Estimation Of Net Radiation Research Articles

Estimation of surface net radiation in the boreal forest and northern prairie from shortwave flux measurements

The relationships between net radiation and surface shortwave fluxes were explored by using one year of 15 min surface flux data for nine sites in central Canada. The sites were located throughout a 50,000 km2 area, in conjunction with the BOREAS project, across a gradient in surface cover from prairie grassland to boreal forest. When insolation alone was used to estimate instantaneous net radiation, the r2 values at the nine sites ranged from 0.93 to 0.98. The use of the net shortwave flux as a predictor improved the range in r2 values to between 0.96 and 0.99. The root‐mean‐square error across sites varied from 26 to 41 W m−2 without albedo to a range of 18–36 W m−2 when albedo was included. The data for all sites were combined to produce a single linear relationship with an r2 value of 0.97 and a root‐mean‐square error (RMSE) of 29 W m−2 when the surface albedo is included. Cross validations were performed where the data for eight sites were used to predict net radiation at a ninth (excluded) site, revealing little site‐specific bias. Classifications of the data by season and by transmittance provided little improvement in estimated net radiation at any site. Our study suggests that a single equation using net shortwave flux alone can be used to estimate net radiation for large portions of the boreal forest and northern prairie, regardless of spatial and temporal variation in surface cover and atmospheric conditions.

Bowen ratio estimates in return flow over the Gulf of Mexico

The Bowen ratio (and associated turbulent fluxes at the ocean‐air interface) has been estimated over the Gulf of Mexico's continental shelf. These estimates are time‐averaged values during the outflow phase of a particular return flow event. The study uses standard buoy data and oceanic soundings from the Louisiana‐Texas Shelf Physical Oceanography Program (LATEX). Since the buoys are not equipped with water vapor sensors, the latent heat flux is determined indirectly. The calculation is made possible by combining the buoy data with a one‐dimensional oceanic slab model and climatological estimates of net radiation. The application also requires a locally isothermal ocean. Estimates have been made for the return flow event of November 3–12, 1992, which exhibited maximum ocean‐air temperature differences of ∼10°C. During a 2 day period when the ocean‐air differences were near their maximum and when mixing extended to the seafloor beneath the buoy sites, the Bowen ratio is found to be 0.38±0.06 (0.35±0.08) at buoy sites at depths of 14 (24) m. The corresponding estimates of sensible and latent heat flux are 115±7 (115±7) and 302±48 (329±75) W m−2, respectively. These values compare favorably with results from other field programs that used bulk aerodynamic methods to estimate turbulent fluxes in cold air outflow. The methodology appears to be generally applicable to return flow events in the Gulf of Mexico.

Analysis and modeling of the radiation budget and net radiation of a sandhills wetland

The surface radiation budget consists of the streams of incoming and outgoing shortwave and longwave radiation flux. Net radiation. the algebraic sum of these four terms, is very significant because it represents the amount of energy available to drive surface climatic, hydrologic, and biological processes. Heterogeneity of the surface is an important factor controlling the spatial variability of the radiation budget. Wetland surfaces such as those in the Nebraska Sandhills are a complex mosaic of cover types and surface conditions and, thus, have potential for great variability in their surface radiation and energy microclimate. In this study, radiation budget data collected using fixed measurement stations and mobile instruments were used to assess the degree and causes of variability in the radiation regime within a typical Sandhills wetland. The site was stratified into four subsystems, high and low marsh, subirrigated meadow, and open water. Results show that there is no significant variability of radiation regime within the marsh, but the marsh, meadow, and water are all separable from each other. Surface albedo appears, to be a significant factor separating the subsystems, while the longwave energy balance functions as an “homogenizing” influence. Further analysis indicates that accurate estimates of net radiation can be made using only measurements of solar radiation.

