Low-level climatological wind fields over the La Plata River region of South America synthesized with a mesoscale boundary-layer numerical model

A primitive equation, dry, hydrostatic and incompressible mesoscale boundary layer model (MBLM) is used to simulate the high-resolution low-level wind field climatology over the La Plata River region in South America. The horizontal model domain (350 km x 316 km) has 79x58 points, with a horizontal resolution of 0.05 degrees. The model climatology is the ensemble result of a series of daily forecasts obtained by forcing the model with limited local observations. Each ensemble member produces a daily forecast that participates in the definition of the wind climatology with a probability calculated with the local observations. The upper boundary condition is taken from the local radiosonde observation at Ezeiza airport, and the lower boundary condition consists of a surface heating function calculated with the temperature observations of the surface weather stations in the region. The study conducted during the period 1960-1990 reveals an overall good agreement between the observed and the modeled surface wind climatology at five weather stations of the region. The MBLM model represents very well the differences in the wind speed magnitudes and predominant wind direction sectors throughout a region that displays a strong sea-land breeze daily cycle. The average root mean squared value of the model relative error is 31% for wind direction and 23% for wind speed.

A study of climate variability of the low-level wind field over the region is conducted during the period 1960-1990, using the MBLM and observations from five weather stations. The observations reveal a significant decrease in the daily amplitude of the land-river surface thermal contrast; an increase of wind frequencies with E component at the surface as well as in the Ezeiza sounding, accompanied by a decrease of the mean wind speed of the latter. In order to evaluate the MBLM model capability to reproduce the observed changes during 1960-1990, two subperiods are chosen, i.e., 1967-1976 and 1982-1991. The wind direction frequency distribution and mean wind speed of the two subperiods are calculated with the MCLM model using the ensemble methodology. The results obtained with the model, despite reaching partial agreement with observations, show clear differences between the changes over land and over the river which are consistent with the observed ones.

The conclusion of the study is that the ensemble method is an appropriate methodology for determining high-resolution, low-level climatological wind fields, with the MBLM model applied to a region with a strong diurnal cycle of surface thermal contrast. The proposed methodology is of particular utility for synthesizing wind fields over regions with limited meteorological observations, since the 192-member matrix can be easily defined with few observing points, as well as in the case of relatively incomplete records.