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JAN PAEGLE and GLENN E. RASCH

Abstract

Numerical models of three-dimensional, diurnally varying, boundary-layer flows are integrated to study the effect of fluctuating pressure gradients and eddy stresses within different circulation systems. The computational problem is reduced by expanding the horizontal dependence of solutions into Taylor series truncated at the first two terms. Within this simplification, sufficient generality is retained to reproduce axially symmetric similarity solutions and solutions of the nonlinear balance equation. For the time-dependent cases, substantial deviations from linearized (horizontally uniform) theory are predicted. Diurnally periodic pressure gradient and eddy stress oscillations cause greatly differing responses for various circulation systems. The magnitude of the balanced vorticity and the nature of the local deformation field have great bearing on the development of boundary-layer jets and secondary vertical circulations.

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Guillermo J. Berri and Jan Paegle

Abstract

A hydrostatic, anelastic mesoscale numerical model is used to study the predictability of a sea breeze type of circulation over La Plata River in South America. The experiments are designed to test the sensitivity of the predictions to uncertainties in the initial wind field. They consist in the comparison of control and perturbed forecasts which differ only in the initial specification of the wind held. In the case of small-domain prediction systematic uncertainties are probably more representative of actual initial uncertainties than are random, uncorrelated uncertainities. In order to study such cases we systematically add to or subtract from the control run at every grid point a constant value equal to the typical error in the measurement of the wind. We also perform other experiments on a larger domain covering most of South America.

The results show that the predictions of the sea breeze type of circulation in the boundary layer around the La Plata River are sensitive to errors in the initial wind held. These results hold both for random uncertainties as well as systematic uncertainties, and lead to pessimistic conclusions about the deterministic predictability of this type of weak low-level flow. This conclusion reverses, however, in those cases where the local observation accuracy exceeds current observation accuracies by a factor of approxirnately 3. A more optimistic conclusion also holds in the vicinity of a very rough lower boundary, such as the Andes Mountains.

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Jan Paegle and Edward Mason Tomlinson

Abstract

Efficient algorithms for the solution of the nonlinear balance equation in a spherical coordinate system are presented. These methods based upon expansion of the longitudinal dependence of the dependent variables in a Fourier series whose coefficients are obtained by means of Gauss elimination. Two different iterative approaches are used on the nonlinear term of the equation. One of these is a generalization of methods originally suggested by Miyakoda (1956) and Shuman (1957a); the other a generalization of a method discussed by Arnason (1958). Thirteen data tests indicate that the former method is slightly more efficient than the latter, and both methods are significantly more efficient than methods based on relaxation.

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Wayman E. Baker and Jan Paegle

Abstract

The influence of tropical wind data on the numerical prediction of ultralong waves is examined. Two data assimilation experiments are performed using the GLAS fourth-order general circulation model. The two experiments are identical except that one utilizes tropical wind data while the other does not. Six forecasts are generated from the initial conditions provided by each experiment.

After two days, a reduction in the extratropical wind error is found in the assimilation experiment with tropical wind data. For the six pairs of forecasts examined, the effect of tropical wind data on the 72 h planetary wave prediction is positive in four cases and negative in two cases over the western half of the Northern Hemisphere. Also, the 72 h planetary wave error appears in a predominantly barotropic mode.

A detailed examination of the 0000 GMT 15 January 1979 case reveals that the planetary waves are more strongly affected by the wind data in the Northern Hemisphere than in the Southern Hemisphere. Examination of the velocity potential suggests the presence of stronger heating gradients in the no-tropical-wind forecast. The differences present in the initial divergent wind field remain largely restricted to the tropics after 72 h whereas significant differences may be seen in the rotational wind component at all latitudes after 72 h.

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Kingtse C. Mo, Julia Nogues-Paegle, and Jan Paegle

Abstract

The physical mechanisms responsible for the onset and maintenance of the 1993 summer floods were examined using the localized Eliassen-Palm flux diagnostics and solutions of a single-level primitive equation model linearized about a meridionally varying basic state. The unusually long persistent summer pattern is linked with the marked transient eddy activity in late May and June. The feedback of eddies in the time mean flow caused a strengthening and eastward extension of the Pacific jet and a strengthening of the jet over North America. Results from the model suggest that the summer pattern may be interpreted as that of a lee trough forced by the Rocky Mountains in the presence of a strong westerly mean flow maintained by the eddies upstream.

