Surface energy balance complexity in GCM land surface models. Part II: coupled simulations

 Global coupled simulations with the Bureau of Meteorology Research Centre climate model and the CHAmeleon Surface Model (CHASM) are used to examine how four general extensions to the representation of the basic land surface energy balance affect simulated land-atmosphere interface variables: evaporation, precipitation, skin temperature and air temperature. The impacts of including separate surface energy balance calculations for: vegetated and non-vegetated portions of the land surface; an explicit parametrisation of canopy resistance; explicit bare ground evaporation; and explicit canopy interception are isolated and quantified. The hypothesis that these aspects of surface energy balance parametrisation do not contain substantial information at the monthly time scale (and are therefore not important to consider in a land surface model) is shown to be false. Considerable sensitivity to each of the four general surface energy balance extensions is identified in average pointwise monthly changes for important land-atmosphere interface variables. Average pointwise changes in monthly precipitation and land evaporation are equal to about 40 and 31–37% of the global-average precipitation and land evaporation respectively. Average pointwise changes for land surface skin temperature and lowest model layer air temperature are about 2 and 0.9 K respectively. The average pointwise change and average pointwise biases are statistically significant at 95% in all cases. Substantial changes to zonally average variables are also identified. We demonstrate how the globally averaged surface resistance parameter can vary from 150 to 25 s/m depending on which aspects of the surface energy balance are treated implicitly. We also show that if interception is treated implicitly, the effective surface resistance must vary geographically in order to capture the behaviour of a model which treats this process explicitly. The implication of these results for the design of land surface models is discussed. Received: 8 July 1999 / Accepted: 1 September 2000     

The role of surface energy balance complexity in land surface models' sensitivity to increasing carbon dioxide

Global simulations with the Bureau of Meteorology Research Centre climate model coupled to the CHAmeleon Surface Model (CHASM) are used to explore the sensitivity of simulated changes in evaporation, precipitation, air temperature and soil moisture resulting from a doubling of carbon dioxide in the atmosphere. Five simulations, using prescribed sea surface temperatures, are conducted which are identical except in the level of complexity used to represent the surface energy balance. The simulation of air temperature, precipitation, evaporation and soil moisture at 1 2 CO₂ and at 2 2 CO₂ are generally sensitive at statistically significant levels to the complexity of the surface energy balance representation (i.e. the level of complexity used to represent these processes affects the simulated climate). However, changes in mean quantities, resulting from a doubling of atmospheric CO₂, are generally insensitive to the surface energy balance complexity. Conversely, changes in the spatial and temporal variance of evaporation and soil moisture are sensitive to the surface energy balance complexity. The addition of explicit canopy interception to the simplest model examined here enables that model to capture the change in the variance of evaporation simulated by the more complex models. In order to simulate changes in the variability of soil moisture, an explicit parameterization of bare soil evaporation is required. Overall, our results increase confidence that the simulation by climate models of the mean impact of increasing CO₂ on climate are reliable. Changes in the variability resulting from increased CO₂ on air temperature, precipitation or evaporation are also likely to be reliable since climate models typically use sufficiently complex land surface schemes. However, if the impact of increased CO₂ on soil moisture is required, then a more complex surface energy balance representation may be needed in order to capture changes in variability. Overall, our results imply that the level of complexity used by most climate models to represent the surface energy balance is appropriate and does not contribute significant uncertainty in the simulation of changes resulting from increasing CO₂. Our results only relate to surface energy balance complexity, and major uncertainties remain in how to model the surface hydrology and changes in the physiology, structural characteristics and distribution of vegetation. Future developments of land surface models should therefore focus on improving the representation of these processes.     

The contribution of the land surface energy balance complexity to differences in means, variances and extremes using the AMIP-II methodology