The Estimation of Emitted and Reflected Energy over the Central Pacific Using Meteorological Parameters

The authors conducted a quantitative investigation of the relationship between meteorological parameters (chiefly cloud cover) and the principal components of the earth's radiation budget. The area of study is the tropical central Pacific, where considerable variation in cloudiness and flux was observed from 1985 to 1989. The observed variations were in response to the El Nino-Southern Oscillation event of 1987. International Satellite Cloud Climatology Project (ISCCP) C2 parameters were used as independent variables in the development of multiple linear models to predict Earth Radiation Budget Experiment (ERBE) outgoing longwave radiation (OLR) and shortwave (SW) fluxes. Net radiation estimates were obtained from OLR and SW predicted fluxes. The technique of all-subsets regression was used to determine which combination of ISCCP C2 parameters could best predict OLR and SW fluxes. The models were developed for the years 1985 and 1989 (non-ENSO years) and tested by both month and year on the years 1986 through 1988. Predicted fluxes were obtained for three 15° latitude zones, north (7.5° to 22.5°N), central (7.5°S to 7.5°N), and south (7.5°S to 22.5°S). Over the 60 months of data, explained variances (R 2 ) of over 90% for the development and test periods were typical. An increase in the accuracy of the OLR models was observed when noncloud variables were included. This accuracy improvement was most apparent when cloud amounts were either very low or very high and homogeneous. Biases (predicted-observed) were all less than 4 Wm −2 and rms estimates were within the range of uncertainty for ERBE monthly mean flux estimates. Flux estimates based on linear models could serve as a means of extending long-term radiation balance datasets during intervals of time with limited satellite coverage

Meteorological variability and air‐sea fluxes at a closely spaced array of surface moorings

Data from an array of five surface moorings, deployed in a region of transient ocean fronts with separations of between 17 and 55 km, were used to document meteorological variability at an open ocean site and to look for evidence of links between the surface meteorology and air‐sea fluxes and the gradients in sea surface temperatures. Averages over the array were also compared to historical and model‐produced data. Spatial variability in surface meteorology was observed infrequently, and much ofthat was associated with atmospheric fronts and periods of low winds. Air‐sea fluxes computed using the bulk formulae included spatial variability linked to the ocean, but spatial gradients in ocean surface velocities introduced a larger signal in the bulk formulae fluxes than did the gradients in sea surface temperature. The greatest differences between the observations and climatological data were seen in relative humidity and sea surface temperature. The bulk estimate of net longwave radiation was the heat flux component most different from the climatology. Wind stress time series from numerical weather prediction models were in best agreement with the observations at periods of a few days and longer.

An approach to the estimation of surface net radiation in mountain areas using remote sensing and digital terrain data

An approach is proposed to estimate the net radiation load at the surface in mountain areas. The components of the radiation balance are derived using a radiative transfer model combined with remotely sensed and digital terrain data. Integrated shortwave (0.28–6.00 µm) and longwave irradiances (3.00–100.00 µm) are computed using a modified version of the Practical Improved Flux Method (PIFM) of Zdunkowski et al. (1982) which makes use of digital topographic data in order to account for slope, aspect, and shading effects. Surface albedo and thermal exitance estimates are obtained using Landsat Thematic Mapper (TM) and digital terrain data combined with the LOWTRAN 7 atmospheric model (Kneizys et al., 1988). LOWTRAN 7 is utilized together with a set of terrain modeling programs to compute direct and diffuse sky irradiance for selected TM bands, and to remove atmospheric effects within the visible, near-infrared, mid-infrared, and thermal infrared bands of Landsat TM. Model testing in the Colorado alpine show a generally good correspondence between estimated values and field measurements obtained over comparable tundra surfaces during several field campaigns. The method is finally used to produce 1) maps of the components of the radiation balance at the time of Landsat TM overflight and 2) maps of daily totals of shortwave irradiance and net shortwave radiation on a typical summer day in the Colorado Rocky Mountains (i.e. including cloud cover effects). The results indicate that the proposed approach is particularly suitable for obtaining estimates of net radiation at the surface from the toposcale to the regional scale.

