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WILLIAM D. BONNER and JAN PAEGLE

Abstract

Analysis of 1 week's data in August 1960 shows significant diurnal variations in surface geostrophic wind over the south-central United States. The oscillation in the southerly component (V g) is driven by the response of the thermal wind to the diurnal temperature cycle over sloping terrain. A smaller oscillation in U g derives from spatial variations in the amplitude of the diurnal pressure wave. The amplitude of the oscillation in V g is about 3 to 5 m sec–1 at the surface, decaying exponentially with height to near 0 at 2 km.

Examination of 11 yr of summertime rawinsonde data at Fort Worth, Tex., shows a very regular diurnal variation in boundary layer wind with maximum amplitude of about 3 m sec–1 at 600 m above the ground. This oscillation is forced by periodic variations in both eddy viscosity and geostrophic wind. Using a simplified model of the boundary layer, we obtain solutions for the diurnally periodic wind resulting from “reasonable” variations in eddy viscosity and “observed” variations in geostrophic wind.

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Matilde Nicolini, Kim M. Waldron, and Jan Paegle

Abstract

Heavy convection that occurred over Tulsa, Oklahoma, during the evening and night of 26/27 May 1984 is studied through a series of numerical experiments with a regional forecast model. The emphasis is on diurnal oscillations of the boundary-layer flow and the sensitivity of model predictions of nocturnal low-level jets to the presence or absence of cloud radiative interaction and latent heating. The nocturnal low-level jet appears to be established mainly by boundary-layer processes and to be significantly amplified by the latent heat of condensation, which increases during the night over the lower Great Plains. The presence of cloud radiative interactions appears to have less impact on the prediction of the low-level jet and nocturnal convection for the present case, but it is important for the timing of the nocturnal precipitation. The regional model underestimates the extreme precipitation rates observed over Tulsa. The possibility that this underestimation is a consequence of insufficient resolution is investigated by running a high-resolution, convective-scale cloud model initialized with and driven by the regional model forecast state. In the nested mode, the regional model with a grid size of 50 km and the convection model with a grid size of 0.6 km produce local rainfall totals on the order of 10 cm over time periods on the order of 2 h, in agreement with the heavy observed precipitation.

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Jan Paegle, Chi-Dong Zhang, and David P. Baumhefner

Abstract

Evidence is presented for a highly regular seasonal rearrangement of the long-wave pattern over North America from the winter to the spring. The change involves a reversal of the trough-ridge pattern between the two seasons wherein the anticyclone observed in the winter climatology over and west of the Rocky Mountains reverses to a spring trough, and the winter lee cyclone changes to a spring ridge over the eastern United States. These changes, which represent a westward shift of the long-wave pattern from winter to spring, are accompanied by westward shifts of the subtropical jet core from the east to the west coast, and of the tropical rain maxima from the Amazon Basin to the tropical East Pacific Ocean.

We investigate the possibility that these westward displacements are dynamically related in a series of ten-day integrations of a general circulation model over four separate ensembles, each consisting of ten cases, during the winter and spring. The data for these ensembles is taken from the Global Weather Experiment, and is used to initialize the NCAR general circulation model. This model maintains the winter-spring patterns observed in the western hemisphere when run in a control mode. The tropical precipitation distribution around the Eastern Pacific and the Amazon Basin is then modified to produce a winterlike rainfall pattern for the spring ensemble. The resulting extratropical changes in the forecast wind and height field resemble the observed winter pattern, rather than the spring distribution of the control. When the tropical precipitation of the winter cases is modified to simulate the distribution observed in spring, the resulting experiments produce forecast changes that resemble the extratropical pattern of spring, although the apparent response is not as great as the observed seasonal changes. The statistical reliability of the winter experiments appears to be higher than that of the spring experiments.

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Julio Buchmann, Jan Paegle, Lawrence Buja, and R. E. Dickinson

Abstract

A series of experiments using real-data general circulation model integrations is performed to study the impact of remote tropical Pacific heating modifications upon the rainfall over the Amazon Basin. In one set of experiments, a heating term is added to the thermodynamic equation in the western tropical Pacific Ocean, and in the second set, the sea surface temperatures are cooled in the eastern Pacific Ocean. The rainfall of northern sections of South America decreases in the first set of experiments and increases in the second set of experiments. Examination of the circulation changes for the second set of experiments suggests that the remote links occur through equatorially trapped flow modifications, perhaps related to the east-west Walker cells, rather than through midlatitude teleconnections via Hadley cells. The time evolution of these patterns suggests them to be clearly relevant for medium range weather prediction in the tropics.

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M. G. WURTELE, JAN PAEGLE, and ANITA SIELECKI

Abstract

Open boundaries are desirable when the region of interest of a computation is a localized area of a much larger domain. Boundary conditions are developed for the linear storm-surge equations (without Coriolis effects) that permit disturbances to pass out of the region of computation with negligible reflection. These conditions, based on the concept of Riemann invariants, are applied in one- and two-space dimensions. Examples of the flow around a sea mound, a shelf, and an island are given. Selected comparisons are made with Sommerfeld's radiation condition, advanced by Vastano and Reid.

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Jan Paegle, Wilford G. Zdunkowski, and Ronald M. Welch

Abstract

The Crank-Nicholson method may not give useful results in detailed prediction of the thermal planetary boundary layer unless tune steps on the order of 10 s are used. In similar problems, lower order time differencing methods give reasonable results with time steps as large as 300 s. The reason for the superior behavior of the lower order schemes relative to straightforward application of the Crank-Nicholson technique is due to a better treatment of short waves which appear to be critically important in nonlinear terms.

