Convection in PIDCAP Part I: Evaluating LAM convection

Summary One goal of BALTEX is to quantify the energy and water cycle over the Baltic region through LAMs. The convective components of these cycles cannot be explicitly modelled and thus are parameterized. In this two-part paper we consider the total convective heat flux c as key sub-gridscale quantity. We use it, in the present Part I, for a model intercomparison. We have four European LAMs (EM, BM, REMO and HIRLAM) at our disposal and run them, both in the prognostic and in the diagnostic mode, for a 9-day target period (Aug/Sep 1995) located within the convectively active part of PIDCAP. We find in the prognostic mode that the vertical profile of c is in most cases characterized by boundary layer convection. The flux is strongest at the surface ( W/m²) and becomes weaker at higher levels. In three deep convection experiments we select columns with a maximum number of lightning events, a maximum precipitation rate and a maximum c. These events are rare (order of 1 % of all columns) but show free atmosphere fluxes of up to  W/m². This result is supported by the net condensation rate CON which is complementary to c. The differences between the LAMs in terms of c and CON are small but significant. From the analysed gridscale fields of the LAMs we further derive, in the diagnostic mode, profiles of c, CON and other convective parameters and compare them with their forecast equivalents. The comparison is best for the HIRLAM model. This suggests that the physical package of HIRLAM, due to its hybrid convection scheme STRACO, may be a bit more realistic than those of EM, BM and REMO. Conclusion of Part I is that the total convective heat flux c is useful to make the model physics comparable across LAMs. In Part II, the problem how to diagnose c from observed gridscale data will be reviewed. Received January 8, 2001 Revised April 26, 2001     

Impact of time sampling on atmospheric energy budget residuals

Summary Diagnostic time-mean budgets of energy and water are evaluated in many atmospheric process studies. The errors of budget-derived quantities like sub-gridscale fluxes or diabatic heating are governed by the errors of the budgets. Here we consider 3D-budgets on the meso-β scale over Europe. They are compiled from analyses of state quantities available from forecast centres. In the present study we found that the mandatory 6 hours sampling interval between synoptic observations is the main error source for routine time-mean budgets. The errors have been quantified (i) by first sampling forecast data of the German Europamodell every 5 minutes and averaging them over 12 hours (reference budget), and (ii) by sampling the same data only every 6 hours and averaging these also over 12 hours (routine budget). With this method we find that routine budgets in single atmospheric meso-β scale columns show relative random errors of typically 200% and systematic errors of up to 20%, exclusively due to undersampling. Thus routine budgets, if applied to specific days at individual locations, cannot be expected to yield useful results, except perhaps for cases with extremely strong signal. Compositing over several hundreds of columns with similar weather reduces the random budget error down to about 50%; this seems to be the best one can achieve for routine budgets. The systematic error of some budget quantities is caused by a correlation between the time of occurence of certain processes (mainly convection) and the sampling times. While this error cannot be reduced through compositing, we find that it can be crudely estimated by using different time averaging methods. As application for this method we determine sub-gridscale budget quantities over the BALTEX catchment (August-September 1995) for an ensemble of convectively active and an ensemble of rain-active columns. For the ensemble mean profiles we find, in terms of the diagnosed sub-gridscale test quantities diabatic heating and vertical moist enthalpy flux divergence, that their accuracy is sufficient to detect statistically significant differences between both ensembles. The diabatic heating is about the same for both ensembles, while the flux divergence in the convective ensemble is about three times as large as in the rain ensemble. Received November 7, 2001 Revised April 4, 2001     

Convection in PIDCAP Part II: DIAMOD — A standard for diagnosing convective quantities

Summary For the purpose of specifying the strength of atmospheric convection we review how correlation fluxes and phase transition terms are generated from first principles, i.e., from a thermodynamic set of equations. They may be used either for a forecast (prognostic mode) or for an analysis (diagnostic mode). We identify the total convective heat flux c as key parameter for quantifying convection. Other convective quantities can be derived from c. In the prognostic mode the convective quantities are parameterized. In the diagnostic mode they are indirectly inferred. Here we concentrate on the diagnostic mode. We review how c can be estimated from the routinely analyzed gridscale budget of moist enthalpy by solving a first-order linear differential equation, referred to as convection equation. A novel aspect of the present standard of the convection equation is that the ice phase has now been included. The pertinent algorithm runs under the acronym DIAMOD (diagnostic model). The difference to a LAM is that DIAMOD takes the gridscale tendency as observed input while in a LAM it is output. We demonstrate the use of c (and of the net condensation rate CON) through an observation simulation experiment. Gridscale budget data from a forecast run with the Deutschland–Modell are taken as perfect input for DIAMOD; we study the corresponding output for several approximations of the convection quation. The vertical profiles of c (and to a lesser extent those of CON) are relatively robust and precise (error margins of a few percent) while the accuracy of the other convective quantities is inferior. Conclusion is that parameterization schemes of different LAMs can be compared with each other as well as validated in terms of c and CON. Application of this technique has been demonstrated in Part I of this study. Received January 8, 2001 Revised April 26, 2001     

