Terms Of Surface Energy Balance Research Articles

Quenching extinction of solid sphere diffusion flames induced by a sudden removal of gravity

The response of a diffusion flame around a PMMA solid sphere to a sudden transition to zero-gravity is investigated both experimentally and numerically. The initial flame is established in normal gravity with a low speed forced flow in a 17% oxygen by volume atmosphere. An abrupt (step change) normal-to-zero gravity transition occurs when the test package is released in the drop tower. The dynamic flame response is recorded by video and modeled numerically. By the end of the 5.18-s drop, the flame tip retreats, and the flame base may either remain stabilized near the forward stagnation region or extinguish depending on several parameters. The two parameters investigated are: forced flow velocity and the degree of preheating in the surface layer of the solid sample. The amount of preheating or equivalently the conductive flame heat loss rate to the solid interior is varied by controlling the duration of the normal gravity burning before releasing to microgravity. The heat loss is quantified by using embedded thermocouples to measure the solid subsurface temperature gradient near the stagnation point. The detailed numerical model reveals details of the flow field and flame structure including oscillatory extinction and quantifies the various transient gas-solid surface energy balance terms.

Estimation of the terms acting on local 1 h surface temperature variations in Paris region: the specific contribution of clouds

Abstract. Local short-term temperature variations at the surface are mainly dominated by small-scale processes coupled through the surface energy balance terms, which are well known but whose specific contribution and importance on the hourly scale still need to be further analyzed. A method to determine each of these terms based almost exclusively on observations is presented in this paper, with the main objective being to estimate their importance in hourly near-surface temperature variations at the SIRTA observatory, near Paris. Almost all terms are estimated from the multi-year dataset SIRTA-ReOBS, following a few parametrizations. The four main terms acting on temperature variations are radiative forcing (separated into clear-sky and cloudy-sky radiation), atmospheric heat exchange, ground heat exchange, and advection. Compared to direct measurements of hourly temperature variations, it is shown that the sum of the four terms gives a good estimate of the hourly temperature variations, allowing a better assessment of the contribution of each term to the variation, with an accurate diurnal and annual cycle representation, especially for the radiative terms. A random forest analysis shows that whatever the season, clouds are the main modulator of the clear-sky radiation for 1 h temperature variations during the day and mainly drive these 1 h temperature variations during the night. Then, the specific role of clouds is analyzed exclusively in cloudy conditions considering the behavior of some classical meteorological variables along with lidar profiles. Cloud radiative effect in shortwave and longwave and lidar profiles show a consistent seasonality during the daytime, with a dominance of mid- and high-level clouds detected at the SIRTA observatory, which also affects near-surface temperatures and upward sensible heat flux. During the nighttime, despite cloudy conditions and having a strong cloud longwave radiative effect, temperatures are the lowest and are therefore mostly controlled by larger-scale processes at this time.

Refined urban canopy parameters and their impacts on simulation of urbanization-induced climate change

Changes in land cover and urban canopy structure caused by urbanization have important effects on regional climate. In this study, the Weather Research and Forecasting model is used to explore the influences of urbanization on local climate change in Guangzhou, a core city in the rapid urbanization region of the Pearl River Delta of China. Global Land Cover data from 2000 and 2018 are adopted to represent urban expansion, and urban canopy parameters (UCPs) established by a novel method are incorporated to represent changes in urban density. Model results reveal that the updated land cover and refined UCPs describe the surface information more accurately. The 10-m wind speed is reduced by 1–4 m/s and the 2-m air temperature is increased by 2–4 °C in urban areas, which brings the simulation results closer to observations. In terms of surface energy balance, upward longwave radiation is increased by 14.3 W/m2 and the land surface temperature is increased by 0–25 k; hence the net all-wave radiation is decreased by 15.6 W/m2. Meanwhile, the urban boundary layer height is increased by 192.2 m on average, but along with the decreased mean wind speed, the boundary layer ventilation is reduced by 500–3500 m2/s.

