Satellite-Based Estimation of Daily Average Net Radiation under Clear-Sky Conditions

ABSTRACT Daily average net radiation （DANR） is an important variable for estimating evapotranspiration from satellite data at regional scales, and is used for atmospheric and hydrologic modeling, as well as ecosystem management. A scheme is proposed to estimate the DANR over large heterogeneous areas under clear-sky conditions using only remotely sensed data. The method was designed to overcome the dependence of DANR estimates on ground data, and to map spatially consistent and reasonably distributed DANR, by using various land and atmospheric data products retrieved from MODIS （Moderate Resolution Imaging Spectroradiometer） data. An improved sinusoidal model was used to retrieve the diurnal variations of downward shortwave radiation using a single instantaneous value from satellites. The downward shortwave component of DANR was directly obtained from this instantaneous value, and the upward shortwave component was estimated using satellite-derived albedo products. Four observations of air temperature from MOD07_L2 and MYD07_L2 data products were used to derive the downward longwave component of DANR, while the upward longwave component was estimated using the land surface temperature （LST） and the surface emissivity from MOD1 l_L2. Compared to in situ observations at the cropland and grassland sites located in Tongyu, northern China, the root mean square error （RMSE） of DANR estimated for both sites under clear-sky conditions was 37 W m-2 and 40 W m-2, respectively. The errors in estimation of DANR were comparable to those from previous satellite-based methods. Our estimates can be used for studying the surface radiation balance and evapotranspiration.

Assessment of the methods for determining net radiation at different time-scales of meteorological variables

When modeling the soil/atmosphere interaction,it is of paramount importance to determine the net radiation flux.There are two common calculation methods for this purpose.Method 1 relies on use of air temperature,while Method 2 relies on use of both air and soil temperatures.Nowadays,there has been no consensus on the application of these two methods.In this study,the half-hourly data of solar radiation recorded at an experimental embankment are used to calculate the net radiation and long-wave radiation at different time-scales(half-hourly,hourly,and daily) using the two methods.The results show that,compared with Method 2 which has been widely adopted in agronomical,geotechnical and geo-environmental applications.Method 1 is more feasible for its simplicity and accuracy at shorter time-scale.Moreover,in case of longer time-scale,daily for instance,less variations of net radiation and long-wave radiation are obtained,suggesting that no detailed soil temperature variations can be obtained.In other words,shorter time-scales are preferred in determining net radiation flux.

Relationship between Net Radiation and Broadband Solar Radiation in the Tibetan Plateau

The characteristics of net radiation （Rn） （0.3-10 μm） in Lhasa and Haibei in the Tibetan Plateau were analyzed based on long-term in-situ measurements of surface radiation data. The monthly average of daily Rn reached a minimum during the winter period followed by an increase until May and then a decline until January. This variation is consistent with solar activity. The annual mean daily total Rn values were 0.92 MJ m^-2 d^-1 and 0.66 MJ m-2 d-1 in Lhasa and Haibei, respectively. A relationship between Rn and broadband solar radiation （Rs） was demonstrated by a good linear correlation at the two sites. Rn can be an accurate estimate from Rs. The estimated Rn values were similar to the observed values, and the relative deviations between the estimates and measurements of Rn were 2.8% and 3.8% in Lhasa and Haibei, respectively. The application of the Rn estimating model to other locations showed that it could provide acceptable estimated Rn values from the Rs data. Furthermore, we analyzed the influence of clouds on Rn by different clear index （Ks）, defined as the ratio of Rs to the extraterrestrial solar irradiance on a horizontal surface. The results indicate that more accurate results are associated with increased cloudy conditions. The influence of the albedo was also considered, but its inclusion in the model resulted in only a slight improvement. Because surface albedo is not usually measured, an expression based solely ou global solar radiation could be of more extensive use.

Relevant Analysis of Grassland Temperature and Ground Net Radiation in Guilin

[Objective] The aim was to study the relevance of grassland temperature and ground net radiation in Guilin.[Method] By dint of ground observation data and net radiation of national benchmark climate station in Guilin from 2007 to 2009,the changes of grassland temperature and ground net radiation were expounded and their relations were pointed out.[Result] The annual changes trends of grassland temperature and ground net radiation in Guilin were basically the same.Monthly average maximum value all appeared in summer(July to August).Monthly average lowest value appeared in winter(December to next January);monthly average grassland temperature and monthly average ground net radiation had positive relation.Grassland temperature and ground net radiation had basically same distribution in four seasons.The average largest value of ground net radiation in summer was the largest and average smallest value was the smallest;the average largest value of ground net radiation in cloudy days was the smallest and average minimum value was the largest;the daily difference was the largest in sunny day and daily difference was the smallest in cloudy day.The daily changes trend of grassland temperature and ground net radiation in different weather state were basically the same;when it was sunny or cloudy,the daily largest value of grassland temperature and ground net radiation occurred between 15:00 and 19:00;when it was overcast,there was no distinct peak and daily changes.The largest value of the daily extreme value of grassland temperature and ground net radiation took place from 12:00 to 15:00.The daily lowest value took place from 20:00 to 07:00 on the next day.[Conclusion] The study provided reference for the analysis of temperature changes in Guilin.

