Energy Budget Components Research Articles

Land-Atmosphere Energy Transfer and Surface Boundary Layer Characteristics in the Rongbu Valley on the Northern Slope of Mt. Everest

The land-atmosphere interaction processes are unique at Mt. Everest as a result of high elevation. Based on turbulent data collected from April 2005 to March 2006 with the eddy covariance method at Quzong in the Rongbu Valley on the northern slope of Mt. Everest, land-atmosphere energy budget before and after the southwest monsoon onset and surface layer turbulent characteristics are studied for the first time. It is found that energy budget components (net radiation flux, sensible heat flux, latent heat flux, and soil heat flux) and surface heating field have strong diurnal and seasonal variations. In particular, under the influence of the southwest monsoon, the characteristics of surface parameters can be clearly identified. From pre-monsoon to monsoon season, the sensible heat flux decreases whereas the latent heat flux increases. The latent heat flux and Evaporative Fraction at Quzong are relatively high most of the year. Furthermore, the intensity of heating source in the wet season (from June to August) is much greater than that in the dry season (from October to December). The relationship between normalized standard deviation of wind speed and atmospheric stability, variations of normalized standard deviation of temperature, and humidity with atmospheric stability are analyzed using the Monin-Obukhov similarity theory. The result reveals that the normalized standard deviation of velocity components of the three-dimensional wind speed follows similarity relationships in the convective and near-neutral surface layer, but does not seem to be valid in stable surface layer. However, the normalized standard deviation of temperature and humidity does not obey Monin-Obukhov similarity theory in the entire interval of atmospheric stratifications.

Quantifying the Spatial Variability of Surface Fluxes Using Data from the 2002 International H2O Project

Spatial variability in the exchange of energy and moisture is a key control on numerous atmospheric, hydrologic, and environmental processes. Using observations made on fair weather days during the 2002 International H2O Project, four methods for quantifying the spatial variability of surface fluxes are investigated. The first two methods utilize applied statistical techniques to describe the spatial variability of the surface fluxes, while the third method is a geostatistical technique rooted in variography. Typically, the methods yield similar results, with median values of horizontal variability consistent to within 5%. The geostatistical technique, however, provides much more information than the other statistical methods; it not only provides an estimate of the spatial variability, but also provides estimates of the total variability, the non-spatial variability due to measurement error, and the range of spatial correlation among the data points. The fourth method is based on the relationship between the components of the surface energy budget. This method describes the variability in the fluxes in terms of the slope of the best-fit line relating the time-averaged latent and sensible heat fluxes from different locations along the flight path. The meaning of the slopes can also be interpreted in terms of the spatial variability in the available energy. For four of the five days analyzed, the key control on the spatial variability in the turbulent heat fluxes was horizontal variability in the soil heat flux. In turn, the soil heat flux varied as a function of surface properties including surface temperature, soil moisture content, and leaf area index. On the remaining day, 25 May, the primary control was the variability in net radiation.

The effect of temperature on growth and energy budget of the polychaete, Neanthes japonica Izuka

Growth and energy budget of the polychaete, Neanthes japonica, at various temperatures (17, 20, 23, 26 and 29°) were investigated in this study. The growth, as indicated by final dry weight and specific growth rate (SGR), increased with increasing temperature, with the maximum level at 26°C, and then decreased significantly at 29°C. A similar trend was observed in feeding rate, food conversion efficiency (FCE) and apparent digestive rate (ADR). However, no significant differences were detected in ADR among all the temperature treatments. In the pattern of energy allocation, faeces energy was only a small component of energy budget and had little influence on the proportion of food energy allocated to growth. The metabolic energy accounted for a large portion of energy intake for each temperature treatment. The nitrogen excretion was appreciable with changing temperature. The two expenditure terms (respiration energy and excretion energy) in energy budget were the major factors influencing the proportion of food energy allocated to growth. These results revealed that temperature affected the growth of N. japonica mainly by influencing feeding rate and FCE. In addition, regression equations describing the relationship between feeding rate, faecal production, SGR, FCE and temperature were obtained. The optimum temperatures for feeding rate, FCE and SGR were estimated at 25.01°C, 24.24°C and 24.73°C, respectively, from the regression equations.