This paper explores the relationship between the complexity of the land surface energy balance parameterization and the simulation of means, variances and extremes in a climate model. We used the BMRC climate model combined with the protocol of AMIP-II to perform six ensemble simulations for each of four levels of surface energy balance complexity. Our results were then compared with other AMIP-II results in terms of the mean, variance and extremes of temperatures and precipitation. In terms of the zonally-averaged mean and the maximum temperatures and precipitation, the surface energy balance complexity did not systematically affect the BMRC climate model results. The zonal minimum temperature was affected by the inclusion of tiling and/or a temporally variable canopy conductance. We found no evidence that surface energy balance complexity affected the globally- or zonally-averaged variances. Some quite large differences were identified in the probability density functions of maximum (10 K) and minimum (4 K) temperature caused by surface tiling and/or the inclusion of a time-varying canopy conductance. With these included, the model simulated a higher probability of cooler minima and warmer maxima and therefore a different diurnal temperature range. Adding interception of precipitation led to an increase in the likelihood of more extreme precipitation. Thus, provided interception, surface tiling and a time-variable stomatal conductance are included in a land surface model, the impact of other uncertainties in the parameterization of the surface energy balance are unlikely to limit the use of climate models for simulating changes in the extremes. Most published results indicating changes to precipitation and temperature extremes due to increasing carbon dioxide are therefore unlikely to be significantly limited by uncertainty in how to parameterize the surface energy balance. Given that the variations in surface energy balance complexity included in our experiments approximates the range included in the AMIP-II models, we conclude that it this is unlikely to explain the differences found between the AMIP-II simulations. This does not mean that AMIP-II differences are not caused to a significant degree by differences in their respective LSMs, rather it limits the potential role of the land surface to non-surface energy balance components, or components (such as carbon) that are not considered here.     

Testing a GCM land surface scheme against catchment-scale runoff data

A GCM land surface scheme was used, in off-line mode, to simulate the runoff, latent and sensible heat fluxes for two distinct Australian catchments using observed atmospheric forcing. The tropical Jardine River catchment is 2500 km² and has an annual rainfall of 1700 mm y^–1 while the Canning River catchment is 540 km², has a Mediterranean climate (annual rainfall of 800 mm y^–1) and is ephemeral for half the year. It was found that the standard version of a land surface scheme developed for a GCM, and initialised as for incorporation into a GCM, simulated similar latent and sensible heat fluxes compared to a basin-scale hydrological model (MODHYDROLOG) which was calibrated for each catchment. However, the standard version of the land surface scheme grossly overestimated the observed peak runoff in the wet Jardine River catchment at the expense of runoff later in the season. Increasing the soil water storage permitted the land surface scheme to simulate observed runoff quite well, but led to a different simulation of latent and sensible heat compared to MODHYDROLOG. It is concluded that this 2-layer land surface scheme was unable to simulate both catchments realistically. The land surface scheme was then extended to a three-layer model. In terms of runoff, the resulting control simulations with soil depths chosen as for the GCM were better than the best simulations obtained with the two-layer model. The three-layer model simulated similar latent and sensible heat for both catchments compared to MODHYDROLOG. Unfortunately, for the ephemeral Canning River catchment, the land surface scheme was unable to time the observed runoff peak correctly. A tentative conclusion would be that this GCM land surface scheme may be able to simulate the present day state of some larger and wetter catchments but not catchments with peaky hydrographs and zero flows for part of the year. This conclusion requires examination with a range of GCM land surface schemes against a range of catchments. Received: 9 June 1995 / Accepted: 4 April 1996     

Global warming potentials modified for surface radiative forcing for use in surface energy balance models

The radiative impact of greenhouse gases in warming the Earth varies significantly, depending on whether one considers the forcing at the tropopause or at the surface. Compared to the former, the surface forcing for some greenhouse gases is reduced by the interference of water vapour. Hence, we calculate alternative surface global warming potentials (SGWPs) that are derived from the surface forcing radiation of greenhouse gases for potential use in surface radiative energy balance models (SREBMs). For gases with a large water vapour overlap, the SGWPs are typically 30% smaller than current GWPs; for gases with relatively little overlap, the SGWPs are larger by more than 33%. These results may be used in conjunction with SREBMs as an additional means of calculating climate change, and may lead to an altered emissions budget compared to that outlined by the current Kyoto agreement.     

植被冠层截留对地表水分和能量平衡影响的数值模拟

利用NCAR_CLM4.0模式,通过有无植被冠层截留的试验对比分析,讨论了植被冠层截留对全球陆面水分和能量平衡产生的潜在影响.结果表明:就全球水分平衡而言,不考虑植被冠层截留时,全球平均土壤总含水量、表面径流和次表面径流增加,蒸散发减少.空间分布特征表明,低纬地区各水分平衡分量全年维持较高的差值分布,并随季节变化沿赤道南北振荡;北半球中高纬高值区有春季扩张、夏季极盛、秋冬季撤退的趋势.冠层截留消失后冠层蒸发的消失是蒸散发减弱的主要原因.对于能量平衡而言,不考虑冠层截留时,全球感热通量增加,冠层感热的增加明显大于地面感热的减少;潜热减少.此外,不同植被类型对不考虑冠层截留后产生的响应存在明显差异.     

