Partitioning Of Net Radiation Research Articles

Hydroclimatological impact of century‐long drainage in midwestern United States: CCSM sensitivity experiments

Community Climate System Model (CCSM3) sensitivity experiments are performed to investigate the impact of past wetland drainage on the hydroclimatology of midwestern United States (MW USA). Coupled land surface and atmospheric components of CCSM3 are used at T85 (∼140 km) horizontal grid mesh size to create four control model experiments. These include: (1) 355 ppm CO2 for year 1990 excluding wetland (present condition), (2) 355 ppm CO2 for 1990 including wetland, (3) 289 ppm CO2 for year 1870 excluding wetland, and (4) 289 ppm CO2 for year 1870 including wetland. The CCSM3 control run for the present condition is validated with high‐resolution North American Regional Reanalysis (NARR) data for the Midwest region. Validation results show that CCSM3 is reasonable in simulating at‐surface incoming solar radiation, net short‐wave radiation, and 2 m air temperature. However, partitioning of net radiation into sensible and latent heat fluxes is imprecise, and summer precipitation is largely underestimated in CCSM3. To remove any biases in CCSM3 output, results from sensitivity experiments are analyzed in terms of difference in monthly climatology. Sensitivity experiment results show significant changes in summer sensible and latent heat fluxes due to wetland drainage. Near‐surface (2 m) air temperature has significantly increased, and convective precipitation has decreased by a small amount (∼50 mm) during the summer. Except for 2 m air temperature which is affected by both greenhouse gas emission–based climate change and wetland drainage over the last century, all other climatic variables are primarily affected by wetland drainage in the region.

Hydrological behaviour of grasslands of the Sandhills of Nebraska: water and energy‐balance assessment from measurements, treatments, and modelling

AbstractUnderstanding energy and water balance processes in the Sandhills is crucial to assess the land–atmosphere feedback effects. The Sandhills located in western Nebraska covers a vast grassland ecosystem with limited variability in vegetation and soil. However, the combined effect of topography, land cover, and micrometeorology by subjecting the land surface to various disturbances and treatments is rarely studied. The NOAH Land Surface Model (LSM) was used to estimate net radiation, latent, sensible, and ground heat (GH) fluxes as well as water balance components for two growing seasons between 2005 and 2006 in various plots at the Grasslands Destabilization Experimental (GDEX) site where these plots were subjected to four different treatments and located at two topographical locations, namely high and low positions. The simulated results of net radiation and GH fluxes correlated well with measurements. While the amount of precipitation received was between 900 and 1000 mm for both seasons, on a daily and sub‐daily time scale the partitioning of net radiation into latent, sensible, and GH fluxes showed high variability across the plots, primarily driven by vegetation and soil moisture. Total evapotranspiration and soil moisture averages suggested the influence of vegetation and the timing of precipitation also in controlling various land surface processes in the Sandhills. This study provides a framework for using the LSM to quantify the feedback effects and emphasizes the importance of microtopography and land treatments in the model environment. Copyright © 2009 John Wiley & Sons, Ltd.

Impact of sub-grid variability of precipitation and canopy water storage on hydrological processes in a coupled land–atmosphere model

The impact of sub-grid variability of precipitation and canopy water storage is investigated by applying a new canopy interception scheme into the Community Atmosphere Model version 3 (CAM3) coupled with the Community Land Model version 3 (CLM3). Including such sub-grid variability alters the partitioning of net radiation between sensible heat flux and latent heat flux on land surface, which leads to changes in precipitation through various pathways/mechanisms. The areas with most substantial changes are Amazonia and Central Africa where convective rain is dominant and vegetation is very dense. In these areas, precipitation during December–January–February is increased by up to 2 mm/day. This increase is due to the enhanced large-scale circulation and atmospheric instability caused by including the sub-grid variability. Cloud feedback plays an important role in modifying the large-scale circulation and atmospheric instability. Turning off cloud feedback mitigates the changes in surface convergence and boundary layer height caused by inclusion of sub-grid variability of precipitation and water storage canopy, which moderate the effect on precipitation.