Downward longwave radiation estimates for clear and all-sky conditions in the Sertãozinho region of São Paulo, Brazil

The accurate estimate of the surface longwave fluxes contribution is important for the calculation of the surface radiation budget, which in turn controls all the components of the surface energy budget, such as evaporation and the sensible heat fluxes. This study evaluates the performance of the various downward longwave radiation parameterizations for clear and all-sky days applied to the Sertaozinho region in Sao Paulo, Brazil. Equations have been adjusted to the observations of longwave radiation. The adjusted equations were evaluated for every hour throughout the day and the results showed good fits for most of the day, except near dawn and sunset, followed by nighttime. The seasonal variation was studied by comparing the dry period against the rainy period in the dataset. The least square linear regressions resulted in coefficients equal to the coefficients found for the complete period, both in the dry period and in the rainy period. It is expected that the best fit equation to the observed data for this site be used to produce estimates in other regions of the State of Sao Paulo, where such information is not available.

Combined analysis of energy and water balances to estimate latent heat flux of a sudanian small catchment

Actual evapotranspiration is one of the major components of both energy and water budgets, but is often difficult to monitor over long period with sufficient accuracy. Within the framework of the “AMMA-CATCH” program, a project dedicated to the study of the West African Monsoon, a large aperture scintillometer has been installed in a small catchment (12 km 2 ), located in the North of Benin, a region exposed to sudanian climate. The present study is an attempt to estimate the latent heat flux over this small but heterogeneous catchment based on scintillation and ground observations. The analysis covers the end of the dry season (lasting from February to April 2006). During this period two isolated rainfall events occurred, giving a unique opportunity to study energy and water budgets simultaneously. The comparison between the average sensible heat flux derived from scintillometer observations and the one obtained with conventional eddy correlation shows a relatively good agreement, where the scattering is mainly explained by differences in footprint associated with both instruments. A relevant hourly residual latent heat flux is then obtained through the energy balance equation, with careful attention brought to the net radiation, and the ground heat fluxes. The residual of the energy budget equation is compared to soil water losses from vadose zone and water table, in order to evaluate whether this estimation is consistent with the water budget of the ground. Daily soil water depletion within the first meter of the surface shows a similar dynamic as the one calculated from the energy balance equation, but exhibits a constant 1 mm/day lag. The excess of actual evapotranspiration is supposed to be explained by water table losses and root extraction by trees. Finally, this study shows how combined energy and water budget analysis can help to better understand water transfers at the watershed scale.

Towards an understanding of coupled physical and biological processes in the cultivated Sahel – 1. Energy and water

This paper presents an analysis of the coupled cycling of energy and water by semi-arid Sahelian surfaces, based on two years of continuous vertical flux measurements from two homogeneous recording stations in the Wankama catchment, in the West Niger meso-site of the AMMA project. The two stations, sited in a millet field and in a semi-natural fallow savanna plot, sample the two dominant land cover types in this area typical of the cultivated Sahel. The 2-year study period enables an analysis of seasonal variations over two full wet–dry seasons cycles, characterized by two contrasted rain seasons that allow capturing a part of the interannual variability. All components of the surface energy budget (four-component radiation budget, soil heat flux and temperature, eddy fluxes) are measured independently, allowing for a quality check through analysis of the energy balance closure. Water cycle monitoring includes rainfall, evapotranspiration (from vapour eddy flux), and soil moisture at six depths. The main modes of observed variability are described, for the various energy and hydrological variables investigated. Results point to the dominant role of water in the energy cycle variability, be it seasonal, interannual, or between land cover types. Rainfall is responsible for nearly as much seasonal variations of most energy-related variables as solar forcing. Depending on water availability and plant requirements, evapotranspiration pre-empts the energy available from surface forcing radiation, over the other dependent processes (sensible and ground heat, outgoing long wave radiation). In the water budget, pre-emption by evapotranspiration leads to very large variability in soil moisture and in deep percolation, seasonally, interannually, and between vegetation types. The wetter 2006 season produced more evapotranspiration than 2005 from the fallow but not from the millet site, reflecting differences in plant development. Rain-season evapotranspiration is nearly always lower at the millet site. Higher soil moisture at this site suggests that this difference arises from lower vegetation requirements rather than from lower infiltration/higher runoff. This difference is partly compensated for during the next dry season. Effects of water and vegetation on the energy budget appear to occur more through latent heat than through albedo. A large part of albedo variability comes from soil wetting and drying. Prior to the onset of monsoon rain, the change in air mass temperature and wind produces, through modulation of sensible heat, a marked chilling effect on the components of the surface energy budget.