The Impact of Soil Moisture Availability upon the Partition of Net Radiation into Sensible and Latent Heat Fluxes

The impact of soil moisture availability on the Bowen ratio and on the partition of net radiation flux into sensible, latent and soil heat fluxes was investigated by using one-dimensional primitive equations with a refined soil parameterization scheme. Simulation results presented that as soil moisture availability increases, the Bowen ratio and the partition of net radiation flux into sensible and soil heat fluxes decrease. The partition of net radiation flux into latent heat flux, however, increases. Quantitative relationships between Bowen ratio and the partitions with soil moisture availability were also given in this study.

Advances in Monitorinq Methods of Downward Surface Net Radiation

Downward surface net radiation,the net energy of the earth's surface obtained through shortwave and longwave radiation process,is the main driving force for material and energy cycle in the whole earth system.In this study,the main research results of monitoring methods of downward surface net radiation at home and abroad in recent decades have been summarized,and main remote sensing radiation products produced according to various sensors have been introduced.Moreover,the monitoring methods of downward shortwave and long-wave radiation have been discussed,and their principles,advantages and disadvantages have been analyzed to provide scientific references for further study of downward surface solar net radiation in future.

Estimation of Evapotranspiration by Various Net Radiation Estimation Formulae for Non-Irrigated Grass in Brazil

The objective of this study was to assess the accuracy of estimating evapotranspiration (ET) using the FAO-56 Penman-Monteith (FAO-56-PM) model, with measured and estimated net radiation (Rnmeasured and Rnestimated, respectively), the latter obtained via five different models. We used meteorological data collected between August 2005 and June 2008, on a daily basis and on a seasonal basis (wet vs. dry seasons). The following data were collected: temperature;relative humidity;global global solar radiation (Rs);wind speed and soil heat flux. The atmospheric pressure was determined by aneroid barograph, and sunshine duration was quantified with a Campbell-Stokes recorder. In addition to the sensor readings (Rnmeasured), five different models were used in order to obtain the Rnestimated. Four of those models consider the effects of cloud cover: the original Brunt model;the FAO-24 model for wet climates;the FAO-24 model for dry climates, and the FAO-56 model. The fifth was a linear regression model based on Rs. In estimating the daily ET0 with the FAO-56-PM model, Rnmeasured can be replaced by Rnestimated, in accordance with the FAO-24 model for dry climates, with a relative error of 2.9%, or with the FAO-56 model, with an error of 4.9%, when Rs is measured, regardless of the season. The Rnestimated obtained with the fifth model has a relatively high error. The original Brunt model and FAO-24 model for wet climates performed more poorly than did the other models in estimating the Rn and ET0. In overcast conditions, the original Brunt model, the FAO-24 model for wet climates, the FAO-24 model for dry climates, the FAO-56 model and the model of linear regression with Rs as the predictor variable tended to overestimate Rn and ET, those estimates becoming progressively more accurate as the cloud cover diminished.

Significant discrepancies of land surface daily net radiation among ten remotely sensed and reanalysis products

Land surface all-wave net radiation(R_(n))is crucial in determining Earth’s climate by contributing to the surface radiation budget.This study evaluated seven satellite and three reanalysis long-term land surface R_(n)products under different spatial scales,spatial and temporal variations,and different conditions.The results showed that during 2000-2018,Global Land Surface Satellite Product(GLASS)-Moderate Resolution Imaging Spectroradiometer(MODIS)performed the best(RMSE=25.54 Wm^(-2),bias=-1.26 Wm^(-2)),followed by ERA5(the fifth-generation of European Centre for Medium-Range Weather Forecast Reanalysis)(RMSE=32.17 Wm^(-2),bias=-4.88 Wm^(-2))and GLASS-AVHRR(Advanced Very-High-Resolution Radiometer)(RMSE=33.10 Wm^(-2),bias=4.03 Wm^(-2)).During 1983-2018,GLASS-AVHRR and ERA5 ranked top and performed similarly,with RMSE values of 31.70 and 33.08 Wm^(-2)and biases of-4.56 and 3.48 Wm^(-2),respectively.The averaged multi-annual mean R_(n)over the global land surface of satellite products was higher than that of reanalysis products by about 10~30 Wm^(-2).These products differed remarkably in long-term trends variations,particularly pre-2000,but no significant trends were observed.Discrepancies were more frequent in satellite data,while reanalysis products showed smoother variations.Large discrepancies were found in regions with high latitudes,reflectance,and elevation which could be attributed to input radiative components,meteorological variables(e.g.,cloud properties,aerosol optical thickness),and applicability of the algorithms used.While further research is needed for detailed insights.