The atmospheric energy budget and implications for surface fluxes and ocean heat transports

Comprehensive diagnostic comparisons and evaluations have been carried out with the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) and European Centre for Medium Range Weather Forecasts (ECMWF) reanalyses of the vertically integrated atmospheric energy budgets. For 1979 to 1993 the focus is on the monthly means of the divergence of the atmospheric energy transports. For February 1985 to April 1989, when there are reliable top-of-the-atmosphere (TOA) radiation data from the Earth Radiation Budget Experiment (ERBE), the implied monthly mean surface fluxes are derived and compared with those from the assimilating models and from the Comprehensive Ocean Atmosphere Data Set (COADS), both locally and zonally integrated, to deduce the implied ocean meridional heat transports. While broadscale aspects and some details of both the divergence of atmospheric energy and the surface flux climatological means are reproducible, especially in the zonal means, differences are also readily apparent. Systematic differences are typically ∼20 W m⁻². The evaluation highlights the poor results over land. Land imbalances indicate local errors in the divergence of the atmospheric energy transports for monthly means on scales of 500 km (T31) of 30 W m⁻² in both reanalyses and ∼50 W m⁻² in areas of high topography and over Antarctica for NCEP/NCAR. Over the oceans in the extratropics, the monthly mean anomaly time series of the vertically integrated total energy divergence from the two reanalyses correspond reasonably well, with correlations exceeding 0.7. A common monthly mean climate signal of about 40 W m⁻² is inferred along with local errors of 25 to 30 W m⁻² in most extratropical regions. Except for large scales, there is no useful common signal in the tropics, and reproducibility is especially poor in regions of active convection and where stratocumulus prevails. Although time series of monthly anomalies of surface bulk fluxes from the two models and COADS agree very well over the northern extratropical oceans, the total fields all contain large systematic biases which make them unsuitable for determining ocean heat transports. TOA biases in absorbed shortwave, outgoing longwave and net radiation from both reanalysis models are substantial (>20 W m⁻² in the tropics) and indicate that clouds are a primary source of problems in the model fluxes, both at the surface and the TOA. Time series of monthly COADS surface fluxes are shown to be unreliable south of about 20^∘N where there are fewer than 25 observations per 5^∘ square per month. Only the derived surface fluxes give reasonable implied meridional ocean heat transports. Received: 21 March 2000 / Accepted: 21 June 2000     

气候模式中云的次网格结构对全球辐射影响的研究

利用一种用于大尺度天气、气候模式的随机云产生器(SCG)和独立气柱近似(ICA)辐射算法,研究了次网格云的水平结构以及垂直重叠结构对全球辐射场的影响.比较了水平非均匀云(IHCLD)和水平均匀云(HCLD)的辐射场差异以及云的最大.随机重叠(MRO)和一般重叠(GenO)的辐射场差异.结果显示,与HCLD相比,IHCLD一方面可增加地面净短波辐射通量,纬向平均最大值(约1W/m~2)和次大值(约0.6 W/m~2)分别位于高纬度低云密集地区和对流旺盛的热带地区;另一方面可增加大气顶的净长波辐射通量,纬向平均最大值(0.3 W/m~2)出现在热带地区.不同的重叠结构对短波和长波辐射收支也有很大的影响.MRO和GenO的短波辐射通量差异在热带辐合带最大.达到30-40W/m~2,在高纬度低云带的纬向平均也可达到5W/m~2左右;长波辐射通量差异具有相似的地区分布,但量值相对较小.不同重叠结构可以造成大气上下层的辐射加热率差异,影响大气热力层结.云的水平和垂直结构对有云区域辐射收支的影响将改变大气热力、动力状况以及水汽条件,从而影响模拟的气候系统的演变.文中采用单向云-辐射计算,排除了与气候系统其他过程复杂的相互作用,从而使其结果具有一定的普适性,可为不同大尺度模式进行次网格云辐射参数化提供参考.     