Cod and climate in a latitudinal cline: physiological analyses of climate effects in marine fishes

Characteristics of temperature-dependent metabolic adaptation, as well as their impli- cations for climate-dependent energy budgets, biogeography and fitness are reviewed and analysed for populations of Atlantic cod Gadus morhua in relation to findings in other fish species from north- ern and southern hemispheres, especially various species of eelpout (Zoarcidae). The analysis builds on the recently posited concept of oxygen- and capacity-dependent thermal tolerance in aquatic ectotherms. Mechanistic physiological studies are used to explain both functional differences between populations and current observations (such as the northward movement of cod, or the changes in seasonal abundance of eelpout due to global warming). Available data support the hypothesis that natural selection favours individuals that maximize growth and energy efficiency at the expense of ranges of thermal tolerance. The levels of energy turnover are subject to the con- straints of resource availability and temperature variability. Temperature variability in the cold, such as in the sub-Arctic, causes a rise in maintenance costs at the expense of growth, but possibly in favour of motility, and thus of foraging capacity. These different trends are mirrored in higher capac- ities for gene expression of key enzymes involved in aerobic metabolism (during cold acclimation) in northern as compared to southern cod populations of the East Atlantic. However, detailed patterns, as well as many of the underlying mechanisms, remain underexplored, especially with respect to the suggested hierarchy of energy allocation to energy budget components.

Surface energy balance of seasonal snow cover for snow-melt estimation in N-W Himalaya

This study describes time series analysis of snow-melt, radiation data and energy balance for a seasonal snow cover at Dhundi field station of SASE, which lies in Pir Panjal range of the N-W Himalaya, for a winter season from 13 January to 12 April 2005. The analysis shows that mean snow surface temperature remains very close to the melting temperature of snow. It was found close to −1°C for the complete observational period which makes the snow pack at Dhundi moist from its beginning. The average air temperature over this period was found to be 3.5°C with hourly average variation from −5.5°C to 13°C. The snow surface at this station received a mean short wave radiation of 430W m−2, out of which 298W m−2 was reflected back by the snow surface with mean albedo value of 0.70. The high average temperature and more absorption of solar radiation resulted in higher thermal state of the snowpack which was further responsible for faster and higher densification of the snowpack. Net radiation energy was the major component of surface energy budget with a mean value of 83W m−2. Bulk transfer model was used to calculate turbulent fluxes. The net energy was utilized for satisfying cold content and snow-melt by using measured snow surface temperature and density of snow pack. The mean square error between calculated and measured daily snow-melt was found to be approximately 6.6mm of water equivalent.