Soil Moisture–Atmosphere Interactions during the 2003 European Summer Heat Wave

Abstract The role of land surface–related processes and feedbacks during the record-breaking 2003 European summer heat wave is explored with a regional climate model. All simulations are driven by lateral boundary conditions and sea surface temperatures from the ECMWF operational analysis and 40-yr ECMWF Re-Analysis (ERA-40), thereby prescribing the large-scale circulation. In particular, the contribution of soil moisture anomalies and their interactions with the atmosphere through latent and sensible heat fluxes is investigated. Sensitivity experiments are performed by perturbing spring soil moisture in order to determine its influence on the formation of the heat wave. A multiyear regional climate simulation for 1970–2000 using a fixed model setup is used as the reference period. A large precipitation deficit together with early vegetation green-up and strong positive radiative anomalies in the months preceding the extreme summer event contributed to an early and rapid loss of soil moisture, which exceeded the multiyear average by far. The exceptionally high temperature anomalies, most pronounced in June and August 2003, were initiated by persistent anticyclonic circulation anomalies that enabled a dominance of the local heat balance. In this experiment the hottest phase in early August is realistically simulated despite the absence of an anomaly in total surface net radiation. This indicates an important role of the partitioning of net radiation in latent and sensible heat fluxes, which is to a large extent controlled by soil moisture. The lack of soil moisture strongly reduced latent cooling and thereby amplified the surface temperature anomalies. The evaluation of the experiments with perturbed spring soil moisture shows that this quantity is an important parameter for the evolution of European heat waves. Simulations indicate that without soil moisture anomalies the summer heat anomalies could have been reduced by around 40% in some regions. Moreover, drought conditions are revealed to influence the tropospheric circulation by producing a surface heat low and enhanced ridging in the midtroposphere. This suggests a positive feedback mechanism between soil moisture, continental-scale circulation, and temperature.

Modelling surface energy fluxes over maize using a two-source patch model and radiometric soil and canopy temperature observations

Models estimating surface energy fluxes over partial canopy cover with thermal remote sensing must account for significant differences between the radiometric temperatures and turbulent exchange rates associated with the soil and canopy components of the thermal pixel scene. Recent progress in separating soil and canopy temperatures from dual angle composite radiometric temperature measurements has encouraged the development of two-source (soil and canopy) approaches to estimating surface energy fluxes given observations of component soil and canopy temperatures. A Simplified Two-Source Energy Balance (STSEB) model has been developed using a “patch” treatment of the surface flux sources, which does not allow interaction between the soil and vegetation canopy components. A simple algorithm to predict the net radiation partitioning between the soil and vegetation is introduced as part of the STSEB patch modelling scheme. The feasibility of the STSEB approach under a full range in fractional vegetation cover conditions is explored using data collected over a maize (corn) crop in Beltsville Maryland, USA during the 2004 summer growing season. Measurements of soil and canopy component temperatures as well as the effective composite temperature were collected over the course of the growing season from crop emergence to cob development. Comparison with tower flux measurements yielded root-mean-square-difference values between 15 and 50 W m − 2 for the retrieval of the net radiation, soil, sensible and latent heat fluxes. A detailed sensitivity analysis of the STSEB approach to typical uncertainties in the required inputs was also conducted indicating greatest model sensitivity to soil and canopy temperature uncertainties with relative errors reaching ∼ 30% in latent heat flux estimates. With algorithms proposed to infer component temperatures from bi-angular satellite observations, the STSEB model has the capability of being applied operationally.

Temporal variations in heat fluxes over a central European city centre

Energy fluxes have been measured over an area near the centre of the city of Łodź, Poland, since November 2000. The site was selected because the building style (surface cover and morphology) is typical of European cities, yet distinct from the majority of cities where energy balance observations have been studied thus far. The multi-year dataset permits consideration of temporal changes in energy balance partitioning over a wide range of seasonal and synoptic conditions and of the role of heat storage and anthropogenic fluxes in the energy balance. Partitioning of net radiation into the turbulent fluxes is consistent in the two years, with the largest differences occurring due to differing precipitation. The monthly ensemble diurnal cycles of the turbulent fluxes over the two years are similar. The largest differences occur during the July–September period, and are attributable to greater net radiation and lower rainfall in 2002. The latent heat flux accounts for approximately 40% of the turbulent heat transfer on an annual basis. The average daily daytime Bowen ratio and its variability are slightly reduced during the summer (growing) season. Anthropogenic heat is a significant input to the urban energy balance in the winter. The fluxes observed in this study are consistent with results from other urban sites.

