Goddard Laboratory For Atmospheres Research Articles

An Assessment of the NCEP, NASA, and ECMWF Reanalyses over the Tropical West Pacific Warm Pool

Three very different views of the mean structure and variability of deep convection over the tropical east Indian and west Pacific Oceans, provided by three different reanalysis datasets for 1980–93, are highlighted. The datasets were generated at the National Centers for Environmental Prediction, the National Aeronautics and Space Administration's Goddard Laboratory for Atmospheres, and the European Centre for Medium-Range Weather Forecasts (ECMWF). Precipitation, outgoing longwave radiation (OLR), and 200-mb wind divergence fields from the three datasets are compared with one another and with satellite observations. Climatological means as well as interannual and intraseasonal (30–70 day) variability are discussed. For brevity the focus is restricted to northern winter (DJF). The internal consistency of the datasets is high, in the sense that the geographical extremes of rainfall, OLR, and divergence in each dataset correspond closely to one another. On the other hand, the external consistency, that is, the agreement between the datasets, is so low as to defy a simple summary. Indeed, the differences are such as to raise fundamental questions concerning 1) whether there is a single or a split ITCZ over the west Pacific Ocean with a strong northern branch, 2) whether there is more convection to the west or the east of Sumatra over the equatorial Indian Ocean, and 3) whether there is a relative minimum of convection near New Guinea. Geographical maps of interannual and intraseasonal variances also show similar order 1 uncertainties, as do regressions against the principal component time series of the Madden–Julian oscillation. The annual cycle of convection is also different in each reanalysis. Overall, the ECMWF reanalysis compares best with observations in this region, but it too has important errors. Finally, it is noted that although 200-mb divergence fields in the three datasets are highly inconsistent with one another, the 200-mb vorticity fields are highly consistent. This reaffirms the relevance of diagnosing divergence from knowledge of the vorticity using the method described in Sardeshmukh (1993). This would yield divergence fields from the three datasets that are not only more consistent with each other, but also more consistent with the 200-mb vorticity balance. Further, as proxies of deep convection, they would help resolve many of the issues raised above.

A comparison of TOVS ocean skin and surface air temperatures with other data sets

A data set of geophysical fields has been processed at the Goddard Laboratory for Atmospheres from Tiros operational vertical sounder (TOVS) high‐resolution infrared sounder 2 (HIRS2) and microwave sounding unit (MSU) radiance observations. The retrieval system is the physically based Pathfinder Path A methodology. The data set currently covers the period 1985–1995. We compare monthly means of TOVS skin temperature Ts and surface air temperature Ta to the Tropical Ocean Global Atmosphere (TOGA) buoy observations, National Center for Environmental Prediction (NCEP) Ts and the Comprehensive Ocean Atmosphere Data Set (COADS) Ts and Ta for the time period 1987–1989. We also compare the Ta data to the European Center for Medium‐Range Weather Forecasts (ECMWF) 2 m temperature. The TOVS Ts has a mean cold bias of ≤0.5 K relative to NCEP Ts, while the Ta has a mean cold bias of ≤1.0 K in the tropics relative to both ECMWF and COADS Ta. The bias in the TOVS Ta is largely due to the diurnal sampling of NOAA 10 observations. The bias in the Ts has additional contributions from a possible cool skin effect as well as retrieval errors associated with cloud clearing and accounting for water vapor attenuation and reflected solar radiation. Because of the holistic retrieval of TOVS Ts and Ta there is a near cancellation of biases due to diurnal sampling and cloud clearing, so that the derived Ts‐Ta have higher accuracy than the individual Ts and Ta and give a more realistic representation of the evolution of known features of the ocean surface circulation than does the COADS. These results indicate that TOVS Ts and Ta may be suitable for climate variability studies involving air‐sea interactions. The data set is available on a daily, 5‐daily, or monthly mean basis with a spatial resolution of 1° by 1°.