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Wilford G. Zdunkowski, Ronald M. Welch, and Jan Paegle

Abstract

A dynamic-numerical model is utilized to study the impact of air pollution on the temperature and wind distributions of the planetary boundary layer. The mathematical model uses a rather complete radiative treatment which comprises the entire solar and infrared spectrum ranging from 0.29 to 100 µm. In the solar spectral range, the absorption by water vapor, nitrogen dioxide and industrial haze is fully accounted for in addition to multiple scattering by air molecules and haze particles. In the spectral region of the strong absorption hands of the infrared emission spectrum, the effect of aerosol is very small and is disregarded. The emissivity method is applied here, allowing full treatment of the overlapping effects of water vapor and carbon dioxide. In the window region, however, the effect of aerosol and water vapor absorption and emission is taken into account in addition to multiple scattering by aerosol particles. The radiative treatment accounts for the influence of relative humidity on the particle distribution function and on the complex index of refraction of the aerosol. The spherical harmonic method is used to handle the scattering problem.

The dynamical part of the analysis consists of the numerical solution of a coupled system of partial differential equations comprising the equation of horizontal mean motion, the thermodynamic equations of the air and the soil, and the transport equations of moisture and pollution. Various models of the exchange coefficient are used to study the impact of model assumptions on the computed distributions of temperature, pollutant material and wind. It is found that the choice of the exchange model is not critical but has some effect on the model computations. The present calculations show that the maximum impact of air pollution on the evolution of temperature and wind profiles is highly significant, thus verifying the previous conclusions of Zdunkowski and McQuage (1972).

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Julio Buchmann, Jan Paegle, Lawrence E. Buja, and Robert E. Dickinson

Abstract

Severe droughts occurred over eastern sections of North America and central sections of South America in 1986 and 1988. We summarize data suggesting that both periods were characterized by above-normal tropical Atlantic sea surface temperatures and convection, and investigate the response of a general circulation model to positive heating anomalies in the tropical Atlantic sector. An eight-case control ensemble of 30 day global predictions is made starting from the atmospheric state observed on 1 January of each year from 1977 through 1984. The same eight cases are integrated in a second experimental ensemble that is identical to the first control ensemble, except that a heating term is added to the thermodynamic equation in a region centered at 30°W, 6.6°N. This is intended to simulate the latent heating of enhanced tropical Atlantic convection. The third ensemble is identical to the second, except the heating is centered at 6.6°S.

Both heated ensembles produce reductions of forecast precipitation over most of North and South America, but these appear to have greater statistical significance over North America. Here the greatest precipitation reductions are forecast over the southern and eastern United States, and this response does not change substantially between the two experiments. The South American response is more sensitive to the placement of the heating anomaly. When the anomaly is located north of the equator, drying occurs over northeast Brazil; meanwhile this region receives increased rainfall when the anomaly is located south of the equator. Both experiment ensembles display a region of reduced rainfall over the Andes Mountains, and over southern portions of Brazil. However, only the former region is statistically significant above the 95% confidence level. The present usage of real initial data and an ensemble of cases permits us to draw quantitatively meaningful estimates of the time scale of response and case-to-case variability. For presently tested cases, the South American response is evident by day 5, but exhibits substantial intersample variability, and the North American response is fully established by day 10, and exhibits less intersample variability. The model drying effects can be explained only partly by enhanced local subsidence; much of the rainfall reduction appears to be related to a reorientation of the synoptic scale wave pattern in which the lower tropospheric circulation is unfavorable for water vapor inflow from source regions over the tropical Atlantic and Amazon Basin.

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Julio Buchmann, Lawrence E. Buja, Jan Paegle, and Robert E. Dickinson

Abstract

A series of real-data experiments is performed with a general circulation model to study the sensitivity of extended range rain forecasts over the Americas to the structure and magnitude of tropical beating anomalies. The emphasis is upon heat inputs over the tropical Atlantic, which have shown significant drying influences over North America in the author's prior simulations. The heating imposed in the prior experiments, that is, shown to be excessive by a factor of 2, is compared with the condensation heating rates that naturally occur in the forecast model. Present experiments reduce the imposed anomaly by a factor of 3 and also impose sea surface temperature decreases over the eastern tropical Pacific Ocean. The new experimental results are in many ways consistent with the author's prior results. The dry North American response is statistically more significant than the South American response and occurs at least as frequently in the different members of the experimental ensembles as in our prior experiments. The drying effect is accentuated by the presence of east Pacific cooling, but this does not appear to be the dominant influence. Over tropical South America, the Pacific and Atlantic modifications produce compensating influences, with the former dominating, and allow increased rainfall over the Amazon Basin.

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Kim M. Waldron, Jan Paegle, and John D. Horel

Abstract

Numerical filters required to control spatial computational modes in a limited-area model (LAM) that uses the unstaggered. A grid are developed and tested over the complex topography of the Great Basin of the western United States. The filters are founded upon Fourier expansions of forecast deviation fields and function equally effectively for both periodic and aperiodic local structures. Unlike other spatial filters, the approach used here avoids any direct contamination of larger scales. Provided that the shortest resolved wavelength of two grid intervals is removed, the results do not depend strongly on the range of filtered short waves or on the type and order of horizontal space difference approximations.

This approach leads naturally to methods in which the large scales predicted by an ambient outer model can be directly incorporated within the complete domain of the inner LAM, rather than just through conditions applied at the lateral boundaries of the LAM. This technique has some similarities to methods used both in operational regional models in Japan and in recent regional research models at the National Centers for Environmental Prediction (formerly National Meteorological Center) of the United States. Several methods to incorporate the large scales into the LAM are evaluated in a winter storm case study and in an ensemble of seven forecasts.

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