Ecosystem and understory water and energy exchange for a mature, naturally regenerated pine flatwoods forest in north Florida

Eddy covariance was used to measure energy fluxes from July 2000 – June 2002 above the tree canopy and above the understory in a mature, naturally regenerated slash pine (Pinus elliottii Engelm. var. elliottii) – longleaf pine (Pinus palustris Mill.) flatwoods forest. Understory latent energy (λE) and sensible heat (H) fluxes accounted for 45% and 55% of whole-ecosystem fluxes, respectively, with strong seasonal variation in the proportion of λE attributable to the understory. The partitioning of net radiation (Rnet) to λE and H also changed seasonally, with half-hourly mean ecosystem H in the winter peaking at 175 W·m–2, almost twice as large as λE. In contrast, half-hourly ecosystem λE and H remained almost equal throughout the day in July and August, with mean midday peaks of approximately 200 W·m–2. Maximum hourly evapotranspiration (ET) in the months of July and August was 0.32 and 0.29 mm·h–1 for 2000 and 2001, respectively. For a variety of environmental conditions, mean daily ET was approximately 2.7 mm in the summer and 1.3 mm in the winter. Annual ET for the first year was 832 mm, or 87% of annual precipitation (956 mm). Although leaf area index was higher in the second year, annual ET was only 676 mm, which is considerably lower than that of the previous year, but it still accounted for approximately the same proportion (84%) of the much lower annual precipitation (811 mm). Canopy conductance declined as soils dried, changing patterns of partitioning of Rnet to λE.

Fire effects on net radiation and energy partitioning: Contrasting responses of tundra and boreal forest ecosystems

The net radiation available to drive surface‐atmosphere exchange is strongly influenced by albedo and surface temperature. Tower‐based microclimatic and eddy covariance measurements in typical Alaskan black spruce and tundra ecosystems before and immediately after fire indicated a 10% decrease in net radiation over the burned spruce stand but a 12% increase in net radiation over the burned tundra surface. In both cases, there was increased partitioning of net radiation into sensible heat flux. In terms of absolute fluxes, however, fire increased average sensible heating over tundra by ∼50 W m−2 but caused little change in average sensible heat flux over the black spruce forest. This difference in fire effects occurred because fire altered the canopy characteristics (including surface roughness) more strongly in the forest than in the tundra, leading to a greater reduction in surface‐atmosphere coupling over the forest.

Structural complexity and land‐surface energy exchange along a gradient from arctic tundra to boreal forest

Abstract:Question: Current climate changes in the Alaskan Arctic, which are characterized by increases in temperature and length of growing season, could alter vegetation structure, especially through increases in shrub cover or the movement of treeline. These changes in vegetation structure have consequences for the climate system. What is the relationship between structural complexity and partitioning of surface energy along a gradient from tundra through shrub tundra to closed canopy forest?Location: Arctic tundra‐boreal forest transition in the Alaskan Arctic.Methods: Along this gradient of increasing canopy complexity, we measured key vegetation characteristics, including community composition, biomass, cover, height, leaf area index and stem area index. We relate these vegetation characteristics to albedo and the partitioning of net radiation into ground, latent, and sensible heating fluxes.Results: Canopy complexity increased along the sequence from tundra to forest due to the addition of new plant functional types. This led to non‐linear changes in biomass, cover, and height in the understory. The increased canopy complexity resulted in reduced ground heat fluxes, relatively conserved latent heat fluxes and increased sensible heat fluxes. The localized warming associated with increased sensible heating over more complex canopies may amplify regional warming, causing further vegetation change in the Alaskan Arctic.