EFFECT OF VIEWING GEOMETRY ON REMOTELY SENSED NET RADIATION MEASUREMENTS FOR VEGETATED AND NON-VEGETATED SURFACES

Increased use of remotely sensed data for estimation of land-surface climatological variables such as the radiation and energy balance has underscored the need to understand errors inherent in remote sensing systems. The effects of viewing geometry on the signal received at the sensor have been examined for a variety of surface types, but systematic study of how sensor viewing geometry may introduce error into remote measurements of surface climate parameters is lacking. In this paper, close-range remote sensing is used to assess error caused by viewing geometry in remote measurements of net radiation, an important surface climate variable. Estimates of net radiation were made by inserting remotely sensed values of albedo and surface temperature into the surface radiation budget equation. Two contrasting surfaces, one vegetated and one bare, were evaluated at seven view angles and five azimuth angles. The remote net radiation measurements for each combination of view and azimuth angle then were compared t...

A model of net radiation over suburban snowpacks

Fieldwork by Xu and Buttle ( Proc. East. Snow Conf. 43, 173–184, 1987) has identified enhanced net radiation around homes as a contributing factor to the increased snowmelt rates observed in suburban environments relative to rural areas. A simple steady-state building climatology model was developed and linked to a ripe snowpack model to examine the effect of suburban homes on the net radiation balance over the sorrounding snowpacks. An error estimation of simulated hourly net radiation was determined by comparing model-predicted and observed net radiation at two suburban homes in Peterborough, Ontario, during a snowmelt period in March 1985. Results demonstrate that the model effectively simulates the complex mean hourly daytime net radiation inputs over ripe suburban snowpacks under all-sky conditions.

Validation of a rediation model for estimation of longwave net radiation at the surface

Validation of a rediation model for estimation of longwave net radiation at the surface

Precipitation Rates in the Tropics Based on theQ1-Budget Method: 1 June 1984–31 May 1987

The 'apparent' heat source method (Q1 budget) is used to compute the total derivative of dry static energy from 30 deg N to 30 deg S for the period June 1, 1984-May 31, 1987. The dataset is produced from the ECMWF global analyses and consists of twice-daily values of temperature, geopotential height, horizontal wind components, and vertical velocity at increments of 2.5 x 2.5 deg lat/long at seven pressure levels. Vertically integrated values of ds/dt, which are equal to total diabatic heating, Q1, are combined with estimates of net columnar radiation and surface sensible heat exchange to compute mean monthly precipitation rates, P0, as the residual in the Q1 budget. The accuracy of these P0 values is thoroughly examined, and it is suggested that the technique produces reliable estimates of precipitation over tropical oceanic areas on a monthly basis. Time series of mean monthly P0 for several geographic regions of the Southern Hemisphere tropics and the equatorial western Pacific (TOGA-COARE region) reveal that (1) the South Pacific convergence zone has the highest precipitation rates in the Southern Hemisphere; (2) a clear and distinct seasonal cycle is prominent in all regions; and (3) the 1986-87 ENSO event is easily identified, particularly in the TOGA-COARE region.

Estimation of net radiation and surface heat fluxes using NOAA-7 satellite infrared data during fair-weather cloudy situations of Mesogers-84 experiment

Estimation of radiation during fair weather cloudy situations of the MESOGERS-84 experiment has been examined using micrometeorological observations and satellite data. Diurnal variation of cloudiness is empirically determined using satellite information as a function of global radiation, and relationships between net incoming radiation and global radiation are analyzed. Particularly, it has been found that a very simple relationship between global radiation, cloudiness and net radiative heat flux proposed by Nielsenet al. (1981) can be used with satellite data and applied to the Mesogers region in Southwest France. The different relationships between cloudiness and radiation are utilized to modify and to validate Taconet'set al. model (1986) to get fluxes related to a cloudy situation without advection.

Spectral estimates of net radiation and soil heat flux

Conventional methods of measuring surface energy balance are point measurements and represent only a small area. Remote sensing offers a potential means of measuring outgoing fluxes over large areas at the spatial resolution of the sensor. The objective of this study was to estimate net radiation (R n) and soil heat flux (G) using remotely sensed multispectral data acquired from an aircraft over large agricultural fields. Ground-based instruments measured R n and G at nine locations along the flight lines. Incoming fluxes were also measured by ground-based instruments. Outgoing fluxes were estimated using remotely sensed data. Remote R n, estimated as the algebraic sum of incoming and outgoing fluxes, slightly underestimated R n measured by the ground-based net radiometers. The mean absolute errors for remote R n minus measured R n were less than 7%. Remote G, estimated as a function of a spectral vegetation index and remote R n, slightly overestimated measured G; however, the mean absolute error for remote G was 13%. Some of the differences between measured and remote values of R n and G are associated with differences in instrument designs and measurement techniques. The root mean square error for available energy (R n - G) was 12%. Thus, methods using both ground-based and remotely sensed data can provide reliable estimates of the available energy which can be partitioned into sensible and latent heat under nonadvective conditions.