Spatial variation of net radiation and its contribution to energy balance closures in grassland ecosystems

Introduction:Low energy balance closure(EBC)at a particular eddy-covariance flux site has increased the uncertainties of carbon,water,and energy measurements and has thus hampered the urgent research of scaling up and modeling analyses through site combinations in regional or global flux networks.Methods:A series of manipulative experiments were conducted in this study to explore the role of net radiation(Rn)in the EBC in relation to spatial variability of vegetation characteristics,source area,and sensor type in three sites of the Inner Mongolian grassland of northern China.Results:At all three sites,the residual fluxes of EBC peaked consistently at 110 W m^(-2).The spatial variability in net radiation was 19 W m^(-2)(5%of R_(n))during the day and 7 W m^(-2)(16%)at night,with an average of 13 W m^(-2)(11%)from eight plot measurements across the three sites.Large area measurements of Rn significantly increased by 9 W m^(-2)during the day and decreased by 4 W m^(-2)at night in the unclipped treatments.Net radiation decreased by 25 W m^(-2)(6%of Rn)at midday and 81 MJ m^(-2)(6%)during a growing season with heavier regular clipping than that in unclipped treatments.The Rn was lower by 11–21 W m^(-2)(~20–40%of Rn)measured by CNR1 than by Q7.1 at night,while there was only 6 W m^(-2)(~1–2%of Rn)difference during the daytime between these two types of commonly used net radiometers.Conclusions:Overall,the inclusion of the uncertainty in available energy accounted for 65%of the~110 W m^(-2)shortfalls in the lack of closure.Clearly,the unclosed energy balance at these three grassland sites remains significant,with unexplored mechanisms for future research.

CLIMATOLOGICAL STUDIES ON NET RADIATION IN CHINA

An established climatic calculation scheme for estimating the components of surface radiation budget is conducted at 223 stations throughout the country. The spatial patterns of net radiation (NR) and its components suggest that the high-value belt is steadily situated over the Yarlung Zangbo Basin in the south- ern Qinghai-Xizang Plateau and over the eastern Inner Mongolian Plateau, with the low-value belt over Sichuan-Guizhou mountainous terrain. The pattern changes a little with the transition of seasons. Tem- porally, the NR's annual changes, especially its maximizing month, are closely related to the coming of rainy season. The relative computation error is 2.7% for global radiation (GR). 4.0% for surface albedo (SA), 9.4% for effective radiation (ER), and 8.9% for NR.

CLIMATIC CHARACTERISTICS OF ATMOSPHERIC NET RADIATION OVER CHINA

In terms of ERBE and ISCCP data.and measured/calculated surface net radiation, computation is performed of the climatic characteristics of ANR(atmospheric net radiation)across China alongside with the discussion and relations to other two kinds of radiation and contributing factors.Evidence suggests a high linear correlation of ANR with atmosphere-absorbed shortwave radiation.whereby can be established a general expression for ANR,which decreases more sharply as a function of altitude and increases slightly with latitude in summer,and changes uniformly in winter.Eventually,a comparison is made of the findings presented in this paper and literatures regarding the ANR pattern and magnitudes,indicating their great difference.

CLIMATIC CHARACTERISTICS OF NET RADIATION IN THE EARTH-ATMOSPHERE SYSTEM OVER CHINA

Investigated are effects of the total cloudiness and other factors on earth-atmosphere net radia- tion(EANR)and analyzed is its relation to other components and ground surface net radiation in the context of ERBE and ISCCP.Evidence suggests that planetary scale albedo and earth-atmo- sphere short wave absorption radiation have maximum effect on the net radiation under study,with the influence of cloud and latitude displayed predominantly through the two factors;OLR has rela- tively weak effect;the earth-atmosphere net radiation is well correlated with surface net radiation. Analysis is also performed of the geographic distribution of the earth-atmosphere net radiation throughout China,and the annual curve of the net radiation on a local basis is marked by high (low)value in summer(winter)with the impact of factors.including total cloudiness responsible largely for the shift of the months with maximum.