Structural complexity and land-surface energy exchange along a gradient from arctic tundra to boreal forest

Question: Current climate changes in the Alaskan Arctic, which are characterized by increases in temperature and length of growing season, could alter vegetation structure, especially through increases in shrub cover or the movement of treeline. These changes in vegetation structure have consequences for the climate system. What is the relationship between structural complexity and partitioning of surface energy along a gradient from tundra through shrub tundra to closed canopy forest? Location: Arctic tundra-boreal forest transition in the Alaskan Arctic. Methods: Along this gradient of increasing canopy complexity, we measured key vegetation characteristics, including community composition, biomass, cover, height, leaf area index and stem area index. We relate these vegetation characteristics to albedo and the partitioning of net radiation into ground, latent, and sensible heating fluxes. Results: Canopy complexity increased along the sequence from tundra to forest due to the addition of new plant functional types. This led to non-linear changes in biomass, cover, and height in the understory. The increased canopy complexity resulted in reduced ground heat fluxes, relatively conserved latent heat fluxes and increased sensible heat fluxes. The localized warming associated with increased sensible heating over more complex canopies may amplify regional warming, causing further vegetation change in the Alaskan Arctic.

Surface energy exchanges and interactions with thunderstorms during the Maritime Continent Thunderstorm Experiment (MCTEX)

Inputs of heat and moisture through surface energy exchanges are important in the evolution of diurnally modulated thunderstorms. As part of the Maritime Continent Thunderstorm Experiment (MCTEX) conducted on the Tiwi Islands, northern Australia, we measured the surface energy exchanges above the dominant surface types (savanna, grassland, forest and shallow tidal strait). Measurements were made during the monsoon transition period (October–December) using the Bowen ratio and the aerodynamic techniques. A full radiation balance was measured at the Savanna site, where the surface albedo was determined as 0.19. The surface energy exchanges showed broad similarities between the sites, with comparable high net radiation totals averaging 11.8 MJ.m−2.day−1 over the terrestrial sites. The substrate heat fluxes were large at the Tidal strait (4.21 MJ.m−2.day−1) due to the high rates of energy absorption into water. The partitioning of net radiation into sensible and latent heat fluxes was controlled primarily by surface characteristics such as soil moisture and vegetation cover, with Bowen ratios averaging 1.08, 0.65, 0.5 and 0.40 for grassland, forest, savanna and tidal strait sites, respectively. A higher Bowen ratio was recorded at the Grassland site as this was during an earlier period that was much drier. Strong relationships were observed between the total surface convective fluxes and boundary layer development (e.g., the lifting condensation level) and between the magnitude of the daytime surface sensible heat flux and the onset time of thunderstorm convection. Observed terrestrial surface fluxes of moisture (averaging 165–265 W.m−2) were deemed insufficient for observed thunderstorm initiation and suggest that sea breeze advection of moisture is necessary. Tiwi Island thunderstorms were shown to represent a dynamic system where surface fluxes are important in generating a boundary layer sufficient to initiate the thunderstorms but where feedbacks between thunderstorms and surface fluxes eventually help terminate the storms.

Fire effects on surface‐atmosphere energy exchange in Alaskan black spruce ecosystems: Implications for feedbacks to regional climate

Although fire is crucial to the functioning and diversity of boreal forests, the second largest biome on Earth, there are few detailed studies of the effects of disturbance on surface‐atmosphere interactions in these regions. We conducted tower‐based micrometeorological measurements in summer over six recently burned black spruce stands that varied in age between 0 and 14 years. Results are presented for nonprecipitating conditions. There was an initial reduction of minimum albedo following fire from ∼0.09 to 0.06 followed by a rapid increase to 0.135 as the cover of successional vegetation increased. For clear‐sky conditions near local solar noon, the combined effects of increased surface temperature due to a reduced surface‐atmospheric coupling and albedo reduced midday net radiation by 9.3% (∼70 W m−2) for the first decade of succession. The average daily net radiation declined by 5.5%. Near noon, the relative partitioning of net radiation into ground heat flux doubled compared to nearby unburned stands, although on a daily basis the increase was only significant for the first few years following fire. Reduced net radiation, enhanced ground heat flux, and reduced Bowen ratio values following the first decade of succession could lead to a midday reduction of approximately 80 W m−2 in sensible heat flux compared to an unburned stand. Across a fire‐scar boundary, the contrast in sensible heating and roughness is sufficiently large to induce mesoscale circulations and possibly trigger convective development. Because fire scars frequently exceed 104 ha, locally cooler conditions would prevail with suppressed planetary boundary layer development. Given the abundance of fire disturbance throughout interior Alaska, these effects may contribute to the regional climate patterns.