Composites from cases similar to that of the 1993 summer exhibit strong low-level southerly flow cast of the Rockies and suggest that the low-level jet may be an important mechanism to sustain the anomalous rainfall.

It is concluded that the effect of the eddies in maintaining a strong upper-level zonal flow, the role of the Rockies in sustaining a lee trough, and an associated low-level jet that brings in tropical moisture are essential ingredients in developing and maintaining floodlike conditions over the central United States.

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Jan Paegle, Kingtse C. Mo, and Julia Nogués-Paegle

Abstract

Regional summertime atmospheric conditions of 1993 are analyzed with the University of Utah Local Area Model (ULAM) by nudging boundary values and large internal scales of the local model toward values produced by the Nested Grid Model (NCEP/NOAA) initial analyses and forecasts archived at 6-h intervals. The approach allows the local ULAM to develop finer-scale structures in the precipitation and circulation forecasts than those resolved by the NGM. The study focuses on the influence of surface evaporation upon rainfall and low-level flow in regional simulations. Much of the rainfall simulated in the control experiment occurred from the late afternoon to early morning hours, with a pronounced midday minimum over the flood region.

The moisture flux from the south due to the low-level jet (LLJ) provides much of the moisture source for the precipitation, and it is shown that the net moisture influx is significantly larger than the rainfall rate over the flood region. As a consequence, modifications of surface evaporation apparently are relatively more important in changing the buoyancy and resulting LLJ strength than they are in providing additional moisture to the already plentiful moisture influx from the Gulf of Mexico. This suggests that accurate surface evaporation in the Great Plains is necessary for accurate simulation of dynamic support for rainfall.

The LLJ and especially its diurnal oscillation increase for drier surface conditions in the vicinity of the jet core, providing more effective convergence patterns to support rainfall in these cases than in cases of stronger surface evaporation. This appears to be a more important mechanism for rainfall release over the Mississippi River basin than moistening through local evapotranspiration, although the latter also contributes to more rainfall when this moistening occurs downwind of the jet core.

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Julia Nogués-Paegle, Kingtse C. Mo, and Jan Paegle

Abstract

Ten years of forecasts during Northern Hemisphere winter produced by the NCEP–NCAR reanalysis project are analyzed and their errors are documented with a focus on forecasts over South America. Previous studies have documented a seesaw pattern in the South Atlantic Convergence Zone (SACZ). Events associated with a strong SACZ are periods when the subtropical plains of South America exhibit precipitation deficits. When the SACZ weakens, precipitation in the plains is abundant. The forecast errors during these periods are examined separately. An error climatology is also obtained based on all available forecasts for January and February.

It is found that cases with weak SACZ are characterized by relatively smaller forecast errors in the Americas and neighboring oceans during the first three days than are strong SACZ cases. The error growth for these weak SACZ cases is larger, and the forecast errors exceed those in the strong SACZ cases after about 5 days. Upper-tropospheric divergence is systematically underpredicted over tropical and subtropical South America in the 117 studied forecasts, and it is overpredicted over the Caribbean. Similar features are also found in the 6-h forecast precipitation verified against global precipitation index estimates. The 3–8-day model forecasts retain some characteristics of the weak and strong SACZ events, but underpredict their amplitudes. Eight-day predicted upper-tropospheric zonal winds have substantial errors over North America. Integrations with a simple global numerical model suggest that the predicted wind error over North America is due to latent heating errors associated with precipitation forecast errors located nearby. Central and western Pacific errors do not appear to be as important for wind forecast errors in this region. Weak SACZ events are characterized by stronger low-level jets and more poleward moisture transport east of the Andes than are strong SACZ events in the NCEP–NCAR reanalysis. This distinction is substantially weakened in the 8-day forecasts. Integrations with a global model suggest that inadequate simulation of the distribution of latent heating and of radiative heating may have contributed to the inability of the NCEP forecast model to adequately distinguish low-level flow structures during positive and negative events

