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NASA NTRS1989

Dehydration in the lower Antarctic stratosphere during late winter and early spring, 1987

The history of minimum temperatures at 50 and 70 mb is examined from NMC, UK Met O and ECMWF analyses. MSU channel 24 data are similarly inspected. South Pole sonde data are used to calculate saturation humidity mixing ratio as a function of altitude and time throughout 1987. Saturation with respect to ice could be maintained for water mixing ratios of 3.5 ppmv for a period of about 80 days from mid-June to mid-September. Dehydration to mixing ratios of 1 ppmv or less was possible sporadically. Data from the ER-2 flights between 53 S and 72 S are used in conjunction with particle size measurements and air parcel trajectories to demonstrate the dehydration occurring over Antarctica. Water mixing ratios at the latitude of Punta Arens (53 S), in conjunction with tracer measurements and trajectory analysis, show that at potential temperatures from about 325 to 400 K, the dryness (less than 3 ppmv) had its origin over Antarctica rather than in the tropics. Water mixing ratios within the Antarctic vortex varied from 1.5 to 3.8 ppmv, with a strong isentropic gradient being evident in the region of high potential vorticity gradients.

Kelly, K. K.↗

NASA NTRS1990

An intercomparison of intraseasonal variability in general circulation models and observations

Low frequency oscillations appearing in three GCM seasonal cycle integrations are compared with the analyses of the European Center for Medium Range Weather Forecasting (ECMWF). All three models have the same resolution: 4 deg latitude by 5 deg longitude, with 9 levels. The dominant phase speeds and the differential vertical structure of the heating profiles in the GCMs are in general agreement with current theory involving the positive feedback between latent heating and moist static stability. All three GCMs fail to capture the detailed evolution in the different stages of the development and decay of the oscillation. The results suggest that an improvement in the boundary layer moisture processes may be crucial for a better simulation of the oscillation.

Park, Chung-Kyu↗

NASA NTRS1990

The wet tropospheric range correction - Product intercomparisons and the simulated effect for tropical Pacific altimeter retrievals

Four data sets used to correct satellite altimetry for the effects of tropospheric water vapor over the tropical Pacific Ocean are evaluated statistically, with a focus on the period 1979-1983. Satellite altimeter measurements are simulated by degrading model sea-level solutions with selected water-vapor data, and seasonal and interannual variations are investigated. The results are presented in extensive contour maps and characterized in detail. The standard deviation between two satellite-based data sets (Nimbus-7 SMMR and TOVS) was found to be 2-4 cm, while both the rms difference between seasonal cycles of the two sets and the difference between the mean fields were 1-2 cm. Larger differences (4-8 cm) were found between these data sets and those based on operational analyses (ECMWF and Fleet Numerical Oceanography Center). The maximum interannual anomaly in the water-vapor correction was 12 cm, for the eastern Pacific at the peak of the 1982-1983 ENSO event.

Zimbelman, Darrell F.↗

NASA NTRS1989

The role of diabatic heating in maintaining the upper-tropospheric baroclinic zone in the South Pacific

The four-dimensional structure of the region in the South Pacific convergence zone (SPCZ) during January 10-18, 1979 is studied using a modified set of ECMWF FGGE level III-b analyses. The effects of kinematic and thermodynamic processes on maintaining the upper baroclinic region corresponding with the subtropical jet are analyzed. The role of adiabatic and diabatic processes in the maintenance of baroclinicity in the area of the SPCZ is examined using the frontogenetical function. It is observed that the processes affecting the evolution of baroclinicity in the SPCZ region differ from those in the middle latitude; possible reasons for these differences are proposed.

Robertson, Franklin R.↗

NASA NTRS1991

Atlas of the global distribution of atmospheric heating during the global weather experiment

Global distributions of atmospheric heating for the annual cycle of the Global Weather Experiment are estimated from the European Centre for Medium-Range Weather Forecasts (ECMWF) Level 3b data set. Distributions of monthly, seasonally, and annually averaged heating are presented for isentropic and isobaric layers within the troposphere and for the troposphere as a whole. The distributions depict a large-scale structure of atmospheric heating that appears spatially and temporally consistent with known features of the global circulation and the seasonal evolution.