Spatial and temporal patterns of surface–atmosphere energy exchange in a dense urban environment using scintillometry

Spatially integrated measurements of the surface energy balance (SEB) are needed in urban areas to evaluate urban climate models and satellite observations. Scintillometers allow observations of sensible heat flux (QH) over much larger areas than techniques such as eddy covariance (EC), however methods are needed to partition between remaining unmeasured SEB terms. This is the first study to use observed spatial and temporal patterns of QH from a scintillometer network to constrain estimates of remaining SEB terms in a dense, heterogeneous urban environment. Results show that QH dominates the surface energy balance in central London throughout the year, with expected diurnal courses and seasonal trends in QH magnitude related to solar radiation input. Measurements also reveal a clear anthropogenic component of QH with winter (summer) weekday QH values 11.7% (5.1%) higher than weekends. Spatially, QH magnitude is correlated with vegetation and building landcover fraction in the measurement source areas. Spatial analysis provides additional evidence of anthropogenic influence with highest weekday/weekend ratios (1.55) from the City of London. Spatial differences are used to estimate horizontal advection and a novel method to estimate monthly latent heat flux is developed based on observed landcover and wet–dry surface variations in normalized QH. Annual anthropogenic heat emissions are estimated to be 46.3 W m−2 using an energy balance residual approach. The methods presented here have potential to significantly enhance understanding of urban areas, particularly in areas with tall buildings where there are few observational data.

Cloud effects on surface energy and mass balance in the ablation area of Brewster Glacier, New Zealand

Abstract. The effect of clouds on glacier surface energy balance (SEB) has received increased attention in the last decade, but how clouds interact with other meteorological forcing to influence surface mass balance (SMB) is not as well understood. This paper resolves the SEB and SMB at a site in the ablation zone of Brewster Glacier over a 22-month period, using high-quality radiation data to carefully evaluate SEB terms and define clear-sky and overcast conditions. A fundamental change in glacier SEB in cloudy conditions was driven by increased effective sky emissivity and surface vapour pressure, rather than a minimal change in air temperature and wind speed. During overcast conditions, positive net long-wave radiation and latent heat fluxes allowed melt to be maintained through a much greater length of time compared to clear-sky conditions, and led to similar melt in each sky condition. The sensitivity of SMB to changes in air temperature was greatly enhanced in overcast compared to clear-sky conditions due to more frequent melt and changes in precipitation phase that created a strong albedo feedback. During the spring and autumn seasons, the sensitivity during overcast conditions was strongest. To capture these processes, future attempts to explore glacier–climate interactions should aim to resolve the effects of atmospheric moisture (vapour, cloud, and precipitation) on melt as well as accumulation, through enhanced statistical or physically based methods.

Clear‐sky stable boundary layers with low winds over snow‐covered surfaces. Part 2: Process sensitivity

This study evaluates the relative impact of snow‐surface coupling, long‐wave radiation, and turbulent mixing on the development of the stable boundary layer over snow. Observations at three sites are compared to WRF single‐column model (SCM) simulations. All three sites have snow‐covered surfaces but are otherwise contrasting: Cabauw (Netherlands, grass), Sodankylä (Finland, needle‐leaf forest) and Halley (Antarctica, ice shelf). All cases are characterized by stable, clear‐sky, and calm conditions. Part 1 of this study determined the optimal SCM forcing strategy. In this study, the process intensities from that reference are perturbed to study their relative significance and to assess which process could be responsible for the most optimal agreement between model and observation.The analysis reveals a large variability in the modelled atmospheric state and surface parameters. Overall, the modelled gradients of temperature and moisture are underestimated but decreasing the process intensities improves this. The impact is strongest with reduced mixing, though this then causes the model to overestimate the near‐surface wind speed.To study the surface energy balance terms, we use so‐called ‘process diagrams’. The achieved variation between the sensitivity runs indicates the model sensitivity to each process. The overall sensitivity is similar for the three sites but the relative offsets in the position of the sensitivity runs with respect to the observations differ, hampering general recommendations for model improvement. Furthermore, sometimes a meaningful interpretation of observations is troublesome, which hampers the comparison with model results. Radiation is relatively more important at Cabauw and Sodankylä, whilst coupling plays a more important role at Halley.The sensitivity analysis is performed with two boundary‐layer schemes (MYJ, YSU). YSU generates larger, more accurate gradients of atmospheric temperature and humidity, while wind speeds are predicted better with MYJ. The behaviour of an increase in 2 m temperature with decreased mixing is most obvious with YSU.