An investigation of GSWP‐2 Mississippi River basin surface water and energy budgets

This paper demonstrates the usefulness of watershed‐scale surface water and energy budget analysis for evaluating model skill and deficiencies. Mississippi river basin land surface model water and energy budget simulations from the Second Global Soil Wetness Project (GSWP‐2) are compared with available observations from 1986 to 1995. Forced by the same meteorological parameters, the evaluated GSWP‐2 land surface models (SSiB, Noah and CLM2‐TOP) exhibit similarities and differences in depicting basin averaged water and energy balance components. The CLM2‐TOP evaporation approaches the reanalysis and runoff is closer to the gauge measurement, while the Noah model's evaporation is underestimated and the corresponding runoff is overestimated, which could be related to the limitation in the parameterization of turbulent flux and infiltration process. The energy components of the three land surface models have a similar seasonal cycle but with different magnitudes. Despite the seasonal differences, the interannual variations of water and energy budget components are comparable and correlated. Overall, the more complex CLM2‐TOP model outperforms the SSiB and Noah models in reproducing watershed‐scale seasonal and interannual water and energy variations. All three models consistently reveal that the most extreme hydrological events are associated with El Niño‐Southern Oscillation (ENSO) phases. However, the relationship of surface water components to ENSO varies geographically. For future studies, land‐atmosphere coupled processes need to be included to determine the ability of land surface models to represent surface water and energy processes.

An Observational Analysis and Evaluation of Land Surface Model Accuracy in the Nebraska Sand Hills

Abstract In this study, the influence of subsurface water on the energy budget components of three locations with heterogeneous land surfaces in the Nebraska Sand Hills are examined through observations and use of the Noah land surface model (LSM). Observations of the four primary components of the surface energy budget are compared for a wet interdunal meadow valley, a dry interdunal valley, and a dunal upland location. With similar atmospheric forcing at each site, it was determined that differences in the partitioning of the mean diurnal net radiation (Rnet) existed among the three locations due to the influence of varied soil moisture and vegetation through the year. At the wet valley, observations indicated that almost 65% of the mean daily peak Rnet was used for latent heating, due to the relatively higher soil moisture content resulting from an annual upward gradient of subsurface water and denser vegetation. In sharp contrast, the dunal upland site yielded only 21% of the mean daily peak Rnet going to latent heating, and a greater mean diurnal soil heat flux with typically drier soils and sparser vegetation than at the wet valley. The dry valley partition of the peak Rnet fell between the wet valley and dunal upland site, with approximately 50% going to sensible heating and 50% toward latent heating. In addition to the observational analysis, an uncoupled land surface model was forced with the observations from each site to simulate the energy budgets, with no tuning of the model’s fundamental equations and with little adjustment of the model parameters to improve results. While the model was able to reasonably simulate the mean diurnal and annual energy budget components at all locations, in most instances with root-mean-square errors within 20%–25% of the observed values, the lack of explicit treatment of subsurface water within the model limited predictability, particularly at the wet valley site. For instance, only 25% of the peak mean diurnal Rnet went toward latent heating in the model simulation of the wet valley, compared to 65% as estimated by observations. Model evaluation statistics are presented to document the land surface model’s ability to capture the annual and mean diurnal variations in the surface energy budget terms at the dry valley and dunal upland sites, but the absence of subsurface water results in large errors in the wet valley simulation. From these results, a case is made for the future inclusion of the explicit treatment of subsurface water within the Noah LSM to better approximate the prediction of the surface energy budget in such environments.

