited for climate research since they are produced by a fixed assimilation system designed to minimize the spinup in the hydrological cycle. By using a nonvarying system, the variability due to algorithm change is eliminated and geophysical variability can be more confidently isolated.

The analysis incorporates rawinsonde reports, satellite retrievals of geopotential thickness, cloud-motion winds, and aircraft, ship, and rocketsonde reports. At the lower boundary, the assimilating atmospheric general circulation model is constrained by the observed sea surface temperature and soil moisture derived from observed surface air temperature and precipitation fields. The available output data include all prognostic variables and a large number of diagnostic quantities such as heating rates, precipitation, surface fluxes, cloud fraction, and the height of the planetary boundary layer. These variables were chosen to assure a complete budget of the energy and moisture cycles. The assimilated data should also be useful for estimating transport by cumulus processes. The analysis increments (observation minus first guess) and the estimated analysis errors are provided to help the user assess the quality of the data. All quantities are made available every 6 h at the full resolution of the assimilating general circulation model. Selected surface quantities are made available every 3 h.

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