Grassland desertification by grazing and the resulting micrometeorological changes in Inner Mongolia

Overgrazing is one of the most primary causes of desertification in semi-arid zones of China. From 1992 to 1994 we conducted a grazing experiment in Naiman (lat. 42°58′N, long. 120°43′E, 345 m asl), Inner Mongolia, China to elucidate desertification mechanisms. Our experimental field, which was covered with some short grasses, legumes and forbs, included four plots where grazing sheep numbers differed (0, 3, 6, or 9 individuals). Micrometeorological measurement data were analyzed using the Bowen ratio energy balance method. Ground surface reflectivity (albedo) increased with grazing intensity. The overgrazed plot had been desertified after 3 year’s grazing experiment. Albedo can be regarded as an important indicator of potential grassland desertification. When desertification occurs, then albedo will exceed a critical value. Partitioning of net radiation exhibited distinct patterns among the four plots as a result of the interplay between albedo and in situ vegetation. Ratios of the net radiation or net available radiation to solar radiation tended to decrease with increasing grazing intensity. But this pattern of changes was largely affected by soil moisture regime and vegetation status during the measurements. Grazing also remarkably affected wind regimes over the grazing experimental plots through altering surface roughness. Especially, sustained overgrazing decreased surface roughness length so substantially that wind could act directly on sandy grassland surface to initiate desertification. A permissible grazing capacity of the studied temperate semi-arid grassland was also proposed for future grassland management.

Measuring and modelling the radiation balance of a heterogeneous shrubland

ABSTRACTThe accurate quantification of the energy available for sensible and latent heat transfer from plant canopies is essential for the prediction of impacts of climate on vegetation water use and growth. Unlike agricultural fields and extensive forests of more humid zones, vegetation growing in semi‐arid climates is usually sparse creating a heterogeneous surface of shrubs, annuals and bare soil. Under these conditions many of the assumptions of the basic equations used in microclimatology, which assume a uniform vegetated surface, may be violated. It is proposed here that heterogeneous canopies may require a formulation of their energy balance that includes a measure of the canopy complexity in order to both interpret field measurements and be used in predictive models. This paper explores the need for a more complex formulation of the vegetation energy balance through a series of experiments on a sparse clumped shrubland of Retama sphaerocarpa in the Tabernas Desert, Almería, south‐east Spain. These experiments investigated the importance of the radiative properties of each surface on energy balance of soil, annuals and shrubs individually, and the surface as a whole. The study evaluated the use of the fractional vegetative cover (f) and the radiative characteristics of each surface (reflection coefficients and emissivities) for calculating net radiation partitioning between shrubs and bare soil. Results indicated that partitioning of net radiation between components could be accurately calculated from values of fractional vegetative cover, reflection coefficients and emissivities for both bare soil and plant surfaces. A sensitivity analysis showed the importance of specific radiation properties of each surface. Measurements of horizontal long‐wave fluxes between components showed that the location of a plant with respect to other plants made little difference to its long‐wave energy balance. The results also emphasized the importance of soil water content on the energy balance, through its influence on albedo and soil heat storage. This was particularly true when measurements of soil heat flux were used to measure the available energy for soil under shrubs and bare soil because of strong hysteresis cycles. These cycles were larger in the bare soil than in the substrate under shrubs.