Meso-urban meteorological and photochemical modeling of heat island mitigation

A fine-resolution, meso-urban meteorological model was updated and applied, using new data and techniques in this study, in driving fine-resolution photochemical simulations to evaluate the air-quality impacts of urban heat island mitigation. The drag-force-based formulation of the model improves the simulation of fine-resolution meteorological and air-pollutant concentration fields in the urban canopy layer. Compared to the meso scale, the meso-urban modeling of heat island mitigation produces larger localized impacts on meteorology and ozone air quality and captures phenomena of interest that are typically not detectable at the coarser scale. These include, for example, cool islands, heat islands, flow convergence associated with heat island circulation, flow divergence at the leading edge of urban areas, and vertical variation in turbulent kinetic energy budget components within the canopy layer in response to vertical changes in densities of buildings and vegetation. They also include fine-resolution features in the simulated ozone concentration field and its response to surface modifications. Model results show that heat island mitigation is effective in reducing local ozone concentrations. This paper presents results from Sacramento, California, as an example using increased urban albedo as the control mechanism. For the region, episodic conditions, and surface modification scenarios examined in this paper, air temperature is decreased by up to 3 °C. Changes in ozone consist overwhelmingly of decreases but can also involve increases with the latter being confined to small areas and short time intervals. While larger reductions in ozone are detected, decreases of up to 5–10 ppb are more representative and the daily maximum 8-h average can be decreased by up to 13%.

Observations and model estimates of diurnal water temperature dynamics in mosquito breeding sites in western Kenya

AbstractWater temperature is an important determinant of the growth and development of malaria mosquito immatures. To gain a better understanding of the daily temperature dynamics of malaria mosquito breeding sites and of the relationships between meteorological variables and water temperature, three clear water pools (diameter × depth: 0·16 × 0·04, 0·32 × 0·16 and 0·96 × 0·32 m) were created in Kenya. Continuous water temperature measurements at various depths were combined with weather data collections from a meteorological station. The water pools were homothermic, but the top water layer differed by up to about 2 °C in temperature, depending on weather conditions. Although the daily mean temperature of all water pools was similar (27·4–28·1 °C), the average recorded difference between the daily minimum and maximum temperature was 14·4 °C in the smallest versus 7·1 °C in the largest water pool. Average water temperature corresponded well with various meteorological variables. The temperature of each water pool was continuously higher than the air temperature. A model was developed that predicts the diurnal water temperature dynamics accurately, based on the estimated energy budget components of these water pools. The air–water interface appeared the most important boundary for energy exchange processes and on average 82–89% of the total energy was gained and lost at this boundary. Besides energy loss to longwave radiation, loss due to evaporation was high; the average estimated daily evaporation ranged from 4·2 mm in the smallest to 3·7 mm in the largest water pool. Copyright © 2008 John Wiley & Sons, Ltd.

Ground‐based remote sensing of precipitation in the Arctic

In this paper, a simple technique is described for application of radar to retrievals of vertical profiles of precipitation in the Arctic. The method is applied specifically to data from the DOE Atmospheric Radiation Measurement (ARM) North Slope of Alaska–Adjacent Arctic Ocean (NSA‐AAO) site near Barrow, Alaska. Profiles of precipitation characteristic size are obtained by developing a relationship between particle size and particle fall speed derived from 35 GHz Doppler radar. The relationship depends on an assumed size‐ and temperature‐dependent particle morphology tailored to the Arctic. Error analysis indicates that typical uncertainties in the precipitation retrieval rate are, to 95% confidence, about 40% for ice and 15% for liquid, although they can be as high as a factor of 2. Monthly averaged precipitation retrievals show strong seasonal variation and compare to within respective uncertainties with local ground‐based National Weather Service and ARM NSA‐AAO surface measurements. Measurements of precipitation profiles show substantial evaporation of precipitation to the atmosphere below cloud base. Associated atmospheric cooling can be several K d−1, suggesting it may be a substantial component of the boundary layer energy budget in the Arctic.

Evaluating the effects of historical land cover change on summertime weather and climate in New Jersey: Land cover and surface energy budget changes