On the maintenance and initiation of the intraseasonal oscillation in the NCEP/NCAR reanalysis and in the GLA and UKMO AMIP simulations

In this study, satellite-derived outgoing longwave radiation (OLR) and the reanalysis from the National Centers for Environmental Prediction/National Center for Atmospheric Research are used as verification data in a study of intraseasonal variability in the Goddard Laboratory for Atmospheres (GLA) and the United Kingdom Meteorological Office (UKMO) atmospheric general circulation models. These models simulated the most realistic intraseasonal oscillations (IO) of the 15 Atmospheric Model Intercomparison Project models previously analyzed. During the active phase of the intraseasonal oscillation, convection is observed to migrate from the Indian Ocean to the western/central Pacific Ocean, and into the South Pacific Convergence Zone (SPCZ). The simulated convection, particularly in the GLA model, is most realistic over the western/central Pacific Ocean and the SPCZ. In the reanalysis, the baroclinic structure of the IO is evident in the eddy-stream function, and eastward migration of the anticyclone/cyclone pairs occurs in conjunction with the eastward development of convection. Both the GLA and UKMO models exhibit a baroclinic structure on intraseasonal time scales. The GLA model is more realistic than the UKMO model at simulating the eastward migration of the anticyclone/cyclone pairs when the convection is active over the western/central Pacific. In the UKMO model, the main heating is located off the equator, which contributes to the irregular structures seen in this model on intraseasonal time scales. The maintenance and initiation of the intraseasonal oscillation has also been investigated. Analysis of the latent heat flux indicates that evaporative wind feedback is not the dominant mechanism for promoting the eastward propagation of the intraseasonal oscillation since evaporation to the west of the convection dominants. The data suggest a wave-CISK (conditional instability of the secondkind) type mechanism, although the contribution by frictional convergence is not apparent. In the GLA model, enhanced evaporation tends to develop in-place over the west Pacific warm pool, while in the UKMO simulation westward propagation of enhanced evaporation is evident. It is suggested that lack of an interactive ocean may be associated with the models systematic failure to simulate the eastward transition of convection from the Indian Ocean into the western Pacific Ocean. This hypothesis is based upon the examination of observed sea surface temperature (SST) and its relationship to the active phase of the intraseasonal oscillation, which indicates that the IO may evolve as a coupled ocean-atmosphere mode. The eastward propagation of convection appears to be related to the gradient of SST, with above normal SST to the east of the convection maintaining the eastward evolution, and decreasing SST near the western portion of the convective envelope being associated with the cessation of convection.

Comparison and Satellite Assessment of NASA/DAO and NCEP–NCAR Reanalyses over Tropical Ocean: Atmospheric Hydrology and Radiation

Abstract This study compares the atmospheric reanalyses that have been produced independently at the Data Assimilation Office (DAO) of Goddard Laboratory for Atmospheres and at the National Centers for Environmental Prediction (NCEP). These reanalyses were produced by using a frozen state-of-the-art version of the global data assimilation system developed at these two centers. For the period 1987–88 and for the tropical oceanic regions of 30°S–30°N, surface and atmospheric fields related to atmospheric hydrology and radiation are compared and assessed, wherever possible, with satellite data. Some common biases as well as discrepancies between the two independent reassimilation products are highlighted. Considering both annual averages and interannual variability (1987–88), discrepancies between DAO and NCEP reanalysis in water vapor, precipitation, and clear-sky longwave radiation at the top of the atmosphere are generally smaller than discrepancies that exist between corresponding satellite estimates. Am...

