A Model to Estimate Global Radiation in Complex Terrain

Global radiation is an important parameter necessary for most ecological models. However, in situ data barely meets the needs of modelling mountainous ecosystems since most field stations are located in flat areas. Consequently, it is usually necessary to extrapolate radiation measurements obtained from an adjacent flat area to the complex terrain of concern. The distribution of radiation in complex terrain depends upon two factors: the local atmospheric conditions, which determine the radiation potentially available to a supposed flat surface in a given location, and the topographic effects on this possible radiation. The latter have been included in detail in most radiation models for complex terrain, but the former are often only simply treated as constant or estimated by over-simplified empirical algorithms. In this paper we propose a novel model that uses a parametric atmospheric model to calculate the potential radiation for a supposed flat surface in a given location, and then account for topographic effects. Direct radiation, diffuse radiation and reflected radiation are calculated separately in the model due to the distinctive characteristics of and the effects by topography. Based on the parametric model, this paper has investigated the relationship between radiation transmittance, clearness indices and altitude under a series of water vapour content and turbidity conditions. This combines three ratios, Rb, Rd, and Rr, defined as the direct radiation, diffuse radiation and reflected radiation received by the arbitrary surface, respectively, to their counterparts in the horizontal surface, to estimate the global radiation for any given location. The model has been validated with data from measurements in National Park Berchtesgaden, Germany, where six measurement sites with various altitudes and topographic characteristics have been deployed. The r2 of modelled and measured hourly global radiation are greater than 0.90 in all six sites, with RMSE varies from 16 to 100 W m−2. Sensitivity analysis revealed that the model was not sensitive to change in water vapour content, which suggests the possibility to use an exponential algorithm of water vapour content when there is no in situ water vapour content information in complex terrains. The NRMSE was only reduced by 0.04, on average, in five of the six sites when water vapour content information was calculated from the in situ air temperature and relative humidity measurements.

[1]  C. Gueymard Analysis of monthly average atmospheric precipitable water and turbidity in Canada and Northern United States , 1994 .

[2]  Ralph Dubayah,et al.  Topographic Solar Radiation Models for GIS , 1995, Int. J. Geogr. Inf. Sci..

[3]  C. G. Justus,et al.  A Model for Solar Spectral Irradiance and Radiance at the Bottom and Top of a Cloudless Atmosphere , 1985 .

[4]  Peter E. Thornton,et al.  Simultaneous estimation of daily solar radiation and humidity from observed temperature and precipitation: an application over complex terrain in Austria. , 2000 .

[5]  Robert E. Davis,et al.  Statistics for the evaluation and comparison of models , 1985 .

[6]  K. I. Kondratʹev Radiation in the atmosphere , 1969 .

[7]  Lucas Alados-Arboledas,et al.  Performance of global to direct/diffuse decomposition models before and after the eruption of Mt. Pinatubo, June 1991 , 1996 .

[8]  Benjamin Y. H. Liu,et al.  The interrelationship and characteristic distribution of direct, diffuse and total solar radiation , 1960 .

[9]  J. Duffie,et al.  Estimation of the diffuse radiation fraction for hourly, daily and monthly-average global radiation , 1982 .

[10]  W. Beckman,et al.  Diffuse fraction correlations , 1990 .

[11]  L. Wald,et al.  On the clear sky model of the ESRA — European Solar Radiation Atlas — with respect to the heliosat method , 2000 .

[12]  L. Alados-Arboledas,et al.  Parametric models to estimate photosynthetically active radiation in Spain. , 2000 .

[13]  J. Orgill,et al.  Correlation equation for hourly diffuse radiation on a horizontal surface , 1976 .

[14]  R. Bird,et al.  Simple Solar Spectral Model for Direct and Diffuse Irradiance on Horizontal and Tilted Planes at the Earth's Surface for Cloudless Atmospheres , 1986 .

[15]  C. Rao,et al.  Photosynthetically active components of global solar radiation: Measurements and model computations , 1984 .

[16]  Quan Wang,et al.  Diffuse PAR irradiance under clear skies in complex alpine terrain , 2005 .

[17]  J. Garrison,et al.  Estimation of precipitable water over the United States for application to the division of solar radiation into its direct and diffuse components , 1990 .

[18]  D. C. Robertson,et al.  MODTRAN: A Moderate Resolution Model for LOWTRAN , 1987 .

[19]  James N. Kochenderfer,et al.  Topography and radiation exchange of a mountainous watershed , 1995 .

[20]  G. Campbell,et al.  On the relationship between incoming solar radiation and daily maximum and minimum temperature , 1984 .

[21]  Oleg Antonić,et al.  Modelling daily topographic solar radiation without site-specific hourly radiation data , 1998 .

[22]  G. D. Robinson Absorption of solar radiation by atmospheric aerosol, as revealed by measurements at the ground , 1962 .

[23]  J. Calbó,et al.  Modelled and measured ratio of PAR to global radiation under cloudless skies , 2002 .

[24]  J. Garrison A study of the division of global irradiance into direct and diffuse irradiance at thirty-three U.S. sites , 1985 .

[25]  John C. Gallant,et al.  TAPES-G: a grid-based terrain analysis program for the environmental sciences , 1996 .

[26]  Yong Q. Tian,et al.  Estimating solar radiation on slopes of arbitrary aspect , 2001 .

[27]  Christian Gueymard,et al.  An atmospheric transmittance model for the calculation of the clear sky beam, diffuse and global photosynthetically active radiation , 1989 .

[28]  S. Running,et al.  An improved algorithm for estimating incident daily solar radiation from measurements of temperature, humidity, and precipitation , 1999 .

[29]  L. Swift,et al.  Algorithm for solar radiation on mountain slopes , 1976 .

[30]  Christian A. Gueymard,et al.  A two-band model for the calculation of clear sky solar irradiance, illuminance, and photosynthetically active radiation at the earth's surface , 1989 .

[31]  M. Iqbal An introduction to solar radiation , 1983 .

[32]  S. Klein Calculation of monthly average insolation on tilted surfaces , 1976 .

[33]  A.A.L. Maduekwe,et al.  Hourly global and diffuse radiation of Lagos, Nigeria. Correlation with some atmospheric parameters , 1994 .

[34]  Christian A. Gueymard,et al.  Turbidity Determination from Broadband Irradiance Measurements: A Detailed Multicoefficient Approach , 1998 .

[35]  A. Berk MODTRAN : A moderate resolution model for LOWTRAN7 , 1989 .