Local climates simulated by two generations of Canadian GCM land surface schemes

Abstract

The performance of two Canadian land surface schemes of widely differing complexity is compared and contrasted in a pair of year‐long simulations using the GCM developed at Atmospheric Environment Service, Canada. The old land surface model incorporates the force‐restore method for soil temperatures and the bucket approximation for soil moisture; the new model, CLASS (Canadian Land Surface Scheme) features three soil layers, an explicitly modelled snow layer, a thermally separate vegetation canopy, and physically‐based calculations of heat and moisture transfers between all of the land surface components and the atmosphere.

It was reported in previous papers that compared with observations, the old scheme tends to generate a climate which is characterized by anomalously high precipitation rates and cold temperatures over land. In this paper, by reference to field measurements and to the energy fluxes and temperatures generated by the two models at local scales, the hypotheses earlier postulated as to the underlying reasons for this are validated. The main factor contributing to the climate anomalies observed with the old scheme is found to be its generation of excessive evaporation rates; this is caused by the fact that the evaporation rate is never directly energy‐limited, the fact that the scaling of the evaporation rale with decreasing soil moisture content underestimates the effect of vegetation stomatal resistance, and the fact that the evaporation rate over bare soil depends not on the surface soil moisture, but on the moisture content of whole modelled soil column. The cold surface temperatures are additionally attributed to systematic errors incurred by the forward‐stepping temperature scheme, and to the fact that soils subjected to subzero temperature forcing in the winter are modelling as freezing completely. Finally, the inability of the old scheme to simulate partially frozen soils means that it proves unable to handle either shallow frost penetration at temperature latitudes, or the development of an active layer in permafrost.     

GCM sensitivity experiments with locally modified land surface properties over tropical South America

Ensembles of 1-year-long experiments with a relatively high-resolution ECMWF model were conducted in order to investigate the impact of modified land surface properties on local, regional and large-scale atmospheric circulations. The modifications consisted of changes to land cover and increased albedo over the northern part of South America. In many respects the experimental design resembles the setting of classical deforestation experiments. The local model response to imposed modifications, which includes a reduction in precipitation as well as in evaporation and an increase in surface temperature, was found to be stronger in dry (July–September, JAS) than in wet (January–March, JFM) season, and in the ensemble with higher albedo value. Local drying is discussed in terms of locally generated overturning that resembles a direct thermal circulation. The effects of this circulation seem to be dominant over the reduction in large-scale moisture supply from the adjacent ocean. On large scales, changes to the Pacific branch of the Walker circulation lead, through modified divergent flow, to a tropics-wide impact on precipitation. In addition to South America, the largest changes are seen in the south Pacific convergence zone in JFM, while the impact on the Atlantic inter-tropical convergence zone is stronger in JAS. In the extratropics, there is little change in precipitation. In the upper troposphere, a distinctive teleconnection wave-pattern could be seen in the Pacific/North American region during JFM. A notable feature in the upper-air model response in JAS is a wave train extending from South America, over the northern Atlantic into Europe. With regard to the interaction between the land surface response and model systematic errors, our results suggest that the erroneous shift of the downward branch of the Pacific/South American Walker circulation is likely to be a cause, rather than a consequence, of the rainfall deficit over South America in the model climatology.     

Tests of three urban energy balance models

Observations of surface characteristics, meteorological conditions and energy balance components from Vancouver, B.C. are used to test the validity of the output from three one-dimensional surface energy balance models. The results show that whereas all of the models provide good simulations of net radiation, none can consistently predict the turbulent fluxes of sensible and latent heat using easily available input data. Inability to handle the role of water availability and its impact on evapotranspiration is identified as the principal problem.     

Global and continental water balance in a GCM

Surface hydrology is recognised as an important component of general circulation climate models. The global and regional climates simulated by such models are demonstrably sensitive to the parameterization of terrestrial hydrologic processes. There exists, therefore, a clear requirement to evaluate different parameterization approaches in terms of the representation of the terrestrial phase of the hydrologic cycle. One potential means of meeting this requirement is by using available continental water-balance summaries. In this study three versions of a GCM, the National Center for Atmospheric Research (NCAR) Community Climate Model Version l (CCM1), differing mainly in spatial resolution and the representation of the surface hydrology, are compared against existing water-balance studies. Additional streamflow data are incorporated as a means of further validating both the water-balance approach and the GCM surface hydrologic parameterization in capturing the gross features of continental-scale hydrology.     

