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.     

陆面过程模式CLASS径流模拟的改进和应用

针对陆面过程模式CLASS(Canadian Land Surface Scheme)产流模拟方面的不足,提出考虑壤中流产流机制的产流模拟方案。利用淮河流域能量和水分循环试验(HUBEX)资料,在史灌河流域对改进前后的模型进行了对比试验。结果表明,产流模拟改进方案大大提高了CLASS的径流模拟能力,同时改善了模式对土壤含水量的模拟。     

Implementation of a large‐scale variable velocity river flow routing algorithm in the Canadian Regional Climate Model (CRCM)

Abstract

Implementation and validation of a flow routing scheme for the North American domain of the Canadian Regional Climate Model (CRCM) is described. A variable velocity flow routing algorithm is used to transport runoff from the land surface to the continental edges and provide freshwater flux forcing for the oceans. The flow routing scheme uses Manning's equation to estimate flow velocities for river channels whose cross‐sections are assumed to be rectangular. Discretization of major North American river basins and their flow directions are obtained at the polar stereographic resolution of the CRCM using 5‐minute global river flow direction data as a template. In the absence of observation‐based gridded estimates of runoff, model runoff estimates from a global simulation of the Variable Infiltration Capacity (VIC) hydrological model (forced with observationbased meteorological data) are used to validate the flow routing scheme. Model results show that the inclusion of flow routing improves the comparison with observation‐based streamflow estimates when compared to the unrouted runoff. Monthly comparison of simulated streamflow with observation‐based estimates, and basin‐wide averaged flow velocities, suggests that the flow routing scheme performs satisfactorily.     

Effect of surface layer thickness on simultaneous transport of heat and water in a bare soil and its implications for land surface schemes

Abstract

A physically‐based multi‐layer numerical model is developed to determine the coupled transport of heat and water in the soil and in the soil‐atmosphere boundary layer. Using inputs of standard weather data and initial soil conditions the model is capable of predicting the surface energy balance components as well as water content and temperature profiles in the soil. It is used to predict these variables for a bare silt loam soil under two tillage treatments, viz. culti‐packed and left loose after disc‐harrowing, and the predicted results are compared with measurements. Very good agreement between the model predictions and measured evaporation and heat fluxes and soil water and temperatures for a ten‐day period shows that the model is capable of simulating the coupled transport of soil heat and soil water and their transfer across the soil surface‐atmosphere interface adequately.

Model predictions were compared with those of CLASS (Canadian Land Surface Scheme). It is shown that CLASS, version 2.6, provides good estimates of evaporation and hence the latent heat flux density, Q_E, under wetter soil conditions, but overestimates Q_E at moderately wet soil conditions and underestimates it under dry soil conditions. Under dry to moderately wet soil conditions the calculation of evaporation from bare soil is very sensitive to the thickness of the top layer particularly as the thickness approaches 10 cm.     

一种陆面过程模式对径流的模拟研究

径流在陆面模式水量平衡计算中占有重要地位,它不但与土壤水的动态变化有关,而且会影响感热、潜热等其他通量的计算结果.作者针对陆面过程模式AVIM(Atmosphere VegetationInteraction Model)对产流描述的不足,改进模式中对径流的参数化方法.并将改进后的模式用于内蒙古的锡林河流域,以检验模式对径流的模拟能力.1991～1994年的径流模拟结果表明,改进后的模式对径流的模拟有较好的改善.     

On improving cold region hydrological processes in the Canadian Land Surface Scheme

Regional and global climate model simulated streamflows for high-latitude regions show systematic biases, particularly in the timing and magnitude of spring peak flows. Though these biases could be related to the snow water equivalent and spring temperature biases in models, a good part of these biases is due to the unaccounted effects of non-uniform infiltration capacity of the frozen ground and other related processes. In this paper, the treatment of frozen water in the Canadian Land Surface Scheme (CLASS), which is used in the Canadian regional and global climate models, is modified to include fractional permeable area, supercooled liquid water and a new formulation for hydraulic conductivity. The impact of these modifications on the regional hydrology, particularly streamflow, is assessed by comparing three simulations performed with the original and two modified versions of CLASS, driven by atmospheric forcing data from the European Centre for Medium-Range Weather Forecast (ECMWF) reanalysis (ERA-Interim) for the 1990–2001 period over a northeast Canadian domain. The two modified versions of CLASS differ in the soil hydraulic conductivity and matric potential formulations, with one version being based on formulations from a previous study and the other one is newly proposed. Results suggest statistically significant decreases in infiltration and therefore soil moisture during the snowmelt season for the simulation with the new hydraulic conductivity and matric potential formulations and fractional permeable area concept compared to the original version of CLASS, which is also reflected in the increased spring surface runoff and streamflows in this simulation with modified CLASS over most of the study domain. The simulated spring peaks and their timing in this simulation are also in better agreement to those observed. This study thus demonstrates the importance of treatment of frozen water for realistic simulation of streamflows.     