The 19th‐century agrarian landscape of New Jersey (NJ) and the surrounding region has been extensively transformed to the present‐day land cover by urbanization, reforestation, and localized areas of deforestation. This study used a mesoscale atmospheric numerical model to investigate the sensitivity of the warm season climate of NJ to these land cover changes. Reconstructed 1880s‐era and present‐day land cover data sets were used as surface boundary conditions for a set of simulations performed with the Regional Atmospheric Modeling System (RAMS). Three‐member ensembles with historical and present‐day land cover were compared to examine the sensitivity of surface air and dew point temperatures, rainfall, and the individual components of the surface energy budget to these land cover changes. Mean temperatures for the present‐day landscape were 0.3–0.6°C warmer than for the historical landscape over a considerable portion of NJ and the surrounding region, with daily maximum temperatures at least 1.0°C warmer over some of the highly urbanized locations. Reforested regions, however, were slightly cooler. Dew point temperatures decreased by 0.3–0.6°C, suggesting drier, less humid near‐surface air for the present‐day landscape. Surface warming was generally associated with repartitioning of net radiation from latent to sensible heat flux, and conversely for cooling. While urbanization was accompanied by strong surface albedo decreases and increases in net shortwave radiation, reforestation and potential changes in forest composition have generally increased albedos and also enhanced landscape heterogeneity. The increased deciduousness of forests may have further reduced net downward longwave radiation.

Emission rates with the boundary layer budget method supported by acoustic remote sensing

Remote sensing can help to derive turbulent fluxes in the atmosphere even if the remote sensing technique used itself is not able to resolve turbulent fluctuations. Turbulent fluxes are important components of atmospheric energy and substance budgets and they play a dominant role in the surface-atmosphere exchange. Apart from high-resolution in situ measurements it is difficult to determine them. This paper presents an inverse method to derive a regionally-averaged vertical turbulent flux of methane from the surface into the atmospheric boundary layer. The determination of aggregated surface emission fluxes of methane from an agricultural area in Southern Germany on an intermediate scale between the local and the regional scale is based on using a boundary layer budget method combined with surface-based acoustic remote sensing of the mixing-layer height with a sodar. The order of magnitude of the resulting methane emission fluxes coincide well with the national methane emission inventory.

A temperature prediction-correction method for estimating surface soil heat flux from soil temperature and moisture data

Surface soil heat flux is a component of surface energy budget and its estimation is needed in land-atmosphere interaction studies. This paper develops a new simple method to estimate soil heat flux from soil temperature and moisture observations. It gives soil temperature profile with the thermal diffusion equation and, then, adjusts the temperature profile with differences between observed and computed soil temperatures. The soil flux is obtained through integrating the soil temperature profile. Compared with previous methods, the new method does not require accurate thermal conductivity. Case studies based on observations, synthetic data, and sensitivity analyses show that the new method is preferable and the results obtained with it are not sensitive to the availability of temperature data in the topsoil. In addition, we pointed out that the soil heat flux measured with a heat-plate can be quite erroneous in magnitude though its phase is accurate.

Vineyard evaporative fraction based on eddy covariance in an arid desert region of Northwest China

Studying farmland evaporative fraction (EF) plays an important role in interpreting the components of energy budget and evapotranspiration (ET). The present study examines the pattern of vineyard EF after monitoring energy components by eddy covariance for 2 years, and estimates the crop ET by EF in the arid desert region of Northwest China. Main results indicate that EF during daytime is nearly constant on sunny days when the available energy exceeds 200 W m −2, but EF becomes relatively unsteady when the available energy is lower than 200 W m −2. Furthermore, daytime average EF is relatively low in the early growth stage, nearly constant in the mid-later stage, and significantly reduced in the later stage; Moreover, mean EF in different periods of daytime is in good agreement with daytime average EF, mean EF during 10:00–15:00 h is relatively close to daytime average EF and mean EF during 14:00–15:00 h is approximately equal to daytime average EF. The estimated daytime ET from mean EF during 14:00–15:00 h is highly correlated to the measured ET by Bowen ratio-energy balance though the value is partially underestimated. This study demonstrated that daytime ET can be estimated from midday EF and the relationship can be used to guide irrigation practice in the arid region.