Deep Valley Radiation and Surface Energy Budget Microclimates. Part I: Radiation

Solar and longwave radiation data are presented for five sites in Colorado's 650 m deep semiarid Brush Creek Valley (39°32′N, 108°24′W) during September 1984. During the sunlit period of a nearly clear day, individual sites received 0.73–0.81 of the theoretical extraterrestrial solar radiation. Incoming solar radiation increased with elevation in the valley. Direct radiation made up 0.86– 0.88 of the downward shortwave flux. On average, 0.12–0.21 of the incoming shortwave radiation was reflected at the individual sites. Strong variations in reflected solar radiation and outgoing longwave radiation occurred from site to site. Because of the large direct beam component, aspect and inclination angles of the valley surfaces had a strong effect on the solar radiation received. Contrasts between a southwest- and northeast-facing sidewall were significant. Shading from surrounding topography produced inter-site differences in both instantaneous and daily total radiation. Inter-site differences in most daily totals on a clear day were larger than standard deviations of the daily totals at a valley floor site computed over a 16-day period of variable weather. The ridgetop site, on account of its unobstructed view of the sky, had a higher average positive net radiation during the day and a higher average negative net radiation during the night than the valley stations. Observations averaged over a 15-day period of variable weather illustrated the general effect of cloudy weather in reducing contrasts in radiation climate among sites. A simple theoretical correction for radiation measured on a horizontal surface provided a useful estimate of net and global radiation on the underlying sloping surface.

The Relationship between the Planetary and Surface Net Radiation: An Update

Abstract In an earlier study, Pinker et al. have shown that the daily average net radiation at the top of the atmosphere is highly correlated with the daily average net radiation at the surface. A regression formalism was derived and tested on independent datasets; it proved to yield estimates of net radiation to accuracies required for climate studies. The objective of the present paper has been to simplify the regression approach for deriving surface net radiation from satellite observations. It was found that the scene brightness alone, as sensed by the GOES satellite, can serve as a predictor of the net radiation at the surface.

正味放射量の季節変化とその推定

正味放射量の季節変化とその推定

Solar and net radiation in Peninsular Malaysia

AbstractLinear regression equations relating the global solar radiation and sunshine duration are derived for a total of 14 stations in Peninsular Malaysia. Mean monthly global solar radiation values for other stations which have only sunshine data are estimated using these regression equations. The intensities of global solar radiation during different times of the day and under different meteorological conditions are discussed. Maps of mean monthly global solar radiation are presented and discussed. It is found that the distribution pattern of global solar radiation follows somewhat with that of rainfall pattern.Net radiation was estimated using empirical equations developed by Chang (1970), Davies (1967). Selirio et al. (1971) and Penman (cited by Chang, 1970). Comparison of the estimated net radiation with the limited net radiation record available shows that the Davies' formula gives the best estimates. Hence Davies' formula is used for the estimation of net radiation. Maps of estimated net radiation are similarly prepared and discussed.