Simulation Errors Associated with the Neglect of Oceanic Salinity in an Atmospheric GCM

Abstract In all the atmospheric general circulation models (GCMs) at the Goddard Laboratory for Atmospheres (GLA), the influence of oceanic salinity on the saturation vapor pressure of seawater is ignored. Since the relative humidity in the oceanic boundary layer is generally high while the saturation vapor pressure of seawater is lowered by salinity, its neglect could have a nontrivial influence on the near-surface specific humidity gradient, a primary determinant of oceanic evaporation. Such an approximation might effect the simulated circulation and rainfall systematically. To evaluate this idea, we carried out a 5-yr-long salinity simulation (S) with the GLA GCM in which the influence of salinity on the saturation vapor pressure of seawater was included. Corresponding to this, a control simulation (C) with the GLA GCM in which the salinity effect was ignored was also available. Analyses of S-minus-C fields have shown some evidence of discernible systematic errors in the global evaporation, boundary la...

Simulation Errors Associated with the Neglect of Oceanic Salinity in an Atmospheric GCM

Abstract In all the atmospheric general circulation models (GCMs) at the Goddard Laboratory for Atmospheres (GLA), the influence of oceanic salinity on the saturation vapor pressure of seawater is ignored. Since the relative humidity in the oceanic boundary layer is generally high while the saturation vapor pressure of seawater is lowered by salinity, its neglect could have a nontrivial influence on the near-surface specific humidity gradient, a primary determinant of oceanic evaporation. Such an approximation might effect the simulated circulation and rainfall systematically. To evaluate this idea, we carried out a 5-yr-long salinity simulation (S) with the GLA GCM in which the influence of salinity on the saturation vapor pressure of seawater was included. Corresponding to this, a control simulation (C) with the GLA GCM in which the salinity effect was ignored was also available. Analyses of S-minus-C fields have shown some evidence of discernible systematic errors in the global evaporation, boundary la...

Interannual variation of atmospheric mass and the southern oscillation

Two reanalysis datasets, one generated by the Goddard Laboratory for Atmospheres for 1982—1993 and the other generated by the National Centers for Environmental Prediction for 1982—1995, are used to examine the relationship between the Southern Oscillation (SO) and the interannual variation of atmospheric mass. Both reanalyses show that atmospheric mass increases (decreases) during the positive (negative) SO phase. Atmospheric mass consists of dry air and moisture. Since dry mass is conserved, the interannual variation of atmospheric mass results from the variation of water vapor pressure. Thus, global atmospheric hydrological processes are analyzed to illustrate how the SO affects the interannual variation of atmospheric mass. During the positive (negative) SO phase, water vapor is converged (diverged) toward (out of) the central-eastern tropical Pacific [where sea surface temperatures (SSTs) are higher (lower) than normal] to maintain (suppress) cumulus convection in that area. An anomalous east-west Walker circulation straddling the Dateline is driven by the anomalous cumulus convection in this region to create positive (negative) surface pressure anomalies over the western tropical Pacific-Indian Ocean, which result in an increase (decrease) in atmospheric mass.

A complementary depiction of the interannual variation of atmospheric circulation associated with ENSO events: Research note

Abstract A simple diagnostic scheme, which combines a low‐pass temporal filter (with an 18‐month cutoff time) with a regular empirical orthogonal function (EOF) analysis, is used to delineate the synchronous evolution of El Nino‐Southern Oscillation‐related (ENSO‐related) modes in various variables of the ocean‐atmosphere system. Based on the causal relation chain of diabatic heating, divergent circulation and rotational flow, the diagnostic scheme extracts ENSO modes from the following data sources: the Pacific sea surface temperature (SST), the past 14‐years (1979–1992) of data generated by the Global Data Assimilation System of the National Meteorological Center, and a 10‐year (1979–1988) general circulation model climate simulation made at the Goddard Laboratory for Atmospheres. The analysis reveals the following: (a) the eigencoefficient time series of the first eigenmodes of selected filtered variables, which explain about 40–50% of their total variance, synchronize with the filtered SST averaged over Area NINO‐3; (b) the spatial structures of the first eigenmodes resemble the ensemble departures associated with ENSO events of these variables from their long term means; and (c) the results show that the proposed scheme can be easily applied to isolate and illustrate the time evolution of ENSO modes which exist in the long term observational database as well as in climate simulations.

