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Phillip A. Arkin and Bernard N. Meisner

Abstract

Estimates of areal- and time-averaged convective precipitation derived from geostationary satellite imagery using a simple thresholding technique are presented. The estimates are based on measurements of the monthly mean fraction of 2.5° × 2.5° areas covered by clouds whose equivalent blackbody temperature in infrared imagery is below 235 K. The transformation between fractional coverage and rainfall amount is based upon comparisons of fractional coverages using a variety of temperature thresholds and spatial and temporal averaging scales with areal averaged rainfall from the GARP Atlantic Tropical Experiment.

Three-year means of the estimated precipitation for the period December 1981-November 1984 are shown for each of the (3-month) calendar seasons and compared with published descriptions of the long-term seasonal mean rainfall fields. Over the tropical oceans agreement is quite good with no evidence of any systematic errors. Over the Americas, long-term means derived from station observations of rainfall show less extensive areas of heavy rainfall than those derived here, and a slight tendency for lower peak values during the rainy season.

The interannual variability during the 3-yr period is described and compared with station observations of rainfall. The relationship between cloud cover and rainfall in the tropics (30d°N-30°S) is found to be similar to that found in previous studies, with a threshold of 235 K giving highest correlations, while observations between 30° and 50° were best correlated with a threshold of 220 K. The large changes in rainfall distribution over South America associated with the 1982-83 ENSO episode and the breaking of the drought in Northeast Brazil during 1984 are clear in the estimates presented here, but the amplitude of the changes is somewhat over-estimated. Warm season rainfall observed over the United States is less than the estimates, except near the Gulf of Mexico and southeast United States coast where the degree of overestimation increases away from the coast.

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N. Phillips, J. Susskind, and L. McMillin

Abstract

NOAA and NASA have conducted a joint simulation study to compare the retrieval accuracy of atmospheric temperature profiles and surface skin temperature retrieved from HIRS2, the current operational infrared temperature sounder, and AMTS, a proposed high spectral resolution infrared sounder. Simulations were conducted in as realistic a manner as practical for both clear and partial cloud conditions. Simulated radiances for both instruments were prepared at the University of Denver. The data were analyzed at NASA using a physical inversion technique and at NOAA using a statistical technique. The retrievals were done under information constraints typical of operational retrievals. Results show significant improvement of AMTS compared to HIRS2 for both clear and cloudy conditions. The improvements are relatively independent of the method used but the physical retrievals outperform the statistical retrievals. The combination AMTS-Physical produced retrievals with temperatures in the lower troposphere having an accuracy of about 1°C and seal/and surface temperatures having an accuracy of about 0.4°C, even under partial cloudiness. Actual results may be somewhat poorer but an instrument designed along the fines of AMTS should still represent a significant improvement over accuracies attainable from instrumentation that is current or scheduled in the near future.

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Bernard N. Meisner and Phillip A. Arkin

Abstract

Three years of three-hourly infrared satellite data from the American geostationary satellites were used to determine the large-scale spatial and temporal variations in the diurnal cycle of tropical convective precipitation. The region examined extended from 50°N to 50°S, 175°E to 25°W. The satellite data were related to convection through the fractional coverage of 2.5° subareas by clouds colder than several threshold temperatures. Seasonal maps showing mean fractional coverage, total diurnal variance in cold clouds, as well as variance associated with the first and second harmonic, respectively, present the results. Seasonal maps showing vectors of the amplitude and phase of the first harmonic are also shown.

In general, our results agreed with previous studies. The mean positions and annual variations of the maxima and minima in tropical convection were accurately depicted. The diurnal cycle over the tropical continents and the other continents during summer was much larger than that over the oceans. In virtually all areas where the diurnal variation was large, the first harmonic explained most of this variance. The interior of South America during summer had an 1800 LST maximum, with coastal and mountain regions showing somewhat earlier maxima. Over the Central American mountains in summer, late evening or early nighttime maxima were apparent, with near noontime maxima over the adjacent waters. The diurnal cycle observed over the United States in summer was also consistent with previous results. Early morning maxima along the Gulf Coast, the Florida peninsula and the Ohio River Valley, afternoon maxima over the western plains and mountains, evening maxima in the upper Mississippi River Valley, and an area of small diurnal variation and ambiguous phase extending southwestward from the Great Lakes were all present in the data.

Substantial diurnal cycles over the oceans were apparent only in the convergence zones. These regions were generally characterized by near-noontime maxima.