A study on the partition of net radiation into heat fluxes on a dry asphalt surface

Variation of the energy budget components of a dry asphalt surface over an annual cycle is discussed based on the analysis of data collected from a micrometeorological road station near Vienna, in Austria. Net radiation ( R N) and ground heat flux ( G) are directly measured. Sensible heat flux ( H) for dry surface conditions is estimated as a residual of the surface energy balance. Throughout the year, ground heat flux is found to be of larger magnitude than that of the sensible heat flux. The partitioning of net radiation into ground and sensible heat fluxes are studied. Variation of G (and H) with R N showed hysteresis loop behaviour with anti-clockwise cycle (clockwise for H) during cooler periods and clockwise cycle (anti-clockwise for H) during warm periods. The coefficients of time-dependent Camuffo and Bernadi formulations relating the net radiation with the heat fluxes are estimated for different periods of the year.

The Sensitivity of a Land Surface Parameterization Scheme to the Choice of Remotely Sensed Land-Cover Datasets

The characteristics of satellite-derived land-cover data for climate models vary depending on sensor properties and processing options. To better understand the first-order effects of differences in land-cover data on a land surface parameterization scheme (VBATS), stand-alone model runs were performed for two adjacent 2.8° X 2.8° GCM grid cells in Wyoming using land cover from two satellite-derived maps (AVHRR, TM) and a global land-cover dataset commonly used in GCMs. The dominant cover type by area differed among the datasets for both grid cells. In the western grid cell, these differences resulted in substantially different surface fluxes simulated by VBATS. At spatial resolutions of 0.2° and 0.4°, the two satellite-derived datasets agreed on only 54%-62% of the land-cover types in both grid cells. Despite this disagreement, the VBATS simulated surface fluxes averaged over the grid cell were similar in the eastern grid cell for the two satellite-derived datasets. In the western grid cell, the partitioning of net radiation into sensible and latent heat fluxes was influenced by the dataset prescriptions of land-cover heterogeneity. In particular, the relative proportions of wet cover types (i.e., inland water and irrigated crop) had an effect on this partitioning, emphasizing the importance of accounting for the presence of wet cover types within a GCM grid cell in arid regions. Spatial aggregation of the satellite-derived datasets reduced the number of land-cover types prescribed for each GCM grid cell. In the western grid cell, the reduction in the number of cover types from 11 to 2 resulted in differences in annual averages of sensible and latent heat fluxes of about 10%. Other simulations involving these datasets suggest that these differences could be reduced if one accounted for the wet cover types. In this respect, fine spatial resolution is required for some cover types, whereas coarser resolution may be adequate for other types. Land-cover classifications for land surface modeling need to be based more on model sensitivities than on traditional vegetation-type schemes.

Energy balance in irrigated wheat in the cerrados region of Central Brazil

To evaluate the water requirements for irrigated wheat, a micrometeorological study was conducted during the dry seasons of 1982 and 1983 at the Cerrados Agricultural Research Center, Brasilia-DF, Brazil The crop was irrigated when the soil water potential at 5 cm rached -60 to -70J/Kg.Bowen ratio measurements were made on an hourly basis during the entire crop season. Total amount of water evapotranspired during the crop cycle and the energy balance terms varied year to year due to regional advection; In 1982, a less advective year, the total amount of water evapotranspired was 345 mm, with approximately 80% of the net radiation dissipated as latent heat, 13% as a sensible heat and 7% as soil heat after the crop obtained a Leaf Area Index (LAI) of 1.5. In 1983, when advection was greater than 1982, total water evapotranspired increased to 385 mm. The partitioning of energy was similar to 1982 during periods of little or no advection. However, during periods of intense advection, latent heat was greater hi net radiation partitioning. Although regional advection increased the water used by the crop, irrigation scheduling based on soil tensiometers was considered adequate and efficient due to the low values of the Bowen ratio obtained.

A two‐source approach for estimating turbulent fluxes using multiple angle thermal infrared observations