EVALUATION OF MODELS FOR ESTIMATION OF NET RADIATION FOR ALPINE SLOPING SURFACES

This paper describes and tests two models for estimating net radiation(or the radiation balance)on sloping surfaces of alpine environments.They are an empirical method based on the linear relationship between net radiation and global solar radiation and a flux-by-flux method involving the estimation of all the individual components of radiation budget independently.The results show that the empirical method is capable of predicting hourly net radiation on sloping sur- faces to within about±53 W m^(-2) under all sky conditions.During clear sky conditions,it could predict net radiation on slopes to within±58 W m^(-2) or 16% of the measured values.The flux-by-flux method,although it did not perform as well as the empirical method,performed adequately and could give estimates of net radiation on slopes with root mean square error of less than 74 W m^(-2)(20%)and a mean bias error of 27 W m^(-2)(7%).

THE CALCULATION AND ANALYSIS OF SLOPE NET RADIATION IN CHINA

By using the climatological calculating method for each component of slope surface net radiation proposed by the authors,calculations and analyses are done of the distribution features of slope net radiation in China with emphasis on the discussion of variations of slope net radiation in typical stations and sites with slope direc- tion,slope,latitude and season.The distribution features of net radiation on the north and south slopes are, for the first time,mapped and discussed,revealing the great difference on the national basis,and thus acquiring a new interesting result that the negative-value area of winter net radiation on the north slope(20°)can reach Yunnan and Guizhou Provinces and middle and upper reaches of the Changjiang River.

Parameterizations of Surface Radiation in the Semiarid Grasslands of Inner Mongolia under Clear-Sky Conditions Using MODIS Data

A set of improved and efficient radiation parameterization schemes for surface radiation balance components under clear- sky conditions was developed by using general surface measurements and MODIS data. The set of schemes was then adapted for regions similar to the present study sites under different grazing intensities and varying degrees of drought in the semiarid grasslands of Inner Mongolia. Specifically, we mainly improved two schemes for estimating downward shortwave and longwave radiation at the surface, which could be applied to regions with certain degrees of drought. The validation datasets were from ground-based observations at various grazing sites during the growing season （May to September） of different drought years, 2005 and 2006. Through comparisons of parameterized versus measured radiation values, the increased or modified factors in the original schemes demonstrated improved estimation accuracy, and the rationalities of input parameters and variables were analyzed. The regional instantaneous net radiation estimations had root-mean-square errors of less than 30 W m-2 compared with ground measurements at the sites during the study period. The statistical results showed the improved schemes are suitable for estimating surface net radiation in regional semiarid areas during the growing season. Analyses of the sensitivity of the schemes to corresponding variables were conducted to ascertain the major error sources of the schemes and potential variables for improving the performance of the schemes in agreement with observations.

Application study of the awning measure to obstruct solar radiation in permafrost regions on the Qinghai-Tibet Plateau

With globe warming, road safety will change dramatically, especially within the Qinghai-Tibet Plateau permafrost regions. Because of higher elevation and better almospheric transparency, the Qinghai-Tibet Plateau has stronger radiation than other regions, which can change the daily variation of ground surface temperature on the Plateau. The awning measure （shading board） is one of the actively protected permafrost measures, which was adopted along the Qinghai-Tibet railway and highway and the Qing-Kang Highway in China. Field test results show that embankment surface month mean net radiation is 60-130 W/m2, but the value is below 20 W/m2 under the shading board, and the reducing level of naRn-al net radiation is 80%-90%. The shading board reduced the heat flow entering into the embankment by 80%,90% or more, with heat entering into the soil on the common embankment, but emitting from the embankment under the shading board. At the same time, ground surface temperature under the shading board is 6-8 ℃ lower than the exposed embankment. Test results show that the shading board measure can rapidly and effectively reduce net radiation and heat flow into the embankment, decrease embankment surface and interior temperature, effectively delay increase rate of soil temperature under globe warming, ensure stability and safety of the embankment, and guarantee unblocked road projects in cold and permafrost regions.

刺槐蒸腾耗水的动态研究(英文)

To know the annual water consumption of forest, it is necessary to acquire the annual transpiration value of stands. This paper is based on the data measured in the typical weather of the growth season, from 1998 to 2000, with the LI 1600 Steady Porometer and the general weather information. The daily variation of transpiration in black locust forest ( Robinia pesudoacacia L.) is modeled by Penman Monteith equation. As a result of the model, a continuous daily transpiration in the growth season was calculated. The net radiation, intercepted by black locust forest canopy, was acquired from a semi empirical equation of measuring net radiation R n with the extinction coefficient k and leaf area index LAI . The canopy integral stomatic resistance is a mimesis with an empirical equation of measuring data. Compared with measuring data, the relative error of the modeled ones is less than 12% averagely. At last, the total transpiration of black locust forest during the period of 1998 and 2000 in the growth season of May to October, as an average transpiration of the different density stands, were 192 46, 187 07 and 195 59?mm respectively.