Observations and snow model simulations of winter energy balance terms within and between different coniferous forests in southern boreal Finland

Variation of canopy properties between different forest types is seldom taken into account in hydrological and climate models, and consideration of variation inside a forest is normally omitted. In this work, three data sets on near surface energy balance terms (incoming shortwave and longwave radiation; air and snow–soil interface temperatures) were collected in the southern boreal coniferous zone in Finland during three winters below different types of forest canopies. The aim was to evaluate the ability of a snow mass and energy balance model with a canopy module to reproduce the observed differences in below-canopy incoming radiations and snow–soil interface temperature. Clear differences were seen between pine and spruce forest sites (higher snow–soil interface temperatures and incoming shortwave fluxes, and lower incoming longwave fluxes at the pine site). Differences were also observed between the sparse and dense pine canopy locations. Canopy parameter values had a great effect on the quality of the model simulations. The combination of optically obtained leaf area index (LAI) values with a needle clumping correction and either optical or empirical sky view fraction (SVF) values as a canopy parameterization gave better correspondence to observations than the use of uncorrected effective LAI and any SVF.

Multiscale Modeling and Evaluation of Urban Surface Energy Balance in the Phoenix Metropolitan Area

AbstractPhysical mechanisms of incongruency between observations and Weather Research and Forecasting (WRF) Model predictions are examined. Limitations of evaluation are constrained by (i) parameterizations of model physics, (ii) parameterizations of input data, (iii) model resolution, and (iv) flux observation resolution. Observations from a new 22.1-m flux tower situated within a residential neighborhood in Phoenix, Arizona, are utilized to evaluate the ability of the urbanized WRF to resolve finescale surface energy balance (SEB) when using the urban classes derived from the 30-m-resolution National Land Cover Database. Modeled SEB response to a large seasonal variation of net radiation forcing was tested during synoptically quiescent periods of high pressure in winter 2011 and premonsoon summer 2012. Results are presented from simulations employing five nested domains down to 333-m horizontal resolution. A comparative analysis of model cases testing parameterization of physical processes was done using four configurations of urban parameterization for the bulk urban scheme versus three representations with the Urban Canopy Model (UCM) scheme, and also for two types of planetary boundary layer parameterization: the local Mellor–Yamada–Janjić scheme and the nonlocal Yonsei University scheme. Diurnal variation in SEB constituent fluxes is examined in relation to surface-layer stability and modeled diagnostic variables. Improvement is found when adapting UCM for Phoenix with reduced errors in the SEB components. Finer model resolution is seen to have insignificant (<1 standard deviation) influence on mean absolute percent difference of 30-min diurnal mean SEB terms.

A New Look at the Summer Arctic Frontal Zone

Abstract A notable characteristic of the summertime Arctic is the existence of a narrow band of strong horizontal temperature gradients spanning the coastlines of Siberia, Alaska, and western Canada that extends through a considerable depth of the troposphere. Past research has associated this summer Arctic Frontal Zone (AFZ) with contrasts in atmospheric heating between the Arctic Ocean and snow-free land, with its regional strength strongly influenced by topography; however, little is known about its variability. In this study, output from the latest generation of global atmospheric reanalyses is used to better constrain and define the summer AFZ, including its spatial and seasonal characteristics. The relative importance of different factors linked to its variability is then evaluated. The AFZ is best expressed in July and is manifested aloft as a separate Arctic jet feature at about 250 hPa. It is clearly associated with differential atmospheric heating, as evidenced by the sharp difference in surface energy balance terms between the Arctic Ocean and land. Furthermore, the AFZ is strongest over the coastline whether observed near the surface or throughout the troposphere. Interannual variations in peak strength of the AFZ are spatially heterogeneous and systematic near the surface (the 2-m level). Spatiotemporal variability is primarily dependent on factors affecting temperature over land, especially variability in cloud cover, surface wind direction, and the timing of the annual snow cover retreat. Local variability in the timing of annual sea ice retreat is also important through its control on temperatures over coastal seas.

