EuroSun2008-13

USING NWP MODELS FOR SOLAR RADIATION ESTIMATES. IN A COMPLEX-TOPOGRAPHY AREA IN SOUTHEASTERN SPAIN

D. Pozo-Vazquez1*, V Lara-Fanego1, H. Al-Samamra1, J. A. Ruiz-Arias1, A. Molina2 and J.

Tovar-Pescador1

1 MATRAS Group, Department of Physics,

2 Department of Computer Sciences
University of Jaen, Campus Lagunillas, 23071, Jaen, Spain
* Corresponding Author, dpozo@ujaen. es

Abstract

The solar radiation plays a major role in the energy exchange process between the atmosphere and the earth surface and is, therefore, a key parameter in a wide range of studies related to agriculture, hydrology, ecosystem modelling or renewable energy. It is known that complex topography significantly modifies radiation fluxes at the earth’s surface. Nevertheless, terrain effects on radiation fluxes induced by aspect, slope, sky view factor and shadowing are normally neglected in numerical models when horizontal resolution is lower than 10 km. As spatial resolutions of mesoscales models increase (1-2 Km) the topographic effect on the solar radiation might be considerable, especially at low solar-height angles. Fine-scale non-hydrostatic numerical models, such as PSU/NCAR MM5, are able to include the effects of the slope and aspect on the solar radiation estimates.

In this work we analyze the reliability of solar radiation estimates provided by the MM5 in complex topography. Particularly, hourly global solar radiation values for clear-sky days were obtained based on several MM5 simulations. The experiment was carried out for an area located within the Sierra-Magina Natural Park (Jaen, Southeastern Spain). This area is characterized by a relatively complex topography, with elevations ranging from 600 to 2100 m. MM5 estimates were tested against field data measured at 11 radiometric station located in an area of 20 km x 20 km inside the Park. The location of these radiometric stations covers a wide range of elevations, aspects and slopes. Four experiment was conducted, one per season, corresponding to three consecutive clear-sky days collected along the year 2006. Two 1 km resolution simulations were carried out for each experiment: one including the MM5 topographic parameterization and one without including these effects. The comparative analysis of the results allows both knowing the effect of topography on MM5 high-resolution solar radiation estimates and how the slope and aspect parameterization of the MM5 deals with this problem. Finally, the results were analyzed on the light of the different topographic characteristics of the 11 stations.

Results showed, firstly that, compared to observations, an important improvement is obtained both for temperature and radiation when including the topographic effects in the MM5 simulations. Additionally, the model tends to underestimate the solar radiation in morning day hours and to overestimate the values in the central day hours. Finally, results showed, that the difference between the estimated and measured solar radiation increases when the topographic complexity increases.

Keywords: Global Solar Radiation, MM5, NWP, Complex Topography, Andalusia.

1. Introduction

The renewable energies have the advantage of a smaller incidence in the environment in comparison with other energy sources; however, their production is conditioned by variations in the weather and in the climate. Therefore, although the renewable sources of energy can liberate us partially from dependence of the fossil fuels, they introduce another complicated dependence: the weather and the climate. In Spain, in the next future a strong increment in the electricity production based on solar resources is expected. This strong increment of the dependence of the renewable energy resources, along with their inherent variability, highlights problems related to the security and management of the supply. The future success of the renewable energy will be associated with an appropriate evaluation of the available resources and a correct forecast of its variability. This keeps not only for homogenous flat terrain, the usual location for thermosolar or great photovoltaic power plant, but also in complex topography areas, where many small PV power plants are located. It is in this context where the detailed knowledge of the available solar energy resources and its variability has a strategic importance.

Along the last decades, numerous methodologies have been proposed to address the problem of mapping the solar radiation. Due to technical and economical constraints both the spatial density and temporal coverage of solar radiation measurement are considerable lower that for the case of other key climate variables, as the temperature or precipitation. Additionally, the solar radiation can show a considerable spatial and temporal variability associated with topographic features. Given these constraints, the methodologies used for solar radiation mapping ranges from to the use of satellite estimates to, more recently, the use of Geographic Information System (GIS)-based solar radiation models and, traditionally, the use of the well known interpolation techniques. The recent developments of Numerical Weather Prediction (NWP) models makes these models a promising tool for solar resources evaluation and forecasting, even for complex topography areas. The main advantage, among other, of these models is that they allows not only to estimates the resources but also to forecast these resources. Nevertheless, the use of NWP models to estimate and forecast the solar radiation is still very limited.

It is known that complex topography significantly modifies radiation fluxes at the earth’s surface. Nevertheless, terrain effects on radiation fluxes induced by aspect, slope, sky view factor and shadowing are normally neglected in NWP when horizontal resolution is lower than 10 km. As spatial resolutions of NWP models increase (1-2 Km) the topographic effect on the solar radiation might be considerable, especially at low solar-height angles. Fine-scale non-hydrostatic numerical models, such as the Fith-Generation Penn State University/National Center for Atmospheric Research (PSU/NCAR) Mesoscale Model (Grell et al., 1994), known as MM5, , are able to include the effects of the slope and aspect on the solar radiation estimates. The MM5, is a non-hydrostatic, vertical sigma-coordinate model designed to simulate mesoscale atmospheric conditions. It allows simulating the climate with spatial resolution up to 1 km. The MM5 allows to optionally take into account the topography effects on the solar radiation at the earth surface. Particularly, the models includes parameterization that takes into account the effect of the slope, aspect and shadow cast caused by the topography on the solar radiation.

In this work we analyze the reliability of solar radiation estimates provided by the MM5 in complex topography areas and evaluate the importance of these topographic parameterization of the MM5. Particularly, hourly global solar radiation MM5 estimates values for clear-sky days were obtained using and no using the topographic parameterization of the MM5 model. The study was carried out for region of Sierra Magina (Jaen), a Natural Park characterized by a relatively complex topography. Results were evaluated using a set radiometric stations located in this Park.

2. Experiment design

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