Although the principle contrasts of continent/ocean, convective/nonconvective, and high/low relief were apparent in each year, substantial interannual fluctuations in the variances of the diurnal cycle were also noted. Some of these fluctuations such as the one that occurred during the 1982–83 ENSO event, could be attributed to shifts in convection. Other interannual variations have no clear explanation and may represent sampling fluctuations.

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N. Phillips, W . Blumen, and O. Coté

This article gives a review of the research done in the Soviet Union through 1959 on the theory and practice of numerical weather prediction by hydrodynamical methods. Russian meteorologists have used the same geostrophic forecast system as have other meteorologists and have carried out a number of test forecasts with electronic computers. Comparatively little has been published so far in objective weather-map analysis, general-circulation experiments and the use of non-geostrophic equations.

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J. G. Charney and N. A. Phillips

Abstract

An n-level generalization of the 2½-dimensional model is derived by specialization of the complete three-dimensional quasi-geostrophic equations. In the case n = 1, it reduces to the two-dimensional single-layer barometric model. In the case N = 2, it reduces to the double-layer barotropic model, or — what is shown to be mathematically equivalent —the 2½-dimensional model. Methods of numerical integration of the 2- and 2½-dimensional equations, and the machine requirements for such integrations, are discussed.

The results of a series of six two-dimensional and six 2½-dimensional forecasts for 12 and 24 hours are presented. Although the 2½-dimensional forecasts are noticeably superior to the two-dimensional forecasts, it is apparent that considerable improvement will be possible with models in which there are fewer artificial constraints. A method of integration is therefore proposed for the n-level generalization of the 2½-dimensional model, and computation schemes are outlined for the general three-dimensional quasi-geostrophic equations. The semi-Lagrangian coordinate system with potential temperature as vertical coordinate is shown to exhibit favorable properties for machine integration.

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A. P. Khain, V. Phillips, N. Benmoshe, and A. Pokrovsky

Abstract

Some observational evidence—such as bimodal drop size distributions, comparatively high concentrations of supercooled drops at upper levels, high concentrations of small ice crystals in cloud anvils leading to high optical depth, and lightning in the eyewalls of hurricanes—indicates that the traditional view of the microphysics of deep tropical maritime clouds requires, possibly, some revisions. In the present study it is shown that the observed phenomena listed above can be attributed to the presence of small cloud condensation nuclei (CCN) with diameters less than about 0.05 μm. An increase in vertical velocity above cloud base can lead to an increase in supersaturation and to activation of the smallest CCN, resulting in production of new droplets several kilometers above the cloud base. A significant increase in supersaturation can be also caused by a decrease in droplet concentration during intense warm rain formation accompanied by an intense vertical velocity. This increase in supersaturation also can trigger in-cloud nucleation and formation of small droplets. Another reason for an increase in supersaturation and in-cloud nucleation can be riming, resulting in a decrease in droplet concentration. It has been shown that successive growth of new nucleated droplets increases supercooled water content and leads to significant ice crystal concentrations aloft. The analysis of the synergetic effect of the smallest CCN and giant CCN on production of supercooled water and ice crystals in cloud anvils allows reconsideration of the role of giant CCN. Significant effects of small aerosols on precipitation and cloud updrafts have been found. The possible role of these small aerosols as well as small aerosols with combination of giant CCN in creating conditions favorable for lightning in deep maritime clouds is discussed.

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D. Cunnold, F. Alyea, N. Phillips, and R. Prinn

Abstract

A three-year integration of a global three-dimensional model including dynamics and simple photo- chemistry is used to predict ozone. Distributions of NO3 and odd hydrogen deduced by McConnell and McElroy are used to incorporate in a simple way the chemical effect of these species. Good agreement with observation is obtained for stratospheric motion patterns, meridional circulations, ozone density as a function of height and latitude, eddy transports of ozone, surface destruction of ozone, and correlations of ozone with other variables. The annual cycle of columnar ozone in high latitudes is present, but at a smaller amplitude than observed. Vertical transport of ozone downward from the main generation level at 30 km is accomplished primarily by small-scale eddy diffusion between 20 and 30 km and again near the ground; large-scale vertical transport dominates inbetween. The model predicts a secondary maximum in ozone mixingg ratio at 45 km somewhat equatorward of the winter-polar-night zone. This feature, recently observed from satellite measurements, is thought to he caused by the temperature-dependence of reaction rates in the Chapman scheme.