Further Development of the Regional Boundary Layer Model to Study the Impacts of Greenery on the Urban Thermal Environment

AbstractA forest canopy model is developed and coupled into the Regional Boundary Layer Model (RBLM) to fully consider the vertical structure of tree morphology. Instead of a slab surface model formerly used to represent trees in RBLM, the new version allows refinement of the radiation budgets as well as sensible and latent heat fluxes and, hence, more precise simulation of the thermal impacts of tree plantings on urban meteorological behavior. By applying this new version of RBLM, sensitivity tests are conducted to explore the potential impacts of different greenery scenarios on the thermal environment in an eastern Chinese city, Suzhou, during hot summer days. Greenings, both tree planting and grass surfacing, are beneficial in cooling the ambient air temperature. In general, tree planting is more beneficial than grass surfacing with the same coverage. In terms of surface energy balance, with tree coverage increasing from 0% to 20%, and then to 40%, the average surface net radiation fluxes at noon (1200 LST) are 591, 512, and 421 W m−2, respectively. Correspondingly, the Bowen ratio is reduced from 8.78 to 1.20 and then to 0.43 as result of the redistribution of solar energy absorbed at the ground. The cooling effect of trees is more significant at noontime and can remarkably lower the daily maximum air temperature in urban areas. The cooling effect of urban greenery increases with its coverage. Using the study results, a tree coverage of around 40% may be a feasible and optimized urban greenery scheme.

A general framework for understanding the response of the water cycle to global warming over land and ocean

Abstract. Climate models project increases in globally averaged atmospheric specific humidity that are close to the Clausius–Clapeyron (CC) value of around 7% K−1 whilst projections for mean annual global precipitation (P) and evaporation (E) are somewhat muted at around 2% K−1. Such global projections are useful summaries but do not provide guidance at local (grid box) scales where impacts occur. To bridge that gap in spatial scale, previous research has shown that the "wet get wetter and dry get drier" relation, Δ(P − E) ∝ P − E, follows CC scaling when the projected changes are averaged over latitudinal zones. Much of the research on projected climate impacts has been based on an implicit assumption that this CC relation also holds at local (grid box) scales but this has not previously been examined. In this paper we find that the simple latitudinal average CC scaling relation does not hold at local (grid box) scales over either ocean or land. This means that in terms of P − E, the climate models do not project that the "wet get wetter and dry get drier" at the local scales that are relevant for agricultural, ecological and hydrologic impacts. In an attempt to develop a simple framework for local-scale analysis we found that the climate model output shows a remarkably close relation to the long-standing Budyko framework of catchment hydrology. We subsequently use the Budyko curve and find that the local-scale changes in P − E projected by climate models are dominated by changes in P while the changes in net irradiance at the surface due to greenhouse forcing are small and only play a minor role in changing the mean annual P − E in the climate model projections. To further understand the apparently small changes in net irradiance we also examine projections of key surface energy balance terms. In terms of global averages, we find that the climate model projections are dominated by changes in only three terms of the surface energy balance: (1) an increase in the incoming long-wave irradiance, and the respective responses (2) in outgoing long-wave irradiance and (3) in the evaporative flux, with the latter change being much smaller than the former two terms and mostly restricted to the oceans. The small fraction of the realised surface forcing that is partitioned into E explains why the hydrologic sensitivity (2% K−1) is so much smaller than CC scaling (7% K−1). Much public and scientific perception about changes in the water cycle has been based on the notion that temperature enhances E. That notion is partly true but has proved an unfortunate starting point because it has led to misleading conclusions about the impacts of climate change on the water cycle. A better general understanding of the potential impacts of climate change on water availability that are projected by climate models will surely be gained by starting with the notion that the greater the enhancement of E, the less the surface temperature increase (and vice versa). That latter notion is based on the conservation of energy and is an underlying basis of climate model projections.

