long term spatiotemporal changes of net radiation flux over iran

1. introduction the process by which the net radiation is transformed to the atmospheric dynamical energy, occur to a large degree at the interface between the surface and the atmosphere. as the driving force for such process, an accurate estimation of the net radiation at the earth’s surface is a necessary input for surface process models. net radiation is a key component in the surface radiation budget. net radiation (rn) at the earth's surface drives the process of evaporation, photosynthesis, and heating of soil and air. net radiation is the difference between the down welling and upwelling radiation fluxes at the surface, including long wave and shortwave radiation. down welling shortwave radiation, rs↓, at the surface results from scattering, emission and absorption within the entire atmospheric column; while upwelling shortwave radiation can be estimated by rs ↓ and surface albedo. down welling long wave, rl↓, and upwelling long wave radiation, rl↑, are characterized by near-surface air temperature, air emissivity, surface temperature and emissivity. net radiation and the overall surface energy budget are important for the development of the planetary boundary-layer. its quantification over hetero-genous land surfaces is crucial to study land–atmosphere interactions. solar radiation data are required in different fields of researches including architecture, active and passive solar energy systems, agriculture, meteorology and climatology. the estimation of net radiation at the earth’s surface is important for a number of applications including climate monitoring, regional solar energy availability assessment for heating and electrical power generation purposes and for the evaluation of the cloud and radiation parameterization used in weather and climate models. measurement of solar radiation is more difficult than other meteorological variables. this is attendant with problems and errors such as technical failure and operation-related problems. different sources of errors related to solar radiation measurement can be categorized into two classes: equipment technical failure and operation-related problems. there exists error and uncertainty with any measurement that can be systematic and/or random. the most common source of error in measuring solar radiation is systematic and resulted from the sensors and their construction. but the most important error is random and is due to operational, maintenance and reading instruments. precise spatiotemporal distribution of net radiation component in arid/ semiarid areas such as iran are critical information for sustainable management of natural resources as well as for a better understanding of water and heat exchange process between the land surface and the atmosphere. such information can be achieved using interpolating measured irradiance by ground stations. but, availability of solar radiation data in iran is limited by the sparsity of the existing networks. however the distribution of surface fluxes over large areas is difficult to obtain from ground measurement alone. therefore, their prediction from reanalysis data is very attractive since it enables large area coverage with a high repetition frequency. the main objective of this study yields accurate estimates the spatiotemporal variability of the net radiation components over iran using reanalysis data. 2. study area the study area for this study is whole extent of iran, which lies approximately between 25_n and 40_n in latitude and between 44_e and 64_e in longitude. based on the koppen climate classification, most parts of iran’s area are categorized as generally having arid (bw) and semi-arid (bs) climates. the important mountains of iran are alborz and zagros, which play an important role in non uniform spatial and temporal distribution of radiation components in iran. 3. material and methods we used daily reanalysis data over iran for the period 2000-2010 derived from climate forecast system reanalysis (csfr) dataset with spatial resolution less than 0.5°lat/ lon. the surface energy balance components for land surface are discussed using: rn-hl+hs+hg=0 where rn is the net radiation flux (wm-2), hg is the soil heat flux (wm-2), hs is the sensible heat flux (wm-2) and hl is the latent heat flux (wm-2). net radiation also known as net irradiative balance, that is the balance of incoming solar radiation and outgoing terrestrial radiation, which varies with latitude and season. net radiation is generally positive by day and negative by night. the net radiation rn is obtained from the radiation balance between net shortwave and net long wave radiation at the land surface. rn=rs↓-rs↑+rl↓-rl↑ here rs↓ is the incoming shortwave radiation (wm-2), rs↑ is the outgoing shortwave radiation (wm2), rl↓ is the incoming shortwave radiation (wm-2) and rl↑ is the outgoing longwave radiation (wm2).in this study, the flux of net radiation (rn) is considered positive when it is directed toward the surface and vice versa. 4. results and discussion precise spatiotemporal distribution of surface energy balance components in arid/ semiarid areas such as iran are critical information for sustainable management of natural resources as well as for a number of applications including climate monitoring, regional solar energy and radiation parameterization used in weather and climate models. in the spring and summer maximum values of net radiation due to increase of solar height were observed over zagros mountain and azarbaijan region. in this regions land surface more warming and reinforce sensible heat flux that cause severe convective precipitation. the highest values of net radiation in period between may to august seems will be seen in southeastern parts of iran but due to monsoon, increase of clouds and decrease of incoming flux, it occurred in central parts of zagros mountain and upper latitudes. in the other hand, the maximum values of this parameter in precipitation period, from october to may, were detected in southeastern parts of iran like iranshahr. 5. conclusion information regarding net radiation regime is essential for policy makers and managers within the context of water resources management, hydrology, agriculture, and environment. hence, it indicates the need for more attention to climate change and different aspects of its effect in the weather regime of iran. the findings presented here on the spatiotemporal variability of iran’s net radiation can be implemented to improve the water and solar energy resources strategies in the study region. future studies would be attractive to examine the probable effects of climate change on the net radiation. in this study we use the csfr network products in two local times daily (06:00, 12:00), to calculate the spatiotemporal variability of the net surface solar radiation over iran. the results of this study indicate that long-term mean net radiation energy flux has a sinusoidal behavior and the net radiation values in 06:00 hour is negative and outgoing of energy more than of incoming flux. the cooling of the surface was observed in this time for whole of the country. moreover, the warming of the surface and positive values were observed at 12:00 hour because the net incoming flux is more than the net outgoing flux and energy saved over surface. overall, the seasonal variability of net radiation flux shows that it affected by height of solar radiance, general circulation and local phenomena. the value of net radiation fluxes have peaked around the summer solstice and the minimum amount that can be seen at the winter solstice. in this time the maximum flux is seen at about 720 watts in june over the central zagros and damavand mountains, while the minimum flux of 200 watts is seen in north of ardabil and kopedagh in december. monthly changes in energy flux that reflect the effects of changes in sun angle atmospheric circulation and local phenomena. in spring and summer, the sun's altitude increases and the maximum flux of energy shifted towards higher latitudes and the zagros and azerbaijan region have more flux. in the spring, the amount of flux in the regions of azerbaijan increases due to warming of the earth's surface and sensible heat flux that resulting in the convection precipitation. in the months of may through august is expected the maximum of this flux to be observed in the southeastern regions of the country. due to the arrival of monsoon systems and increase cloudiness, input fluxes decreased and the maximum flux occurs at higher latitudes, especially in the central zagros. in the rainy season, from october to may, because of the reduced height of the sun and the shift of the maximum flux into the lower latitudes, maximum flux occurs in the southeastern region of the country, especially the east iranshahr. in general, it is necessary to consider the maximum flux in the area east of iranshahr, and really high peaks of the zagros. it seems to be considered regions such as the zagros region, watershed north of khuzestan and the mountains of hezar-o-lalezar to gawkhoni swamp lack of flux. the results also suggest the need for further investigation on local scale, which could be one of the major causes of evaporation and evapotranspiration.