Schaack, Todd K.↗

NASA NTRS1990

Space-based Doppler lidar sampling strategies: Algorithm development and simulated observation experiments

Lidar Atmospheric Wind Sounder (LAWS) Simulation Models (LSM) were developed to evaluate the potential impact of global wind observations on the basic understanding of the Earth's atmosphere and on the predictive skills of current forecast models (GCM and regional scale). Fully integrated top to bottom LAWS Simulation Models for global and regional scale simulations were developed. The algorithm development incorporated the effects of aerosols, water vapor, clouds, terrain, and atmospheric turbulence into the models. Other additions include a new satellite orbiter, signal processor, line of sight uncertainty model, new Multi-Paired Algorithm and wind error analysis code. An atmospheric wind field library containing control fields, meteorological fields, phenomena fields, and new European Center for Medium Range Weather Forecasting (ECMWF) data was also added. The LSM was used to address some key LAWS issues and trades such as accuracy and interpretation of LAWS information, data density, signal strength, cloud obscuration, and temporal data resolution.

Emmitt, G. D.↗

NASA NTRS1991

Potential vorticity index

Based on the European Center For Medium Range Weather Forecasting (ECMWF) First Global Atmospheric Research Program Global Experiment (FGGE) IIIb data set in the 1978 to 1979 winter, a potential vorticity (PV) index was defined as a measure of the zonally averaged, mid-latitude PV gradient on the 300 K isentropic surface in the Northern Hemisphere. The evolution of that index and its relation to teleconnection patterns of 500 mb geopotential height anomaly are studied. The results of the temporal and spatial variation of blocking and cyclogenesis in the 1978 to 1979 winter and its relation to global and local PV gradients were obtained. Complex empirical orthogonal function (EOF) analyses were performed, using the same FGGE data set for the 1978 to 1979 winter, for a representative high latitude band and mid latitude band geopotential height anomalies at 500 mb, phi sub h, phi sub m, and PV gradient at 300 K, delta(Q), at each longitude for the three month period. The focus of current research is the following: (1) to perform Fourier analyses for the first three EOF's of phi sub h, phi sub m, and delta(Q) at given latitude bands, and to find the dominant wavenumbers and frequencies which are responsible for these EOF's; (2) to compare the results from EOF and Fourier analyses which will be used to explore the relations of blocking and cyclogensis with local and global PV gradients; and (3) to study the time dependence of the local PV gradients and relate it to the PV index vacillation cycles observed in the PV index cycle.

Barcilon, Albert↗

NASA NTRS1990

The three-dimensional distribution of atmospheric heating during the GWE

The large-scale three-dimensional time-averaged distribution of heating and its temporal evolution as determined from the European Center for Medium-range Weather Forecasts (ECMWF) Global Weather Experiment (GWE) Level IIIb data set are examined. Global distributions of vertically averaged heating for January, April, July, and October 1979 are presented to illustrate the seasonal evolution of the planetary pattern. Horizontal distributions of heating and vertical profiles from various climatological regimes of the planetary circulation are given and provide insight into the four-dimensional structure of the thermal forcing of the atmosphere. Major features of the global distributions include heating in regions of deep moist convection over places such as South America. Seasonal meridional and zonal heating migration are discussed and it is noted that zonal migration is linked with planetary scale distribution of continents and oceans and the land-sea surface temperature distribution. Various degrees of heating in the troposphere and their consequences are investigated.