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Julio Buchmann, Lawrence E. Buja, Julia Nogués-Paegle, and Jan Paegle

Abstract

A series of real-data integrations of the National Center for Atmospheric Research Community Climate Model with tropical heat anomalies display regions of pronounced subsidence and drying surrounding the anomaly. The present emphasis is upon subsidence and drying centers located several thousand kilometers westward and poleward of the heating. These features are repeatedly found in several different series of medium to extended range forecast experiments, including cases of tropical Atlantic heating and tropical east Pacific heating. This highly predictable sinking response is established within the first five days of these integrations. The normal modes of a set of primitive equations linearized about a resting basic state are used to partition model response into gravity-inertia and Rossby modes. The results show that most of the vertical motion response can be explained by gravity-mode contributions. The sensitivity of the response is examined through a series of numerical experiments with a simple global forecast model. These integrations suggest that the subsiding response surrounding the heated region is somewhat sensitive to the ambient circulation. In particular, the extratropical response tends to be greatest in the winter hemisphere, and it is relatively less sensitive to the precise location of the tropical heating than to the nature of the zonally averaged background flow. Further experimentation suggests that the peak subsidence response is almost linear in the heating amplitude. These experiments also demonstrate that a significant portion of the early response occurs independently of any fluctuations of the vorticity field and therefore is not merely a secondary circulation associated with extratropical Rossby wave responses. The latter response is relatively more sensitive to the presence of longitudinal vorticity gradients, and the dynamical interpretation is then less clear.

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Jan Paegle, Wayman E. Baker, and Julia N. Paegle

Abstract

The global scale divergent and rotational flow components of the Global Weather Experiment (GWE) are diagnosed from three different analyses of the data. The rotational flow shows closer agreement between the analyses than does the divergent flow. Although the major outflow and inflow centers are similarly placed in all analyses, the global kinetic energy of the divergent wind varies by about a factor of 2 between different analyses while the global kinetic energy of the rotational wind varies by only about 10% between the analyses.

A series of real data assimilation experiments has been performed with the GLA general circulation model (GCM) using different amounts of tropical wind data during the First Special Observing Period of the Global Weather Experiment. In experiment 1, all available tropical wind data were used; in the second experiment, tropical wind data were suppressed; while, in the third and fourth experiments, only tropical wind data with westerly and easterly components, respectively, were assimilated.

The rotational wind appears to be more sensitive to the presence or absence of tropical wind data than the divergent wind. It appears that the model, given only extratropical observations, generates excessively strong upper tropospheric westerlies These biases are sufficiently pronounced to amplify the globally integrated rotational flow kinetic energy by about 10% and the global divergent flow kinetic energy by about a factor of 2.

Including only easterly wind data in the tropics is more effective in controlling the model error than including only westerly wind data. This conclusion is especially noteworthy because approximately twice as many upper tropospheric westerly winds were available in these cases as easterly winds.

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Jan Paegle, Julia N. Paegle, and Gregory C. Dodd

Abstract

The origins of atmospheric states that are non-elliptic for the height constrained balance equation are examined from observational perspectives. Such states are commonly present in the deep tropics in Objectively analyzed data sets. In order to analyze the source of this phenomenon, we compute terms of the divergence equation (from which the balance equation derives) for disturbed periods of the GATE experiment. Meaningful residuals cannot be obtained because they are obscured by observational uncertainty of the geopotential gradients that are calculated from the hydrostatic equation using temperature observations. The geopotential fields recomputed from the divergence equation using observed wind data appear to be much better determined, but they still produce fields that are non-elliptic for the height constrained nondivergent balance equation.

For the convective GATE cases, it appears that the essential balance of the divergence equation is between divergent accelerations, deforming accelerations, the divergent pressure form field and friction, while centripetal accelerations (including the Coriolis is effect) are secondary. Thus, the underlying assumption of solenoidal flow in the balance equation is fundamentally wrong in regions of tropical convection. This appears to be the physical source of the poorly posed balance equation in many non-elliptic cases.

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