Quantification and mitigation of long-term impacts of urbanization and climate change in the tropical coastal city of San Juan, Puerto Rico

Urbanization, along with other cases of land cover and land use changes, has significant climate impacts in tropical regions with the added complexity of occurring within the context of global warming (GW). The individual and combined effects of these two factors on the surface energy balance of a tropical city are investigated by the use of an integrated atmospheric modeling approach, taking the San Juan Metropolitan Area (SJMA), Puerto Rico, as the test case. To achieve this goal, an ensemble of climate and weather simulations is performed, with climate scenarios combining urban development and sprawl with regional climate change over the past 50 years and the short-term simulations designed to test the sensitivity to different urban vegetation configurations as mitigating alternatives. As indicator of change, we use the thermal response number (TRN), which is a measure of the sensible heating to the thermal storage of a surface or region, and the Bowen ratio, which is defined as the ratio of sensible to latent heat fluxes. The TRN of the area occupied by the SJMA has decreased as a consequence of replacing the lowland coastal plain vegetation with man-made materials, indicating that it takes less energy to raise the surface temperature of the urban area, whereas the TRN of forested regions has remained virtually unchanged. The GW signal also has effects on the thermal response of the SJMA, where dryer current conditions generate lower TRN values. Differences owing to GW are more evident in the Bowen ratio pattern, mostly associated with the drier present conditions observed and its effects on sensible and latent heat fluxes. In terms of testing different mitigation strategies, the short-term simulations show that the urban area is more efficient in partitioning surface energy balance terms when green roofs are specified, as opposed to including vegetation inside the urban core.

Micrometeorological modeling to understand the thermal anomaly in the sand dunes across the Israel–Egypt border

A thermal anomaly was observed on the remote sensing images in the sand dunes across the Israel–Egypt border. The Israeli side with more vegetation cover has higher (>2°C) land surface temperature (LST) during the day than the Egyptian side where bare sand prevails. This anomaly is very obvious at about noon in the hot dry summer season. A micrometeorological model has been established in terms of surface energy balance for simulating the surface temperature change and heat flux variation of the region. The purpose of this modeling is to understand the mechanism leading to the occurrence of the thermal anomaly and to reveal the key factors controlling the surface temperature change. The characteristic of the model lies in its coupling soil temperature changing simultaneously with soil moisture movement, described as two differential equations. The methodology for the numerical solution of the model has been developed. The required meteorological data and soil parameters were measured at the study region in the hot summer season for the simulation. Two typical and the most important surface patterns of the region are considered: biogenic crust representing the Israeli side and the bare sand representing the Egyptian side. Results from simulation indicate that surface albedo contributes most to the surface temperature difference between the two typical surfaces, which is followed by sub-soil properties (mainly soil moisture) difference. Biogenic crust has lower surface albedo than bare sand. Consequently, it absorbs much more incident sky radiation. During the hot summer season, the region is very dry and vegetation is in dormancy. The canopy of most shrubs reduces to minimum. Even though the Israeli side has more vegetation, the evapo-transpiration contributed by the vegetation is still very small (<7%) in comparison with the net radiation. This small latent heat flux has little effect on the surface energy balance process in the arid environment. LST change in the desert region is mainly controlled by the amount of incident solar energy absorbed by the ground as soil heat. Therefore, the anomalous LST change and thermal variation on both sides of the region can still be explained as the direct result of the obvious albedo difference on both sides. This albedo difference is mainly caused by the sharp contrast of surface composition especially different biogenic crust and bare sand cover rates on both sides. The overgrazing and other anthropogenic stresses are the direct reasons leading to the bare sand (high albedo) prevalent on the Egyptian side while the conservation policy helps the development of biogenic crust and vegetation (low albedo) on the Israeli side.