An Experimental Study of Net Radiation, Its Components and Prediction1

AbstractAccurate estimates of net radiation may be preferable to a direct measurement with a net pyrradiometer, due to sensor cost and maintenance requirements, for certain long‐term applications such as irrigation scheduling. Thus, the objective of this study was to investigate methods of estimating net radiation from readily available meteorological measurements and in the process, to study each component of net radiation.Simultaneous measurements of incoming and outgoing solar and longwave radiation, net radiation, air temperture, and vapor pressure were made over a well watered alfalfa (Medicago sativa L.) crop at the Scottsbluff Agricultural Laboratory of the Univ. of Nebraska Panhandle Station. Air temperature, used in the Stefan‐Boltzmann equation, was a good predictor of daily outgoing longwave radiation. Calculation of incoming longwave radiation using three values of atmospheric emisivity (Idso and Jackson, 1969; Brutsaert, 1975; Idso, 1980) overestimated measurements. Two procedures to calculate daily values of net radiation are outlined and tested on independent data. One method uses two emisivities, one for clear sky and one for cloudy conditions. The second method uses only one emissivity and is more straight forward. Calculation of net radiation from incoming solar radiation, air temperature, and vapor pressure by both methods compares quite well to measured values, r = 0.98 for each method.

An operational net shortwave radiation model for glacier basins

An operational net shortwave radiation model, which may be used to map radiation distributions under clear and cloudy skies, is described. Construction is achieved by modifying existing total spectrum approaches to allow a minimum of observational data to be used in its operation. Application to the Peyto Glacier Basin, Alberta, shows that realistic and diagnostically useful results can be produced from information contained in a topographic map, monitoring of the snowline, and routine weather observations. Net shortwave radiation estimates from the model are found to be a fair approximation of energy equivalents of ablation derived from stake measurements. It is concluded that net shortwave radiation should be a principal component of any realistic meltwater discharge model.

Canopy temperature as a crop water stress indicator

Canopy temperatures, obtained by infrared thermometry, along with wet‐ and dry‐bulb air temperatures and an estimate of net radiation were used in equations derived from energy balance considerations to calculate a crop water stress index (CWSI). Theoretical limits were developed for the canopy air temperature difference as related to the air vapor pressure deficit. The CWSI was shown to be equal to 1 ‐ E/Ep, the ratio of actual to potential evapotranspiration obtained from the Penman‐Monteith equation. Four experimental plots, planted to wheat, received postemergence irrigations at different times to create different degrees of water stress. Pertinent variables were measured between 1340 and 1400 each day (except some weekends). The CWSI, plotted as a function of time, closely paralleled a plot of the extractable soil water in the 0‐ to 1.1‐m zone. The usefulness and limitations of the index are discussed.

On the Heat Budget of Hydrosphere and Atmosphere in the Indian Ocean

The heat budget of the atmosphere-ocean-land system in the Indian Ocean region (30°N–30°S, 30–120°E) is studied on the basis of ocean surface heat flux calculations from long-term ship observations and satellite-derived estimates of net radiation at the top of the atmosphere. The hydrosphere to the north of the equator exports heat at rates of 5 × 1014 W for the year as a whole, and more than 8 × 1014 W during the northern summer (May–October) half-year, respectively. In contrast, the heat budget of the Southern Hemisphere water is dominated by the seasonal storage/depletion of heat transferred through the ocean surface. Oceanic heat export/import is small for this region during both the November–April and May–October half-years, and near zero for the year as a whole. The mean annual net meridional oceanic heat transport is directed southward throughout the study area, reaching a maximum of 8 × 1014 W at 10–15°S. From heat balance considerations, the annual average upwelling north of the equator is calculated to be ∼6 × 10−7 m s−1. Most of the compensatory down-welling must occur outside the tropical Indian Ocean. Residually determined heat export by the atmosphere north of the equator averages 18 and 4 × 10−14 W during the northern summer and winter half-years, respectively. South of the equator the atmosphere exports heat at a mean annual rate of 19 × 10−14 W, with little seasonal variation. During northern summer, the atmospheric energy export from the southern tropical Indian Ocean is largely in the form of latent heat and is directed northward across the equator. The southern tropical Indian Ocean is the major source of the atmospheric water vapor carried across the coastline of southern Asia during the northern summer southwest monsoon. The larger water vapor flux divergence south of the equator at this time is fed by strong evaporation. This is supported by a combination of the seasonal depletion of the local oceanic heat content and oceanic heat import from north of the equator, in addition to the surface net radiation. South of about 10°S, the atmosphere must dispose of both the net radiative heat input at the top of the system and the heat imported within the oceanic water body. In contrast, to the north the atmosphere and hydrosphere make similar contributions to the lateral energy export.