A GCM-based assessment of the global moisture budget and the role of land-surface moisture reservoirs in processing precipitation

The performance of the Canadian Land Surface Scheme (CLASS) when coupled to the CCCma third generation general circulation model is evaluated in an AMIP II simulation. Our primary aim is to understand how CLASS processes moisture and to compare model estimates of moisture budget components with observations. The modelled mean annual precipitation and runoff, and their latitudinal structures, compare well with observations although some discrepancies remain in the simulation of regional values of these quantities. The amplitude and phase of the first harmonic of the precipitation annual cycle also compares well with observations although less well over regions of sparse precipitation and/or high topography. In the model, the canopy plays a major role in processing moisture at the land surface indicating the importance of vegetation in climate. The canopy intercepts a large fraction of the precipitation and provides the medium for returning much moisture back to the atmosphere as evapotranspiration. Though important locally, the snow moisture reservoir plays a relatively minor role in the global moisture budget. It acts primarily as a storage and delay mechanism with winter precipitation released to the ground reservoir on melting. The ground moisture reservoir also plays a major role and processes a similar amount of moisture as the canopy, although in a different manner. The globally averaged model runoff compares well with observation-based estimates, although the model partitioning into surface runoff and drainage does not agree particularly well with the single available observation-based estimate. How moisture is processed at the land surface serves as a basis for model intercomparison and for understanding the modelled moisture budget and its variation and changes with climate change. Only the most basic quantities (precipitation, runoff, and partitioning of runoff into surface runoff and drainage) may be compared with observation-based estimates, however, and the establishment of more complete moisture budget remains an important need.     

Atmospheric moisture budget estimates of regional evapotranspiration from RES‐91

Abstract

Diurnal changes in the local atmospheric moisture budget over the Canadian Prairies are computed using sequential radiosonde soundings from the 1991 Regional Evaporation Study (RES‐91). Previous attempts to estimate evapotranspiration with radiosonde data have used either similarity theory or a moisture budget, but have been confined to the boundary layer in either case. These studies, as well as semi‐empiric operational techniques which use surface‐based data, exclude the effects of moisture advection and energy exchanges between the boundary layer and the free atmosphere, assuming negligible effects on evapotranspiration. The moisture budget method adopted here includes horizontal advection explicitly, and treats vertical fluxes implicitly through a total tropospheric moisture budget.

Comparison of the evapotranspiration estimates with those of other techniques are positive only when results are averaged over several days to weeks. While the advection estimates are a major source of error for the “daily” estimates in this particular study, it is shown that neither advection nor moisture flux through the boundary layer can be ignored in estimating daily evapotranspiration, regardless of the technique used. The results also suggest that evapotranspiration is more variable on a daily basis than other techniques have indicated. With an improved synoptic database now available for advection estimates, the moisture budget technique may provide an excellent ground‐truth method for fine‐tuning techniques for remote sensing of evapotranspiration, and could lead to improved parametrization schemes for both NWP models and GCMs.     

Parametrization of peatland hydraulic properties for the Canadian land surface scheme

Abstract

A hydraulic parametrization is developed for peatland environments in the Canadian Land Surface Scheme (CLASS). Three ‐wetland soil classes account for the typical variation in the hydraulic characteristics of the uppermost 0.5 m of organic soils. Review of the literature reveals that saturated hydraulic conductivity varies from a median of 1.0 × 10^?7m/s in deeply humified sapric peat to 2.8 × 10^?4 m/s in relatively undecomposed fibric peat. Average pore volume fraction ranges from 0.83 to 0.93. Parameters have been designed for the soil moisture characteristic curves for fibric, hemic and sapric peat using the Campbell (1974) equation employed in CLASS, and the van Genuchten (1980) formulation. There is little difference in modelled soil moisture between the two formulations within the range of conditions normally found in peatlands. Validation of modelled water table depth and peat temperature is performed for a fen in northern Québec and a bog in north‐central Minnesota. The new parametrization results in a more realistic simulation of these variables in peatlands than the previous version of CLASS, in which unrealistic mineral soil “equivalents “ were used for wetland soil climate modelling.     