The impact of implementing the bare essentials of surface transfer land surface scheme into the BMRC GCM

This study describes the first order impacts of incorporating a complex land-surface scheme, the bare essentials of surface transfer (BEST), into the Australian Bureau of Meteorology Research Centre (BMRC) global atmospheric general circulation model (GCM). Land seasonal climatologies averaged over the last six years of integrations after equilibrium from the GCM with BEST and without BEST (the control) are compared. The modeled results are evaluated with comprehensive sources of data, including the layer-cloud climatologies from the international satellite cloud climatology project (ISCCP) data from 1983 to 1991 and the surface-observed global data of Warren et al., a five-year climatology of surface albedo estimated from earth radiation budget experiment (ERBE) top-of-the-atmosphere (TOA) radiatioe fluxes, global grid point datasets of precipitation, and the climatological analyses of surface evaporation and albedo. Emphasis is placed on the surface evaluation of simulations of landsurface conditions such as surface roughness, surface albedo and the surface wetness factor, and on their effects on surface evaporation, precipitation, layer-cloud and surface temperature. The improvements due to the inclusion of BEST are: a realistic geographical distribution of surface roughness, a decrease in surface albedo over areas with seasonal snow cover, and an increase in surface albedo over snow-free land. The simulated reduction in surface evaporation due, in part, to the biophysical control of vegetation, is also consistent with the previous studies. Since the control climate has a dry bias, the overall simulations from the GCM with BEST are degraded, except for significant improvements for the northern winter hemisphere because of the realistic vegetation-masking effects. The implications of our results for synergistic developments of other aspects of model parameterization schemes such as boundary layer dynamics, clouds, convection and rainfall are discussed.     

The surface energy balance of a snow cover: comparing measurements to two differentsimulation models

Summary  We compared two one-dimensional simulation models for heat and water fluxes in the soil-snow-atmosphere system with respect to their mathematical formulations of the surface heat exchange and the snow pack evolution. They were chosen as examples of a simple one-layer snow model and a more detailed multiple-layer snow model (SNTHERM). The snow models were combined with the same one-dimensional model for the heat and water balance of the underlying soil (CoupModel). Data from an arable field in central Sweden (Marsta), covering two years (1997–1999) of soil temperature, snow depth and eddy-correlation measurements were successfully compared with the models. Conditions with a snow pack deeper or shallower than 10 cm and bare soil resulted in similar discrepancies. The simulated net radiation and sensible heat flux were in good agreement with that measured during snow-covered periods, except for situations with snowmelt when the downward sensible heat flux was overestimated by 10–20 Wm⁻². The results showed that the uncertainties in parameter values were more important than the model formulation and that both models were useful in evaluating the limitations and uncertainties of the measurements. Received November 1, 1999 Revised April 20, 2000     

Effective parameters of surface energy balance in heterogeneous landscape

This paper addresses the problem of estimating surface fluxes at large scale over heterogeneous terrain, and the corresponding determination of effective surface parameters. Two kinds of formulation are used to calculate the fluxes of sensible and latent heat: the basic diffusion equations (Ohm's law type) and the Penman-Monteith equations. The strategy explored is based upon the principle of flux conservation, which stipulates that the average flux over a large area is simply the area-weighted mean of the contributions from the different patches making up the area. We show that the application of this strategy leads to different averaging schemes for the surface parameters, depending on the type of flux (latent heat, sensible heat) and on the type of formulation used to express the flux. It appears that the effective value of a given parameter must be appraised for each individual application, because it is not unique, but differs according to the magnitude being conserved and the equation used to express this magnitude. Numerical simulations are carried out to test over contrasted areas the aggregation procedures obtained. The areal fluxes estimated from these effective parameters, together with the areal fluxes calculated by means of a simple areal averaging of the parameters, are compared to the true average fluxes, calculated as area-weighted means of the elementary fluxes. The aggregation procedures obtained prove to be much more accurate for estimating areal fluxes and for closing the energy balance equation than those based upon simple areal averaging of the parameters.     