Impact of orographically induced gravity‐wave drag in the GLA GCM

AbstractThe influence of gravity‐wave drag (GWD) parametrization has been investigated in the Goddard Laboratory for Atmospheres general‐circulation model with the resolution of 4° latitude × 5° longitude in the horizontal and 17 sigma layers in the vertical. The simulations showed several overall beneficial effects in the extended‐range forecast as well as the seasonal simulation for both winter and summer conditions. The key results include the following: The mid‐latitude westerlies, which were shifted poleward (equatorward) in northern (southern) winter, have been corrected. Consequently, the systematic bias of low (high) sea level pressure over the north (south) polar region has been largely eliminated. There is a significant improvement in the simulation of the winter hemisphere stationary waves; this can be attributed to the GWD induced Rossby‐wave propagation. Over the northern Pacific region, as the storm track is simulated better, precipitation distribution improves remarkably. In cooperation with the improved regional zonal‐mean flow, an adjustment of the mean meridional circulation is found over the Asian summer‐monsoon region, leading to an improvement in the precipitation distribution. Spurious absorption of gravity waves at the model top (10 mb) has been removed by applying an open upper‐boundary condition. The vertical propagation of the tropospherical gravity wave into the upper stratosphere and mesosphere over mid‐latitudes of the northern hemisphere in winter is consistent with observational studies as well as the middle‐atmospheric dynamics.

Variability of the global precipitable water with a timescale of 90–150 days

A 90–150 day signal is identified in the global precipitable water field generated by the Global Data Assimilation Systems (GDAS) of the Goddard Laboratory for Atmospheres (GLA), National Meteorological Center, and European Centre for Medium‐Range Weather Forecasts. The finding of this intraseasonal signal in global precipitable water is significant for two reasons: (1) it suggests that there is 90–150 day intraseasonal variability in the atmospheric branch of the global hydrological cycle and (2) it provides a useful parameter to test the sensitivity of the GDAS‐generated hydrological data. This newly identified intraseasonal signal in the global precipitable water was verified with Special Sensor Microwave/Imager precipitable water data over oceans and station‐mixing ratio data over the continental United States. Based upon some simple statistical analyses and global and regional composite charts, it was found that the 90–150 day low‐frequency oscillations contained in different GDAS data sets are more coherent with each other in regions with good data coverage but are poorly correlated over the data‐sparse areas. Furthermore, the GLA GDAS provides the most realistic representation of this intraseasonal global precipitable water signal.

Annual variation of the global precipitable water and its maintenance: observation and climate-simulation

The annual variation of the global-mean precipitable water 〈W〉and the associated hydrological cycle were analyzed with the upper-air data generated by the Global Data Assimilation System of the National Meteorological Center for 1981–1991 and the European Centre for Medium Range Weather Forecasts from 1983–1991. It was found that the annual variation of 〈W〉 coincides with that of the Northern Hemisphere precipitable water [W]NH. The hemispheric-mean ([ ]) water budget analysis shows that water vapor is transported from the winter to the summer hemisphere across the equator by the Hadley circulation, and that the annual variations in the water vapor sink [P – E] for both hemispheres also follow the same seasonal march. The amplitudes of the annual variations in these two hydrological processes are comparable in both hemispheres. Thus, the annual variations of [W]NH and [W]SH are the result of slight imbalances between the cross-equator water vapor transport and the water vapor sink, particularly in the spring and fall. The climatological hemispheric-mean water budgets reveal that the Southern Hemisphere is a water vapor source and the Northern Hemisphere is a water vapor sink. The cross-equator water vapor transport constitutes a major source acting to maintain [W]NH, and in turn 〈W〉. The hydrological mechanism maintaining the observed 〈W〉 annual variation is consistent with that obtained from the hydrological cycle in a 10-year (1979–1988) climate simulation done at the Goddard Laboratory for Atmospheres as part of their participation in the Atmospheric Model Intercomparison Project (AMIP). DOI: 10.1034/j.1600-0870.1996.00001.x