The principal deficiency of the model is an underprediction of the spring ozone concentration in high latitudes in the lower stratosphere.

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Joshua M. Boustead, Barbara E. Mayes, William Gargan, Jared L. Leighton, George Phillips, and Philip N. Schumacher

Abstract

Using system-relative composites, based on a dataset of significant tornadoes and null supercell events, environmental conditions associated with occurrences of significant tornadoes near discernible surface boundaries were compared to nontornadic boundary supercells, and warm sector significant tornadoes to nontornadic warm sector supercells, for a portion of the Great Plains. Results indicated that significant boundary tornadoes were associated with the exit region of a 300-hPa jet maximum, while null boundary events were in closer proximity to the 300-hPa jet entrance region. The differences at 300 hPa led to significant differences at the surface, as the null composite indicated deformation and confluence into the surface boundary and enhanced frontogenesis, while this was not present in the boundary significant tornado composite. Significant synoptic differences also were noted between the warm sector tornadoes and the warm sector null events. The warm sector significant tornadoes were associated with a much stronger, negatively tilted synoptic storm system, with the composite tornado in the 300-hPa jet exit region and downstream of increasing values of absolute vorticity. Additional thermodynamic and kinematic parameters pertaining to low-level moisture and environmental winds appeared to be important in distinguishing boundary and warm sector significant tornadoes from nontornadic supercell events. Statistical comparisons between boundary and warm sector significant tornado events showed significant differences in the climatology of their length, width, and date and time of occurrence.

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W. Lawrence Gates, James S. Boyle, Curt Covey, Clyde G. Dease, Charles M. Doutriaux, Robert S. Drach, Michael Fiorino, Peter J. Gleckler, Justin J. Hnilo, Susan M. Marlais, Thomas J. Phillips, Gerald L. Potter, Benjamin D. Santer, Kenneth R. Sperber, Karl E. Taylor, and Dean N. Williams

The Atmospheric Model Intercomparison Project (AMIP), initiated in 1989 under the auspices of the World Climate Research Programme, undertook the systematic validation, diagnosis, and intercomparison of the performance of atmospheric general circulation models. For this purpose all models were required to simulate the evolution of the climate during the decade 1979–88, subject to the observed monthly average temperature and sea ice and a common prescribed atmospheric CO2 concentration and solar constant. By 1995, 31 modeling groups, representing virtually the entire international atmospheric modeling community, had contributed the required standard output of the monthly means of selected statistics. These data have been analyzed by the participating modeling groups, by the Program for Climate Model Diagnosis and Intercomparison, and by the more than two dozen AMIP diagnostic subprojects that have been established to examine specific aspects of the models' performance. Here the analysis and validation of the AMIP results as a whole are summarized in order to document the overall performance of atmospheric general circulation–climate models as of the early 1990s. The infrastructure and plans for continuation of the AMIP project are also reported on.

Although there are apparent model outliers in each simulated variable examined, validation of the AMIP models' ensemble mean shows that the average large-scale seasonal distributions of pressure, temperature, and circulation are reasonably close to what are believed to be the best observational estimates available. The large-scale structure of the ensemble mean precipitation and ocean surface heat flux also resemble the observed estimates but show particularly large intermodel differences in low latitudes. The total cloudiness, on the other hand, is rather poorly simulated, especially in the Southern Hemisphere. The models' simulation of the seasonal cycle (as represented by the amplitude and phase of the first annual harmonic of sea level pressure) closely resembles the observed variation in almost all regions. The ensemble's simulation of the interannual variability of sea level pressure in the tropical Pacific is reasonably close to that observed (except for its underestimate of the amplitude of major El Niños), while the interannual variability is less well simulated in midlatitudes. When analyzed in terms of the variability of the evolution of their combined space–time patterns in comparison to observations, the AMIP models are seen to exhibit a wide range of accuracy, with no single model performing best in all respects considered.

Analysis of the subset of the original AMIP models for which revised versions have subsequently been used to revisit the experiment shows a substantial reduction of the models' systematic errors in simulating cloudiness but only a slight reduction of the mean seasonal errors of most other variables. In order to understand better the nature of these errors and to accelerate the rate of model improvement, an expanded and continuing project (AMIP II) is being undertaken in which analysis and intercomparison will address a wider range of variables and processes, using an improved diagnostic and experimental infrastructure.

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