Parametrization schemes of incident radiation in the North Water polynya

Abstract

An evaluation of the Canadian Land Surface Scheme (CLASS) 3.1 snow cover simulations at four sites included in the Snow Model Intercomparison Project (SnowMIP) revealed that CLASS was able to provide realistic representations of snow cover accumulation, melt and physical properties over a range of snow cover climates. The modified snow aging parametrization in CLASS 3.1 provided improved simulations of snowpack density which resulted in a marked reduction in the root‐mean‐square (rms) error for daily snow depth, and slight improvements in snow surface temperature. CLASS 3.1 still exhibited a tendency to overestimate snow cover duration which is attributed to the way shallow snow ablation is treated. CLASS provided generally realistic simulations of daily and seasonal variation in snow albedo although cold snow albedo was underpredicted by 0.10 to 0.15 at a site with a deep (> 2 m) cold snowpack. CLASS also exhibited a tendency to overpredict late spring snow albedo which was reduced by the addition of a snow layer subroutine that kept track of snow albedo by precipitation event. CLASS had a noticeable cold bias averaging 3°–4°C at two mountain sites included in the comparison. The bias was closely linked to atmospheric stability and could exceed 10°C under conditions of strong radiative cooling and low wind speeds. The CLASS energy deficit under these conditions was determined to be ～20–40 W m^?2 and was mostly accounted for by introducing a windless exchange coefficient into the calculation of sensible heat fluxes following the approach used in a number of other physical snowpack models. CLASS provided realistic simulations of daily snowmelt runoff with the exception of the Weissfluhjoch site which was characterized by a deep cold snowpack. A preliminary assessment of snow water equivalent (SWE) rms error for the 23 models participating in SnowMIP showed that CLASS was one of the better single layer snow models included in the comparison. CLASS performance was comparable to the multi‐layer CROCUS snowpack model in the evaluations carried out in this study.     

Validation and sensitivity of the global hydrologic budget in stand-alone simulations with the ISBA land-surface scheme

 Global soil moisture data of high quality and resolution are not available by direct observation, but are useful as boundary and initial conditions in comprehensive climate models. In the framework of the GSWP (Global Soil Wetness Project), the ISBA land-surface scheme of Météo-France has been forced with meteorological observations and analyses in order to study the feasibility of producing a global soil wetness climatology at a 1°×1° horizontal resolution. A control experiment has been performed from January 1987 to December 1988, using the ISLSCP Initiative I boundary conditions. The annual mean, the standard deviation and the normalised annual harmonic of the hydrologic fields have been computed from the 1987 monthly results. The global maps which are presented summarise the surface hydrologic budget and its annual cycle. The soil wetness index and snow cover distributions have been compared respectively to the results of the ECMWF reanalysis and to satellite and in situ observations. The simulated runoff has been validated against a river flow climatology, suggesting a possible underestimation over some large river basins. Besides the control run, other simulations have been performed in order to study the sensitivity of the hydrologic budget to changes in the surface parameters, the precipitation forcing and the runoff scheme. Such modifications have a significant impact on the partition of total precipitation into evaporation and runoff. The sensitivity of the results suggests that soil moisture remains one of the most difficult climatological parameters to model and that any computed soil wetness climatology must be considered with great caution. Received: 3 January 1997 / Accepted: 19 August 1987     

Closing the Mackenzie basin water budget,water years 1994/95 to 1996/97

Abstract

A particularly elusive science objective for the Mackenzie Global Energy and Water Cycle Experiment (GEWEX) Study (MAGS) has been to close the atmospheric moisture budget and rationalize it against the surface water budget at annual or even monthly timescales. The task, while not difficult in principle, is complicated by two factors. First is the importance of basin snow‐cover, soil and water‐body storage in the surface water budget. Month‐to‐month changes in these components are frequently greater than the atmospheric flux terms, for example, during spring snowmelt. Furthermore, there is approximately a six‐week lag before local changes are evident in the discharge at the mouth of the basin. Second, the coarse resolution of all of the supporting data may add significant systematic errors. For example, the two radiosonde soundings per day available to the project are unlikely to account adequately for all the moisture generated locally through evapotranspiration during the summer convective season.

This analysis will directly address these two main issues by applying hydrologic and atmospheric computations to assess the storage question, and by using additional soundings at a single site to sample the diurnal signature in atmospheric moisture caused by evapotranspiration. Resulting modifications to the atmospheric moisture and surface water budgets then allow near closure of the MAGS monthly water budget within acceptable error limits.     