Low-Frequency Variations in the Atmospheric Branch of the Global Hydrological Cycle

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 [P̂] 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 [P̂] and [∇ · Q̂] vary annually in a coherent way in each hemisphere so that water vapor diverges from the winter hemisphere, where [P̂] 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 〈P̃〉, evaporation 〈Ẽ〉, 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 〈P̃〉 varies coherently in opposite phase for the two hemispheres, while 〈∇ · Q̃ 〉 varies so that water vapor diverges from the hemisphere of maximum 〈P̃〉 to the hemisphere of minimum 〈P〉. Intraseasonal variations of 〈P̃〉, 〈Ẽ〉 and 〈∇ · 〉 are in accord with the eastward propagation of the intra seasonal global divergent circulation.

A Class of the van Leer-type Transport Schemes and Its Application to the Moisture Transport in a General Circulation Model

A generalized form of the second-order van Leer transport scheme is derived. Several constraints to the implied subgrid linear distribution are discussed. A very simple positive-definite scheme can be derived directly from the generalized form. A monotonic version of the scheme is applied to the Goddard Laboratory for Atmospheres (GLA) general circulation model (GCM) for the moisture transport calculations, replacing the original fourth-order center-differencing scheme. Comparisons with the original scheme are made in idealized tests as well as in a summer climate simulation using the full GLA GCM. A distinct advantage of the monotonic transport scheme is its ability to transport sharp gradients without producing spurious oscillations and unphysical negative mixing ratio. Within the context of low-resolution climate simulations, the aforementioned characteristics are demonstrated to be very beneficial in regions where cumulus convection is active. The model-produced precipitation pattern using the new transport scheme is more coherently organized both in time and in space, and correlates better with observations. The side effect of the filling algorithm used in conjunction with the original scheme is also discussed, in the context of idealized tests. The major weakness of the proposed transport scheme with a local monotonic constraint is its substantial implicit diffusion at low resolution. Alternative constraints are discussed to counter this problem.

Climatology and natural variability of the global hydrologic cycle in the GLA atmospheric general circulation model

Time average climatology and low‐frequency variabilities of the global hydrologic cycle (GHC) in the Goddard Laboratory for Atmospheres (GLA) general circulation model (GCM) were investigated in the present work. A 730‐day experiment was conducted with the GLA GCM forced by insolation, sea surface temperature, and ice‐snow undergoing climatological annual cycles. Influences of interactive soil moisture on time average climatology and natural variability of the GHC were also investigated by conducting 365‐day experiments with and without interactive soil moisture. Insolation, sea surface temperature, and ice‐snow were fixed at their July levels in the latter two experiments. Results show that the model's time average hydrologic cycle variables for July in all three experiments agree reasonably well with observations. Except in the case of precipitable water, the zonal average climates of the annual cycle experiment and the two perpetual July experiments are alike, i.e., their differences are within limits of the natural variability of the model's climate. Statistics of various components of the GHC, i.e., water vapor, evaporation, and precipitation, are significantly affected by the presence of interactive soil moisture. Even with fixed external conditions, the GHC exhibits a variety of low‐frequency natural variabilities. A long‐term trend is found in the principal empirical modes of variability of ground wetness, evaporation, and sensible heat. Dominant modes of variability of these quantities over land are physically consistent with one another and with land surface energy balance requirements. The dominant empirical modes of ground wetness variability in the annual cycle and perpetual July experiments have similar spatial patterns. The dominant mode of precipitation variability is found to be closely related to organized convection over the tropical western Pacific Ocean. The precipitation variability has timescales in the range of 2 to 3 months and can be identified with the stationary component of the Madden‐Julian Oscillation. The precipitation mode is not sensitive to the presence of interactive soil moisture but is closely linked to both the rotational and divergent components of atmospheric moisture transport. The present results indicate that globally coherent natural variability of the GHC in the GLA GCM has two basic timescales in the absence of annual cycles of external forcings: a long‐term trend associated with atmosphere‐soil moisture interaction which affects the model atmosphere mostly over midlatitude continental regions and a large‐scale 2‐ to 3‐month variability associated with atmospheric moist processes over the western Pacific Ocean.