Schaack, Todd K.↗

NASA NTRS1991

Low-frequency intraseasonal tropical-extratropical interactions

Using seven years (1981-1987) of ECMWF initialized analyses, the low-frequency (20-70 day) intraseasonal variability during Northern Hemisphere winter is examined, with emphasis on the zonal wind variability in the Pacific sector and on its relationship to the tropical convection and the middle-altitude wave propagation. Particular consideration is given to changes in the propagation characteristics associated with variations in the subtropical jet and the implications for low-frequency variability in middle latitudes. Also investigated is the relationship between the Pacific sector u-wind fluctuations and tropical convection anomalies.

Schubert, Siegfried D.↗

NASA NTRS1991

Temporal variation of the earth's low-degree zonal gravitational field caused by atmospheric mass redistribution - 1980-1988

Temporal variations in the low-degree zonal harmonics of the earth's gravitational field have recently been observed by satellite laser ranging. A host of geophysical processes contribute to these variations. The present paper studies quantitatively a prime contributor, atmospheric mass redistribution, using ECMWF global surface pressure data for the period of 1980-1988. The annual and semiannual amplitudes and phases of the zonal J(l) coefficient with degree l = 2-6 with and without the oceanic inverted-barometer (IB) effect are computed to obtain the predicted effects on the orbit nodal residuals of Lageos and Starlette. These predicted values are then compared with observations. It is found that the atmospheric influence, combined with the hydrological influence agree well with the Lageos observation for the annual term. The corresponding match appears poorer for Starlette.

Chao, B. Fong↗

NASA NTRS1991

Precipitation rates in the tropics based on the Q1-budget method - 1 June 1984-31 May 1987

The 'apparent' heat source method (Q1 budget) is used to compute the total derivative of dry static energy from 30 deg N to 30 deg S for the period June 1, 1984-May 31, 1987. The dataset is produced from the ECMWF global analyses and consists of twice-daily values of temperature, geopotential height, horizontal wind components, and vertical velocity at increments of 2.5 x 2.5 deg lat/long at seven pressure levels. Vertically integrated values of ds/dt, which are equal to total diabatic heating, Q1, are combined with estimates of net columnar radiation and surface sensible heat exchange to compute mean monthly precipitation rates, P0, as the residual in the Q1 budget. The accuracy of these P0 values is thoroughly examined, and it is suggested that the technique produces reliable estimates of precipitation over tropical oceanic areas on a monthly basis. Time series of mean monthly P0 for several geographic regions of the Southern Hemisphere tropics and the equatorial western Pacific (TOGA-COARE region) reveal that (1) the South Pacific convergence zone has the highest precipitation rates in the Southern Hemisphere; (2) a clear and distinct seasonal cycle is prominent in all regions; and (3) the 1986-87 ENSO event is easily identified, particularly in the TOGA-COARE region.

Vincent, Dayton G.↗

NASA NTRS1990

Global analyses of water vapor, cloud and precipitation derived from a diagnostic assimilation of SSM/I geophysical retrievals

An analytical approach is described for diagnostically assimilating moisture data from Special Sensor Microwave Imager (SSM/I) into a global analysis of water vapor, cloud content, and precipitation. In this method, 3D fields of wind and temperature values taken from ECMWF gridded analysis are used to drive moisture conservation equations with parameterized microphysical treatment of vapor, liquid, and ice; the evolving field of water vapor is periodically updated or constrained by SSM/I retrievals of precipitable water. Initial results indicate that this diagnostic model can produce realistic large-scale fields of cloud and precipitation. The resulting water vapor analyses agree well with SSM/I and have an additional advantage of being synoptic.

Robertson, Franklin R.↗

NASA NTRS1992

Study of explosive and nonexplosive cyclogenesis during FGGE

Explosive cyclogenesis during the winter of the First Global GARP Experiment (January-February 1979) is analyzed using the revised European Centre for Medium Range Weather Forecasts (ECMWF) analyses. Explosive cyclogenesis is defined as a decrease in the sea level pressure at the rate of 1 mb/h for at least 12 h. Diagnostics for 23 explosively developing cases and 16 nonexplosive cases are evaluated. Parameters compared include the dry static stability, low-level relative vorticity, vorticity advection, upper-level divergence, and kinematic vertical velocities. These parameters are compared statistically at the initial, 12-, and 24-h time periods. Parameters for which the explosive and nonexplosive cyclone ensembles were statistically separable are the kinematic vertical velocity and the upper-level divergence and vorticity advection. The strong upper-level processes for the explosive cases at the initial time indicate the importance of upper-tropospheric features in producing the stronger vertical motions and more rapid cyclogenesis.

