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Ernesto H. Berbery and Eugene M. Rasmusson

Abstract

Two years of regional analyses based on the Eta Data Assimilation System (EDAS) are used to examine the mesoscale features of the moisture budgets of the Mississippi River basin and its subbasins. Despite the short period, basic aspects of the regional-scale seasonal means, annual cycle, and even diurnal cycle of the atmospheric water cycle are represented. The ability of the Eta Model to resolve mesoscale features of the low-level circulation is an important factor in improving the estimates of moisture flux convergence at regional scales. It appears that the internal consistency of moisture budgets estimated from EDAS analyses for basins of nearly 5 × 105 km2 is comparable to that computed from radiosondes for basins of about 2 × 106 km2 or larger. In other terms, the spatial scale of basins where consistent moisture budgets can be estimated appears to be reduced by almost one order of magnitude.

Area-averaged evaporation estimates (computed as residuals of the moisture budget equation) for basins of about 5 × 105 km2 range from near zero during winter in the northern subbasins to about 5–6 mm day−1 during summer in the southern subbasin. It is suggested that the slightly negative estimates of evaporation in the northern subbasins during winter may partly result from an underestimation of observed precipitation due to the combined effect of wind and solid precipitation. No attempt was made at computing the model’s moisture budget, since changes in the surface parameterizations prevented having a period long enough to achieve stable results. Broad aspects of the diurnal cycle during summer were also examined through nighttime–daytime differences. Consistent with other studies over the central United States, results show that the nighttime development of moisture flux convergence is associated with an increase of intensity of the low-level jet. Interestingly, the nighttime convergence of moisture flux is offset by divergence during daytime and, as a result, overall moisture flux divergence is observed during summer.

A comparative analysis was made of the observed and model forecast precipitation to assess the model’s overall performance during the 2-yr period. It was found that the spatial patterns, intensity, and even the broad aspects of the summertime diurnal cycle of the model forecast precipitation are similar to those observed. Nevertheless, some deficiencies exist: a dry bias was obtained over the central United States during summer and winter; during summer, the southeastern United States had an excess of precipitation similar to that observed in the National Centers for Environmental Prediction global model; during winter, forecast precipitation in the northwestern United States appears to have biases in location and intensity, which can be related to the large-scale component of the model precipitation.

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Ernesto H. Berbery and Estela A. Collini

Abstract

The physical mechanisms associated with precipitation in southeastern South America during spring are investigated using short-term integrations with the regional mesoscale Eta Model. An evaluation of the model’s performance using in situ measurements of precipitation as well as satellite estimates reveals that the model performed satisfactorily in the subtropics and extratropics. Deficiencies in tropical Brazil are partly related to the model’s convective adjustment scheme and possibly to surface parameterizations as well. The model forecasts reproduce all observed centers of precipitation south of about 20°S, although in some cases the magnitude is somewhat smaller. Of particular relevance for this study is the finding that spatial correlations between the model forecast and observed precipitation over Cuenca del Plata are almost as high as those obtained for the Mississippi River basin using forecasts of the National Centers for Environmental Prediction operational Eta Model. Cuenca del Plata is a basin in southeastern South America that is the water resource for a largely populated area and is well known for its agricultural production and other factors that sustain the region’s economies.

An important component of the circulation reproduced in the simulations is the low-level jet east of the Andes that feeds moisture from the Amazon basin to higher latitudes. It has a diurnal cycle with a nighttime maximum that favors increased moisture flux convergence in southeastern South America. This convergence, in turn, is associated with generalized nighttime ascent and precipitation. The results are consistent with previous observational studies that show a nighttime maximum of precipitation over the region. A second regime of precipitation is found toward the eastern coast, where maximum daytime precipitation appears to be associated with a convectively unstable atmosphere, with convection being triggered by a sea–land breeze enhanced by the topography of southern Brazil. These diurnal regimes of precipitation have a significant impact in the atmospheric water cycle in Cuenca del Plata.

The basin-averaged vertically integrated moisture flux convergence is about 4 mm day−1 and almost doubles the spring values for the Mississippi River basin. The large values may be related to the particular conditions of the period under analysis and the stronger low-level jet. The results reported here provide a preliminary description of the basin-averaged moisture flux convergence and its diurnal variability, but basin-averaged precipitation is still the component that needs to be improved. It is assumed that a blend of observations and high-resolution satellite estimates will be needed to complete the description of the atmospheric water cycle.

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