Downscaling the hydrological cycle in the Mackenzie basin with the Canadian regional climate model

Abstract

The Canadian Regional Climate Model (CRCM) has been nested within the Canadian Centre for Climate Modelling and Analysis ‘ second generation General Circulation Model (GCM), for a single month simulation over the Mackenzie River Basin and environs. The purpose of the study is to assess the ability of the higher resolution CRCM to downscale the hydrological cycle of the nesting GCM. A second 1‐month experiment, in which the CRCM was nested within analyzed fields of a global data assimilation system, was also performed to examine the sensitivity of the basin moisture budget to atmospheric lateral boundary forcing.

We have found that the CRCM can produce realistic lee cyclogenesis, preferentially in the Liard sub‐basin, along with associated circulation and precipitation patterns, as well as an improved rainshadow in the lee of the Rocky Mountains compared to the GCM. While these features do quantitatively affect the monthly average climate statistics, the basin scale moisture budgets of the models were remarkably similar, though some of this agreement is due to compensating errors in the GCM. Both models produced excessive precipitation compared to a recent climatology for the region, the cause of which is traced to lateral boundary forcing. A second experiment, identical to the first except that the CRCM was forced with analyzed fields at the lateral boundaries, produced a qualitatively different basin moisture budget, including a much more realistic precipitation field. Errors in the moisture budget of the first experiment appear to be associated with the poor representation of the Aleutian Low in the GCM, and do not appear to be strongly connected to (local) surface processes within the models. This suggests that an effective strategy for modelling the hydrological cycle of the Mackenzie Basin on the fast climate timescale ‐ a major requirement of the Mackenzie GEWEX Study ‐ will involve nesting the CRCM within analyzed (or re‐analyzed) atmospheric fields.     

Modelling spatially distributed snowmelt and meltwater runoff in a small Arctic catchment with a hydrology land‐surface scheme (WATCLASS)

Abstract

The fully distributed hydrology land‐surface scheme WATCLASS is used to simulate spring snowmelt runoff in a small Arctic basin, Trail Valley Creek, dominated by open tundra and shrub tundra vegetation. The model calculates snowmelt rates from a full surface energy balance, and a three‐layer soil model is used to simulate the infiltration into and the exchange of heat and moisture within the ground. The generated meltwater is delivered to the stream channel network by overland flow, interflow, and baseflow and subsequently routed out of the catchment. Subgrid spatial variability is handled by the model through the use of grouped response units (GRUs). The GRUs in WATCLASS are chosen according to vegetation land cover.

Five spring snowmelt periods with a variety of initial end‐of‐winter snow cover and melt conditions were simulated and compared with observed runoff data. In a second step, the model's ability to simulate spatially variable snow covered area (SCA) within the basin was tested by comparing model predictions to remotely sensed SCA. WATCLASS was able to predict runoff volumes (on average within 15% over five years of modelling) as well as timing of snowmelt and meltwater runoff for open tundra fairly accurately. However, the model underestimated melt in the energetically more complex shrub tundra areas of the basin. Furthermore, the observed high spatial variability of the SCA at a 1‐km resolution was not captured well by the model.

Several recommendations are made to improve model performance in Arctic basins, including a more realistic implementation of the gradual deepening of the thawed layer during the spring, and the use of topographic information in the definition of land cover classes for the GRU approach.     

Sensitivity of the hydrological cycle to the parametrization of soil hydrology in a GCM

 The sensitivity of the hydrological cycle to soil hydrology is investigated with the LMD GCM. The reference simulation includes the land-surface scheme SECHIBA, with a two-reservoir scheme for soil water storage and runoff at saturation. We studied a non-linear drainage parametrization, and a distributed surface runoff parametrization, accounting for the subgrid scale variability (SSV) of soil moisture capacity, through a distribution where the shape parameter was b. GCM results show that the drainage parametrization induces significant reductions in soil moisture and evaporation rate compared to the reference simulation. They are related to changes in moisture convergence in the tropics, and to a precipitation decrease in the extratropics. When drainage is implemented, the effect of the SSV parametrization (b=0.2) is also to reduce soil moisture and evaporation rates compared to the simulation with drainage only. These changes are much smaller than the former, but the sensitivity of the hydrological cycle to the SSV parametrization is shown to be larger in dry periods, and to be enhanced by an increase of the shape parameter b. The comparison of simulated total runoffs with observed data shows that the soil hydrological parametrizations does not reduce the GCM systematic errors in the annual water balance, but that they can improve the representation of the total runoff’s annual cycle.     