Wash, Carlyle H.↗

NASA NTRS1992

Polar stratospheric cloud processed air and potential vorticity in the Northern Hemisphere lower stratosphere at mid-latitudes during winter

The present study compares small-scale (less than 100 km) features in ER-2 measurements of ClO, O3, H2O, N2O, and NO(y) outside the lower stratospheric Arctic vortex of 1988-1989 with features on potential vorticity maps from ECMWF. The potential vorticity maps are obtained from T106 analyses and forecasts. Some of the plots were truncated to lower resolution (T63 or T42) which smooths out the finer-scale structure. Comparison of these lower resolution plots shows how much detail is lost by excessive smoothing. It is also evident that the forecast plots lose fine-scale structure due to dissipation in the model resulting mainly from horizontal diffusion. It is concluded that blobs of air on the maps at latitudes between the vortex edge and 25 deg N having potential vorticities characteristic of the vortex did indeed originate from the vortex, but that the real atmosphere is more sharply differentiated than the meteorological analyses, implying that the potential vorticity maps underestimate the amount of peeled-off material.

Tuck, A. F.↗

NASA NTRS1992

A study of eddy-induced fluctuations of the zonal-mean wind using conventional and transformed Eulerian diagnostics

The dynamics of eddy-induced fluctuations of the zonal-mean current in the troposphere and lower stratosphere is investigated using conventional and transformed Eulerian diagnostics utilizing ECMWF FGGE IIIb data. The results obtained support the notion that large convergences of the vertical component of the Eliassen-Palm flux in the middle troposphere at mid- to high-latitudes have much less effect on zonal-mean angular momentum fluctuations than do small divergences of the horizontal component at lower latitudes and higher altitudes.

Pfeffer, Richard L.↗

NASA NTRS1992

An atmospheric energy analysis of the impact of satellite lidar winds and TIROS temperatures in global simulations

A study of the effects on forecast accuracy of adding wind-profiler data is conducted. An observing system simulation test is employed that assumes a sufficient concentration of aerosols to provide global wind profiles (a best case scenario). The simulated data for the series of five day forecasts are produced from a twenty day integration utilizing the ECMWF model, which is also employed to produce the verification forecast for the five day period.

Keller, Linda M.↗

NASA NTRS1992

Comparison of general circulation models to Earth Radiation Budget Experiment data - Computation of clear-sky fluxes

A clear-sky flux computation method is described which is representative of the Earth Radiation Budget Experiment data processing, while at the same time being enough straightforward for implementation in a general circulation model (GCM). The method is a hybrid version of Cess and Potter (1987) Method I and Method II clear-sky top-of-the-atmosphere flux computations for GCMs. The procedure is demonstrated using the ECMWF GCM.

Cess, Robert D.↗

NASA NTRS1993

Effects of vertical distribution of water vapor and temperature on total column water vapor retrieval error

Results are presented of a test of the physically based total column water vapor retrieval algorithm of Wentz (1992) for sensitivity to realistic vertical distributions of temperature and water vapor. The ECMWF monthly averaged temperature and humidity fields are used to simulate the spatial pattern of systematic retrieval error of total column water vapor due to this sensitivity. The estimated systematic error is within 0.1 g/sq cm over about 70 percent of the global ocean area; systematic errors greater than 0.3 g/sq cm are expected to exist only over a few well-defined regions, about 3 percent of the global oceans, assuming that the global mean value is unbiased.

Sun, Jielun↗