The influence of large‐scale atmospheric circulation on the surface energy balance of the King George Island ice cap

AbstractDuring the austral summer 1997–1998 three automatic weather stations were operated at different altitudes on the sub‐Antarctic ice cap of King George Island (South Shetland Islands). Snowmelt was derived from energy balance computations. Turbulent heat fluxes were calculated from meteorological measurements using the bulk aerodynamic approach, with net radiation being measured directly. Modelled ablation rates were compared with readings at ablation stakes and continuously measured snow height at a reference site. Snow depletion and daily snowmelt cycles could be well reproduced by the model. Generally, radiation balance provided the major energy input for snowmelt at all altitudes, whereas sensible heat flux was a second heat source only in lower elevations. The average latent heat flux was negligible over the entire measuring period. A strong altitudinal gradient of available energy for snowmelt was observed. Sensible heat flux as well as latent heat flux decreased with altitude. The measurements showed a strong dependence of surface energy fluxes and ablation rates on large‐scale atmospheric conditions. Synoptic weather situations were analysed based on AVH RR infrared quicklook composite images and surface pressure charts. Maximum melt rates of up to 20 mm per day were recorded during a northwesterly advection event with meridional air mass transport. During this northwesterly advection, the contribution of turbulent heat fluxes to the energy available for snowmelt exceeded that of the radiation balance. For easterly and southerly flows, continentally toned, cold dry air masses dominated surface energy balance terms and did not significantly contribute to ablation. The link between synoptic situations and ablation is especially valuable, as observed climatic changes along the Antarctic Peninsula are attributed to changes in the atmospheric circulation. Therefore, the combination of energy balance calculations and the analysis of synoptic‐scale weather patterns could improve the prediction of ablation rates for climate change scenarios. Copyright © 2001 Royal Meteorological Society

Validation Study of the CAPS Model Land Surface Scheme Using the 1987 Cabauw/PILPS Dataset

Abstract An updated complete and comprehensive description of the land surface parameterization scheme in the Coupled Atmosphere–Plant–Soil (CAPS) model is presented. The CAPS model has been in development at Oregon State University and Phillips Laboratory since 1981. The CAPS model was originally designed for a global atmospheric model, but it has also been used as a stand-alone model for a variety of applications. The land surface scheme in the CAPS model is one of the two dozen schemes that participated in the Project for Intercomparison of Land Surface Parameterization Schemes (PILPS). Some unique features of the CAPS scheme are given in detail. A comprehensive dataset of one year (1987), including atmospheric forcing data and validation data from Cabauw, has been provided for PILPS by the Royal Netherlands Meteorological Institute. Using the Cabauw data, a validation study for the CAPS scheme has been carried out. The scheme’s self-consistencies in terms of surface energy balance and water budget are...

Comparison of regional surface fluxes from boundary‐layer budgets and aircraft measurements above boreal forest

Daytime surface sensible and latent heat fluxes (Hs and λEs) above boreal forest, derived independently from the boundary‐layer budgets of heat and water vapor and by eddy correlation from the Twin Otter aircraft, are compared. The aircraft and boundary‐layer budget values of Hs + λEs underestimated surface available energy by 25% and 4%, respectively, when the sum of the minor surface energy balance terms (storage and photosynthesis) was estimated as 17% of net radiation. The boundary‐layer budget estimate of the regional Bowen ratio (1.05) agreed to within 15% of the aircraft measurement (0.91). Both methods clearly have value for estimating land‐surface fluxes at a regional scale.

Characteristic surface energy balances for different climate types

AbstractAn attempt is made to answer the question of whether the different climate types of the Köppen‐Geiger classification are also characterized by distinct annual courses of the surface energy balance terms. For this purpose the KöppenGeiger climate map was linked with Budyko's atlas of the heat balance of the Earth. For every climate area shown on the Köppen‐Geiger map (and classified by a certain combination of up to three letters) the annual course of the energy balance terms was extracted from Budyko's atlas and plotted as an energy balance diagram. Then the annual courses of the energy balance terms of all areas classified as equable by Köppen‐Geiger (i.e. being characterized by the same combination of up to three letters) and independent of where in the world they occur could be compared. It became evident from this that each climate type (characterized by a certain letter combination) shows a distinct structure for its energy balance diagram. Thus there is an unambiguous relationship between the Köppen‐Geiger classification and the distinctive structure of the annual course of the components of the surface energy balance. There is only one exception: the BSh climate, with its transitional character throughout the world.