具有Horton及Dunne机制的径流模型在VIC模型中的应用(英)

地表径流主要由蓄满(Dunne)和超渗产流(Horton)机制产生;土壤性质的空间变异性、前期土壤水、地形及降水的空间变异性导致不同的径流机制。在研究区域或模型网格内,蓄满产流及超渗产流可能同时出现,缺乏考虑任何一种机制以及土壤性质的次网格空间变率可能导致地表径流的过高或过低估计,从而影响土壤水的计算。利用Philip入渗公式用于时间压缩逼近(TCA)给出了一种径流参数化方法,该方法可以动态实现模型网格中的Horton及Dunne产流机理,它考虑了土壤空间变异性对Horton和Dunne径流的影响。该径流模型应用到基于水文原理的陆面过程模型VIC,在淮河流域及美国宾西法尼亚州的一个流域进行了测试,结果表明:新的参数化方法对地表径流和土壤水分含量的分配起着重要作用,对于改进径流和土壤水的模拟有重要意义。     

GRAPES NOAH-LSM陆面模式水文过程的改进及试验研究

土壤含水量的计算影响着陆面过程的能量平衡和水量平衡,是陆面模式的核心计算要素之一。目前,GRAPES_Meso模式采用的NOAH-LSM(Noah-Land Surface Model)陆面模式既不能有效地表达径流产源面积的变动情况,也不能完整描述水文循环过程。本次试验针对以上问题对其进行了改进:(1)加入蓄水容量曲线,考虑网格内产流面积的变化及土壤含水量的不均匀性;(2)加入汇流模式,以考虑水平二维水分再分配,提高模式对径流和流量模拟能力。选取2008年8月至9月降水进行模拟试验,研究陆面水循环过程对近地面气象要素的影响。结果表明:改进后的模式模拟土壤湿度、2 m温度等近地面气象要素更接近观测值,并最终对降水量以及降水落区也产生了一定的影响。     

Effects of increased CO2 on land water balance from 1850 to 1989

Numerous studies have shown that increased atmospheric CO₂ concentration is one of the most important factors altering land water balance. In this study, we investigated the effects of increased CO₂ on global land water balance using the dataset released by the Coupled Model Intercomparison Project Phase 5 derived from the Canadian Centre for Climate Modelling and Analysis second-generation Earth System Model. The results suggested that the radiative effect of CO₂ was much greater than the physiological effect on the water balance. At the model experiment only integrating CO₂ radiative effect, the precipitation, evapotranspiration (ET) and runoff had significantly increased by 0.37, 0.12 and 0.31 mm year^?2, respectively. Increases of ET and runoff caused a significant decrease of soil water storage by 0.05 mm year^?2. However, the results showed increases of runoff and decreases of precipitation and ET in response to the CO₂ fertilisation effect, which resulted into a small, non-significant decrease in the land water budget. In the Northern Hemisphere, especially on the coasts of Greenland, Northern Asia and Alaska, there were obvious decreases of soil water responding to the CO₂ radiative effect. This trend could result from increased ice–snow melting as a consequence of warmer surface temperature. Although the evidence suggested that variations in soil moisture and snow cover and vegetation feedback made an important contribution to the variations in the land water budget, the effect of other factors, such as aerosols, should not be ignored, implying that more efforts are needed to investigate the effects of these factors on the hydrological cycle and land water balance.     

Prairie agroclimate boundary‐layer model: A simulation of the atmosphere/crop‐soil interface

Abstract

This paper describes a 1‐D agroclimatic model of the atmosphere/crop‐soil interface. Vertical profiles of wind, potential temperature and water vapour are constructed twice daily for the overnight‐low and maximum temperature times by combining 1200 and 0000 UTC upper‐air standard‐level grid‐point data with climatological observations. The vertical structure of the atmospheric boundary layer has a surface constant‐flux layer that is usually topped by a mixed layer by day but not at night. The crop‐soil boundary layer consists of a shallow top‐zone and a growing root‐zone. Vegetation cover and root depth depend upon crop type and phenological stage. Water‐balance accounting tracks the moisture contents of both the top‐ and root‐zones. Evapotranspiration or the vertical flux of water vapour in the atmospheric boundary layer is tied to the evolution of the crop‐soil boundary layer.

The model was calibrated using field data from the Regional Evaporation Study's primary site in an agricultural area of central Saskatchewan. The evolution of 1991's wheat‐soil boundary layer from the crop's heading to ripe stages was then successfully simulated at two additional sites in the same geographical area.