A recently developed two‐source model for computing the surface energy balance with directional radiometric surface temperature observations can be simplified with the use of radiometric surface temperature observations at two substantially different view angles. Such observations have been available from the Along Track Scanning Radiometer (ATSR) aboard the First European Remote Sensing Satellite (ERS‐1) and therefore could be used operationally with this simplified model. Compared to the original model formulation, the new version, 2ANGLE, does not require an estimate of the fraction of “green” or active vegetation, fg, and use of the Priestley‐Taylor parameterization for partitioning of net radiation absorbed by the canopy into latent, LE, and sensible, H, heat in order to obtain a solution. However, if one uses the Priestley‐Taylor parameterization with two radiometric observations at different view angles, then the model, 2ANGLE_PT, does not require a measurement of air temperature. The model‐derived fluxes using multiple angle radiometric surface temperature observations collected during the First International Satellite Land Surface Climatology Project (ISLSCP) Field Experiment (FIFE), which was conducted in the tall grass prairie in eastern Kansas during the summers of 1987 and 1989, are compared to measured surface fluxes. Differences between predicted and measured H using the 2ANGLE and 2ANGLE_PT models were generally significant (i.e., >50%), while the original version, 1ANGLE_PT, yielded differences of 45–50%, on average. The 2ANGLE and 2ANGLE_PT model predictions of LE yielded differences of 25–40% with measured values, while the 1ANGLE_PT model produced differences of 15–20%, on average. Using a simple technique to estimate daytime total LE from the “instantaneous” values with the three versions of the model resulted in differences generally between 20 and 25%. A review of studies analyzing FIFE flux measurement techniques indicates that differences in half‐hourly to hourly observations of H and LE can reach 50% or greater, but typically are between 20 and 30%. Therefore the performance of the models in predicting H ranged from marginal to poor, but was generally satisfactory in estimating LE because in many cases LE was significantly larger than H. The performance of the 2ANGLE_PT model in predicting Hand LE was significantly better than the 2ANGLE model, but not as good as the original 1ANGLE_PT model. A sensitivity analysis with the three models indicated that not only were the 2ANGLE_PT model predictions unaffected by a 3 K uncertainty in the estimation of air temperature using satellite data, but also showed little sensitivity to a 1.5 K uncertainty associated with deriving an atmospherically corrected surface temperature from ATSR observations. Therefore the 2ANGLE_PT model has potential for computing regional‐scale LE operationally with a satellite‐based sensor such as the ATSR.

An intercomparison of single and dual-source vegetation-atmosphere transfer models applied to transpiration from sahelian savannah

Three models of the partitioning of net radiation into latent and sensible heat fluxes over Sahelian savannah are described. Each model has a different configuration of stomatal and aerodynamic resistances. Their performance was assessed by comparison against field measurements of latent heat flux over savannah vegetation consisting of bushes interspersed with a herbaceous understorey. The modelled results indicate that in dry conditions, a Penman-Monteith based single source model performs adequately when predicting the latent heat flux. However, the models with two sources demonstrate that the bushes and herbs have very different responses to local climate. In all the models, evaporation is highly sensitive to stomatal resistance, suggesting that a better understanding of stomatal response would improve the accuracy of the models.

Boreal forest and tundra ecosystems as components of the climate system

The effects of terrestrial ecosystems on the climate system have received most attention in the tropics, where extensive deforestation and burning has altered atmospheric chemistry and land surface climatology. In this paper we examine the biophysical and biogeochemical effects of boreal forest and tundra ecosystems on atmospheric processes. Boreal forests and tundra have an important role in the global budgets of atmospheric CO2 and CH4. However, these biogeochemical interactions are climatically important only at long temporal scales, when terrestrial vegetation undergoes large geographic redistribution in response to climate change. In contrast, by masking the high albedo of snow and through the partitioning of net radiation into sensible and latent heat, boreal forests have a significant impact on the seasonal and annual climatology of much of the Northern Hemisphere. Experiments with the LSX land surface model and the GENESIS climate model show that the boreal forest decreases land surface albedo in the winter, warms surface air temperatures at all times of the year, and increases latent heat flux and atmospheric moisture at all times of the year compared to simulations in which the boreal forest is replaced with bare ground or tundra. These effects are greatest in arctic and sub-arctic regions, but extend to the tropics. This paper shows that land-atmosphere interactions are especially important in arctic and sub-arctic regions, resulting in a coupled system in which the geographic distribution of vegetation affects climate and vice versa. This coupling is most important over long time periods, when changes in the abundance and distribution of boreal forest and tundra ecosystems in response to climatic change influence climate through their carbon storage, albedo, and hydrologic feedbacks.