Examples for the determination of turbulent (sub-synoptic) fluxes with inverse methods

Turbulent vertical fluxes are important components of atmospheric energy and substance budgets and they play a dominant role in the surface-atmosphere exchange. Apart from high-resolution in situ measurements it is difficult to determine them. This paper, which is dedicated to Prof. Dr. Michael Hantel who recently retired, gives a short overview over existing inverse methods - experimental and numerical - to derive vertical profiles of turbulent fluxes in the atmospheric boundary layer and throughout the free atmosphere. Experimental methods comprise direct techniques and indirect parametric techniques, both using surface-based remote sensing devices. The numerical method is the budget method founded on the integration of the budget equation for the transported substance or property. This latter method has been considerably enhanced by Michael Hantel in the last 30 years; especially by developing an independent error measure. From such integration the calculation of vertical turbulent flux profiles is as well possible as the determination of aggregated surface fluxes over horizontally inhomogeneous terrain. The experimental method is most suitable for the local scale; the numerical method is more suitable for the regional and larger scales. Finally a short example for the determination of aggregated surface emission fluxes of methane from an agricultural area in Southern Germany on an intermediate scale between the local and the regional scale using a budget method combined with surface-based remote sensing of the mixing-layer height is presented.

Atmospheric surface layer characteristics of turbulence above the Pantanal wetland regarding the similarity theory

In this manuscript, some micrometeorological characteristics of the Pantanal are presented. This region, one of the greatest wetlands in the world, is located in the central western part of South America. It has very peculiar environmental and ecological characteristics, a great biodiversity and an irregular hydrological cycle, which often presents floods during the wet season and droughts and fires during the dry season. The experimental data were collected in a meteorological tower during the Interdisciplinary Pantanal Experiment (IPE-1), on April–May 1998. This was a transition period between the wet and the dry season, which a shallow water layer was present at several parts of the experimental field, even around the 25 m-height extended meteorological tower. The surface energy budget components associated with the existence of the shallow water layer, its diurnal variability and some turbulent variables related to the surface–atmosphere turbulent exchange processes were investigated. Eddy-correlation and variance methods have been used to estimate turbulent fluxes and to study the validity of the Monin–Obukhov similarity theory for the Pantanal region. A mixed layer slab model has been used to estimate the height of the turbulent mixing-layer and to provide useful information to test the validity of general relationships concerning horizontal wind velocity variances above Pantanal. In general, the dimensionless relationships between turbulent variables and scaling parameters agree well with the ones found in the literature. Some aspects regarding the heat storage in the shallow water layer present interesting information about its role in the surface–atmosphere energy exchanges processes along the day.

Interannual variability of surface energy exchange depends on stand age in a boreal forest fire chronosequence

Understanding how changes in the boreal fire regime will affect high latitude climate requires knowledge of the sensitivity of the surface energy budget to shifts in vegetation cover. We measured components of the surface energy budget in three ecosystems that were part of a fire chronosequence in interior Alaska for 3 years. Our sites were within the perimeter of stand‐replacing fires that occurred in 1999, 1987, and ∼1920 (hereafter referred to as the 1999‐burn, the 1987‐burn, and the control). Vegetation cover consisted primarily of sparse short grasses at the 1999‐burn, aspen and willow (deciduous trees and shrubs) at the 1987‐burn, and black spruce (evergreen conifer trees) at the control. Averaged over the 3 years of our study, annual net radiation decreased by approximately 25% at the 1999‐burn and 30% at the 1987‐burn, relative to the control. Sensible heat decreased by an even larger amount, by approximately 57% for the 1999‐burn and 44% for the 1987‐burn as compared with the control. Climate during spring and summer varied considerably among the 3 years. The three stands responded differently to this climate variability with consequences for surface energy exchange. As a result of earlier snow cover loss in 2003 and 2004, net radiation during spring increased substantially in the recently disturbed stands, but not in the control. In response to a sustained summer drought in 2004, latent heat decreased more in the 1987‐burn during August than in the control. Our results imply that a shift in plant functional types expected to accompany increases in boreal fire activity may amplify interannual climate variability during both spring and summer.