The Effect of SST and Soil Moisture Anomalies on GLA Model Simulations of the 1988 U.S. Summer Drought

A series of simulations of the late spring and early summer of 1988 were conducted in order to study the relative importance of different boundary forcings to the Goddard Laboratory for Atmospheres (GLA) model's simulation of the heat wave and drought over the Great Plains of the United States during this time period. Separate 60-day simulations were generated from 10, 20, and 30 May 1988 with a variety of boundary condition datasets. For the control experiment, climatological boundary conditions were used. This was followed by experiments in which either the observed 1988 sea surface temperatures (SST) or derived 1988 soil moisture values, or both, were used in place of the climatological fields. Additional experiments were conducted in which only tropical or midlatitude SST anomalies were used. The impact of the different boundary forcings was evaluated relative to the control simulations of the precipitation and surface air temperature over the Great Plains. It was found that the tropical SST anomalies had a significant effect in reducing precipitation in this area, while the midlatitude anomalies did not. Due to the prescribed climatological soil moistures for the SST experiments, a significant increase in surface temperature did not occur in these simulations. In contrast, the simulations with the anomalous 1988 soil moistures produced both a larger reduction of precipitation and a significant increase in surface temperature over the Great Plains. The simulations with both anomalous SST and soil moisture showed only a slight augmentation of the heat wave and drought relative to the experiments with anomalous soil moisture alone.

The 10–20-Day Mode of the 1979 Indian Monsoon: Its Relation with the Time Variation of Monsoon Rainfall

Abstract The synoptic structure of the 10–20-day monsoon mode and this intraseasonal monsoon mode's relationship with the Indian monsoon rainfall are examined with the 1979 summer First GARP Global Experiment IIIb data of the European Centre for Medium-Range Weather Forecasts and the daily 1° × 1° rainfall estimates retrieved from the satellite data by the Goddard Laboratory for Atmospheres. The major findings of this study are as follows. 1) The 10–20-day monsoon mode exhibits a double-cell (either double-high or double-low) structure; one cell is centered at about 15°–20°N and the other at the equator. 2) Both cells of the 10–20-day monsoon mode propagate coherently westward along the Indian monsoon trough and along the equator, respectively. 3) Based upon the zonal wind and local Hadley circulation, the vertical structure of the 10–20-day monsoon mode does not exhibit a phase change. 4) A significant rainfall occurs around low centers of the 10–20-day monsoon mode through the modulation of this monsoon...

The vertical structure of diabatic heating associated with the Madden‐Julian oscillation simulated by the Goddard Laboratory for Atmospheres climate model

Previous studies have shown that numerical simulations of the Madden‐Julian oscillation (MJO) are very sensitive to the vertical distribution of diabatic heating. Since atmospheric diabatic heating is generally difficult to estimate, the vertical diabatic heating profile associated with the MJO is not well known. Judged by its propagation properties and spatial structure, the MJO is reasonably well simulated by the nine‐layer Goddard Laboratory for Atmospheres (GLA) general circulation model. Although only a simulation the model MJO may provide an indication of the vertical diabatic heating profile associated with the real oscillation. The diabatic heating structure of the model MJO is illustrated with composite charts made for those times when this low‐frequency mode reaches its maximum and minimum amplitudes. These composite charts compare the vertically integrated diabatic heating with potential functions, the vertical distribution of diabatic heating with the east‐west mass flux function in the tropics, and the vertical profiles of diabatic heating at the centers of maximum and minimum MJO amplitude. Three interesting features of the model MJO's diabatic heating are revealed: (1) the maximum heating rate of this low‐frequency mode is located over the Asian monsoon region and its amplitude is about a half of the maximum value of the seasonal mean heating rate in this region, (2) the vertical diabatic heating rate profile has a maximum at 500 mbar and resembles the seasonal mean total heating profile, and (3) the total diabatic heating is for the most part composed of the latent heat released by cumulus convection.