Evapotranspiration and Its Energy Exchange in Alpine Meadow Ecosystem on the Qinghai-Tibetan Plateau

To understand the water and energy exchange on the Qinghai-Tibetan Plateau, we explored the characteristics of evapotranspiration （ET） and energy fluxes from 2002 to 2005 over a Kobresia meadow ecosystem using the eddy covariance method. The ratio of annual ET to precipitation （P） of meadow ecosystem was about 60%, but varied greatly with the change of season from summer to winter. The annual ET/P in meadow was lower than that in shrub, steppe and wetland ecosystems of this plateau. The incident solar radiation （Rs） received by the meadow was obviously higher than that of lowland in the same latitude; however the ratio of net radiation （Rn） to Rs with average annual value of 0.44 was significantly lower than that in the same latitude. The average annual ET was about 390 mm for 2002-2005, of which more than 80% occurred in growing season from May to September. The energy consumed on the ET was about 44% of net radiation in growing season, which was lower than that of shrub, steppe and wetland on this plateau. This study demonstrates that the Kobresia meadow may prevent the excessive water loss through evapotranspiration from the ecosystem into the atmosphere in comparison to the shrub, steppe and wetland ecosystems of the Qinghai-Tibetan Plateau.

Surface energy balance of Keqicar Glacier, Tianshan Mountains, China, during ablation period

The meteorological data of ablation season in 2005 were recorded by two automatic weather stations on Keqicar Glacier, in the southwest Tianshan Mountains of China. One is operated on the glacier near the equilibrium line with an altitude of 4,265 m （Site A） and another is operated on the glacier ablation area with an altitude of 3,700 m （Site B）. These data were used to analyze the meteorological conditions and the surface energy balance （SEB） of Keqicar Glacier. Net radiation was directly measured, and turbulent heat fluxes were calculated using the bulk aerodynamic approach, including stability correction. The ablation value of 0.68 m w.e. derived from four ablation stakes is in close correspondence to the modeled value of 0.71 m w.e. During the observation period, net radiation accounts for 81.4% of the total energy with its value of 63.3 W/m2. The rest energy source is provided by the sensible heat flux with a value of 14.4 W/m2. Energy is consumed mainly by melting and evaporation, accotmting for 69.5% and 29.7% of the total energy with their values of 54.0 and 23.0 W/m2, respectively. Radiative energy dominates energy exchanges at the glacier-atmosphere interface, governed by the variation in net shortwave radiation. Net short-wave radiation varies significantly due to the effects of cloudiness and the high albedo caused by solid precipitation. Wind speed influences the turbulent heat fluxes distinctively and sensible heat flux and latent heat flux are much larger in July with high wind speed.

CONCENTRATION AND FLUX OF CO_2, TURBULENCE FLUXES AND RADIATION BALANCE IN THE NEAR SURFACE LAYER OVER WHEAT FIELD

The gradient of CO_2 concentration, photosynthetically active radiation (PAR). net radiation. soil heat flux, profiles of wind speed, and air temperature and humidity were measured above a wheat field during May and June 1985 at Beijing Agro-Ecosystems Experimental Station. Beijing, China. Fluxes of carbon dioxide, sensible heat, latent heat and momentum were calculated by using the aerodynamic method. The observation site. equipment, calibration techniques, the errors associated with the measurement, and the computational procedures are described. The results show that the diurnal variations of amplitude of CO_2 concentrations were 103.8 to 27. 0. 86. 3 to 22.8 and 69.8 to 11.6 ppm: the average CO_2 concentrations were 331.5. 339.9 and 364.6 ppm for the photosynthesis type, and 369.6. 364.0 and 375.2 ppm for the respiration type at 1. 2 and 10 m above surface, respectively, from May 14 to June 15. In the daytime, transfer direction of the CO_2 fluxes and gradients is from air to crop canopy, and at noon (1100 to 1300 BT (Beijing Time)) the transfer rate reaches negative maximum value. At night, transfer of CO_2 fluxes and gradients is in the reversed direction and reaches positive maximum in the early morning (0400 to 0600 BT). There are strong negative correlations between CO_2 flux and the net radiation (Rn), available energy (H+LE). photosynthetically active radiation (PAR) and momentum flux (τ).