A Simple Model for Low-Level High-Resolution Atmospheric Circulations over Coastal Areas of Complex Geometry, Suitable for Downscaling Climate Change Scenarios

GUILLERMO J. BERRI ; LAURA SRAIBMAN ; GABRIELA V. MÜLLER

Shanghai

Conferencia; Conference ECSA 53: Estuaries and coastal areas in times of intense change; 2013

A primitive equation, hydrostatic and incompressible mesoscale atmospheric boundary layer model (BLM) has been especially developed for simulating high-resolution low-level wind fields over coastal areas. The model is forced with boundary conditions defined from local weather observations, large scale model outputs, or a combination of both. When BLM runs with observations or short-term weather forecasts from other models, it becomes an excellent tool for diagnosing particular situations. By running BLM with reanalysis of other models or climate change scenarios, it is possible to construct climatological wind fields and evaluate the local effect of large scale atmospheric conditions and their variability. The physical and numerical model formulation, along with high-resolution coast geometry digitization, allows reproducing kilometer scale details of the atmospheric circulation normally unnoticed by other models.   BLM has been used in different studies of the La Plata River region in South America. High-resolution low-level climatological wind fields are calculated as the ensemble result of a series of daily forecasts obtained by forcing the model with limited local observations. This method employs a 192-member matrix that considers 16 wind directions and 12 wind speed classes. 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, 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 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.   The study conducted during a 30-year period shows an overall good agreement between the observed and the modeled surface wind climatological fields at thirteen weather stations in the region. The model represents very well the differences in the wind speed magnitudes and predominant wind direction sectors throughout the La Plata River region that displays a strong sea?land-breeze daily cycle. The surface wind forecasts obtained by forcing BLM with daily regional scale forecasts during a 6-month case study were always more accurate than the surface winds of the regional model. Interestingly, despite the large errors in surface winds of the regional scale forecasts, their outputs were able to drive BLM to obtain surface winds forecasts with smaller errors. The more appropriate definition of the land?river surface temperature contrast along with a high-resolution geometry of the river coasts are the fundamental advantages of BLM for resolving the smaller scale details of the low-level atmospheric circulation.   In conclusion, BLM is an efficient tool for physical downscaling of low-level atmospheric circulations, and the ensemble method is an appropriate methodology for synthesizing high-resolution wind fields, over coastal regions of complex geometry and strong diurnal cycle of surface thermal contrast.