The small ice cap instability in seasonal energy balance models

Results from a two-dimensional energy balance model with a realistic land-ocean distribution show that the small ice cap instability exists in the Southern Hemisphere, but not in the Northern Hemisphere. A series of experiments with a one-dimensional energy balance model with idealized geography are used to study the roles of the seasonal cycle and the land-ocean distribution. The results indicate that the seasonal cycle and land-ocean distribution can influence the strength of the albedo feedback, which is responsible for the small ice cap instability, through two factors: the temperature gradient and the amplitude of the seasonal cycle. The land-ocean distribution in the Southern Hemisphere favors the small ice cap instability, while the land-ocean distribution in the Northern Hemisphere does not. Because of the longitudinal variations of land-ocean distribution in the Northern Hemisphere, the behavior of ice lines in the Northern Hemisphere cannot be simulated and explained by the model with zonally symmetric land-ocean distribution. Model results suggest that the small ice cap instability may be a possible mechanism for the formation of the Antarctic icesheet. The model results cast doubt, however, on the role of the small ice cap instability in Northern Hemisphere glaciations. Offprint requests to: J Huang     

Improved representation of ocean heat content in energy balance models

Climatic Change - Anomaly-diffusing energy balance models (AD-EBMs) are routinely employed to analyze and emulate the warming response of both observed and simulated Earth systems. We demonstrate a...     

The global energy balance from a surface perspective

In the framework of the global energy balance, the radiative energy exchanges between Sun, Earth and space are now accurately quantified from new satellite missions. Much less is known about the magnitude of the energy flows within the climate system and at the Earth surface, which cannot be directly measured by satellites. In addition to satellite observations, here we make extensive use of the growing number of surface observations to constrain the global energy balance not only from space, but also from the surface. We combine these observations with the latest modeling efforts performed for the 5th IPCC assessment report to infer best estimates for the global mean surface radiative components. Our analyses favor global mean downward surface solar and thermal radiation values near 185 and 342 Wm^?2, respectively, which are most compatible with surface observations. Combined with an estimated surface absorbed solar radiation and thermal emission of 161 and 397 Wm^?2, respectively, this leaves 106 Wm^?2 of surface net radiation available globally for distribution amongst the non-radiative surface energy balance components. The climate models overestimate the downward solar and underestimate the downward thermal radiation, thereby simulating nevertheless an adequate global mean surface net radiation by error compensation. This also suggests that, globally, the simulated surface sensible and latent heat fluxes, around 20 and 85 Wm^?2 on average, state realistic values. The findings of this study are compiled into a new global energy balance diagram, which may be able to reconcile currently disputed inconsistencies between energy and water cycle estimates.     

Components of surface energy balance in a temperate grassland ecosystem

Eddy correlation measurements were made of fluxes of moisture, heat and momentum at a tallgrass prairie site near Manhattan, Kansas, U.S.A. during the First ISLSCP Field Experiment (FIFE) in 1987. The study site is dominated by three C₄ grass species: big bluestem (Andropogon gerardii), indiangrass (Sorghastrum nutans), and switchgrass (Panicum virgatum). The stomatal conductance and leaf water potential of these grass species were also measured.In this paper, daily and seasonal variations in the components of the surface energy balance are examined. The aerodynamic and canopy surface conductances for the prairie vegetation are also evaluated.Published as Paper No. 8987, Journal Series, Nebraska Agricultural Research Division.

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The impact of new land surface physics on the GCM simulation of climate and climate sensitivity

 Recent improvements to the Hadley Centre climate model include the introduction of a new land surface scheme called “MOSES” (Met Office Surface Exchange Scheme). MOSES is built on the previous scheme, but incorporates in addition an interactive plant photosynthesis and conductance module, and a new soil thermodynamics scheme which simulates the freezing and melting of soil water, and takes account of the dependence of soil thermal characteristics on the frozen and unfrozen components. The impact of these new features is demonstrated by comparing 1×CO₂ and 2×CO₂ climate simulations carried out using the old (UKMO) and new (MOSES) land surface schemes. MOSES is found to improve the simulation of current climate. Soil water freezing tends to warm the high-latitude land in the northern Hemisphere during autumn and winter, whilst the increased soil water availability in MOSES alleviates a spurious summer drying in the mid-latitudes. The interactive canopy conductance responds directly to CO₂, supressing transpiration as the concentration increases and producing a significant enhancement of the warming due to the radiative effects of CO₂ alone. Received: 16 March 1998 / Accepted: 4 August 1998