Comparison of Ocean Surface Solar Irradiance in the GLA General Circulation Model and Satellite-based Calculations

A global, 7-year satellite-based record of ocean surface solar irradiance (SSI) is used to assess the realism of ocean SSI simulated by the nine-layer Goddard Laboratory for Atmospheres (GLA) General Circulation Model (GCM). January and July climatologies of net SSI produced by the model are compared with corresponding satellite climatologies for the world oceans between 54 deg N and 54 deg S. This comparison of climatologies indicates areas of strengths and weaknesses in the GCM treatment of cloud-radiation interactions, the major source of model uncertainty. Realism of ocean SSI is also important for applications such as incorporating the GLA GCM into a coupled ocean-atmosphere GCM. The results show that the GLA GCM simulates too much SSI in the extratropics and too little in the tropics, especially in the summer hemisphere. These discrepancies reach magnitudes of 60 W/sq m and more. The discrepancies are particularly large in the July case off the western coast of North America. Positive and negative discrepancies in SSI are shown to be consistent with discrepancies in planetary albedo.

On the Atmospheric Branch of the Hydrological Cycle

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.

Line‐by‐line calculations of atmospheric fluxes and cooling rates: Application to water vapor

A model for the accelerated calculation of clear sky fluxes based on the line‐by‐line radiance model FASCODE has been developed and applied to the calculation of cooling rates for atmospheric water vapor. The model achieves computational accuracies for the longwave upwelling and downwelling fluxes of the order of 0.2%, an accuracy well within current limitations imposed by uncertainties in the spectral parameters, the line shape, and the associated continua. For the same treatment of line shape, the Voigt profile with a 10 cm−1 cutoff and no continuum, the results from the present model are in acceptable agreement with those from two other line‐by‐line models reported as part of the intercomparison of radiation codes used in climate models (ICRCCM). For this line profile and the mid‐latitude summer atmosphere, the largest difference between the results from our model and the Goddard Laboratory for Atmospheres (GLA) model occurs for the downwelling flux at the surface, with the present model providing a value greater than that from GLA. The differences are generally consistent with greater atmospheric opacity from the present model, attributable to the inclusion of a self‐broadening component for the half width for water and to finer spectral sampling in the lower‐pressure regime. Utilization of the line shape and associated continuum model included in FASCODE gives results that are significantly different from those provided by the other two models. This radiance model, including contributions from the foreign continuum as well as from a modified self‐continuum, has received extensive validation against measured radiance spectra, an example of which is provided. For the mid‐latitude summer atmosphere the principal contribution from the foreign continuum occurs in the upper troposphere in the 250–350 cm−1 spectral region, whereas the contribution from the self‐continuum, dependent on the square of the water vapor density, is greatest in the lower troposphere. For the mid‐latitude summer atmosphere the foreign continuum contributes 0.4 Kd−1 or 20% to the cooling in the upper troposphere and the self‐continuum contributes 1.9 K d−1 to the cooling rate at the surface due to water vapor. The latter is 0.17 K d−1 less than the cooling rate from the GLA model which is principally due to a modification of the self‐continuum. A significant result that has developed from the present work is the insight into atmospheric radiative processes provided by spectral profiles of the cooling rate. In the spectral domain there exists a mapping between the altitude and the molecular absorption strength as weighted by the Planck function. The extremely high correlation between the outgoing spectral radiance at the top of the atmosphere and the spectral cooling rate profile suggests that measurement of the outgoing spectral radiance can provide important information about atmospheric state that is not available from spectrally integrated quantities. Our results also indicate the critical importance of the spectral region from 100 to 600 cm−1 for the radiative transfer associated with atmospheric water vapor.