Subgrid-scale vertical heat fluxes for the African region

Summary A diagnostic model for complete heat budgets in the free atmosphere is presented and is applied to the African-Atlantic sector between 35°S–30°N for May 1979. The model is based on the conservation equations for latent and sensible heat. These are evaluated in a form integrated over 24 hours in time and over atmospheric boxes of 2.5°×2.5° in horizontal and 100 hPa in vertical direction. Grid-scale input data are the 3D-ECMWF-diagnoses of the FGGE period plus parameterized fields of surface rain, evaporation and sensible Heat flux. This leads to an overspecification of latent and sensible heat budgets for any atmospheric column between surface and top of the atmosphere and thus yields an objective column imbalance. In order to separate the vertical subscale fluxes of rain, moisture and heat in the free atmosphere the model uses a closure assumption for the coupling between moisture and sensible heat flux as well as one for the vertical imbalance profiles; it is demonstrated that the budgets are not too sensitive with respect to these parameterizations.Results are presented in terms of vertical profiles of the subscale vertical fluxes of rain, moisture and heat. These are interpeted as measures of convective activity, with particular emphasis on the ITCZ. May 1979 averages as well as results for a respresentative single day are discussed. The imbalance (=the error) can be sufficiently well separated from the signal. It is shown that the low-level mass flux divergence does not coincide with the position of the ITCZ while the maximum of the subscale fluxes does coincide. Over the continent, it is not the horizontal mass flux convergence which feeds the ITCZ and the rainbelt but rather the subscale moisture flux and its convergence in the vertical. Over the Saharan latitudes, there is considerable convective flux of sensible heat, but not of latent heat. Over the ocean, deep convection in the ITCZ is weaker than over Africa, and it is consistently correlated with upward converging subscale moisture flux. The fields of the subscale vertical fluxes are coherent in space and time. It is argued from these results that the presented diagnostic model is potentially useful for testing parameterizations of convection in general circulation and climate models.With 19 Figures     

The global heat balance: heat transports in the atmosphere and ocean

The heat budget has been computed locally over the entire globe for each month of 1988 using compatible top-of-the-atmosphere radiation from the Earth Radiation Budget Experiment combined with European Centre for Medium Range Weather Forecasts atmospheric data. The effective heat sources and sinks (diabatic heating) and effective moisture sources and sinks for the atmosphere are computed and combined to produce overall estimates of the atmospheric energy divergence and the net flux through the Earth's surface. On an annual mean basis, this is directly related to the divergence of the ocean heat transport, and new computations of the ocean heat transport are made for the ocean basins. Results are presented for January and July, and the annual mean for 1988, along with a comprehensive discussion of errors. While the current results are believed to be the best available at present, there are substantial shortcomings remaining in the estimates of the atmospheric heat and moisture budgets. The issues, which are also present in all previous studies, arise from the diurnal cycle, problems with atmospheric divergence, vertical resolution, spurious mass imbalances, initialized versus uninitialized atmospheric analyses, and postprocessing to produce the atmospheric archive on pressure surfaces. Over land, additional problems arise from the complex surface topography, so that computed surface fluxes are more reliable over the oceans. The use of zonal means to compute ocean transports is shown to produce misleading results because a considerable part of the implied ocean transports is through the land. The need to compute the heat budget locally is demonstrated and results indicate lower ocean transports than in previous residual calculations which are therefore more compatible with direct ocean estimates. A Poisson equation is solved with appropriate boundary conditions of zero normal heat flux through the continental boundaries to obtain the ocean heat transport. Because of the poor observational data base, adjustments to the surface fluxes are necessary over the southern oceans. Error bars are estimated based on the large-scale spurious residuals over land of 30 W m^–2 over 1000 km scales (10¹² m²). In the Atlantic Ocean, a northward transport emerges at all latitudes with peak values of 1.1±0.2 PW (1 standard error) at 20 to 30°N. Comparable values are achieved in the Pacific at 20°N, so that the total is 2.1±0.3 PW. The peak southward transport is at 15 to 20°S of 1.9±0.3 PW made up of strong components from both the Pacific and Indian Oceans and with a heat flux from the Pacific into the Indian Ocean in the Indonesian throughflow. The pattern of poleward heat fluxes is suggestive of a strong role for Ekman transports in the tropical regions.     

二流—四流球谐函数谱展开累加辐射传输方案在全球气候模式中的应用

本文首先构建了二流—四流球谐函数谱展开累加辐射传输的新方案,然后将其应用于国家气候中心第二代大气环流模式BCC_AGCM2.0.1的新版本中,并与模式中原有的Eddington累加方案进行了比较。由于新方案本质上是单层Eddington近似方案在四流上的推广。因此新方案在计算精度上要优于原方案。通过在全球气候模式中的应用与比较,本文发现新方案对气候模拟会产生比较大的影响。在晴空条件下,新方案计算的在南纬30°到60°区间、北大西洋东北部以及非洲北部的撒哈拉沙漠区域的地表向下年平均短波辐射通量要小于原方案结果,最大差别可以达到3.5 W/m2;同时,新方案计算的在南纬30°到60°区间和北大西洋东北部的大气顶向上年平均短波辐射通量要大于原方案结果,最大差别达到3 W/m2。在有云大气情况下,新方案计算的地表向下年平均短波辐射通量要小于原方案结果,并随着纬度的增加,新旧两种方案的差别逐渐变大,在南北极时达到最大5.5 W/m2;同时,新方案计算的在赤道区域的大气顶的年平均短波向上辐射通量要小于原方案结果,最大差别为2.5 W/m2,而在南北纬30°到60°区间,新方案计算的在大气顶的年平均短波向上辐射通量则要大于原方案结果,最大差别为1.5 W/m2。新方案计算的年平均短波加热率普遍高于原方案结果,特别是在800 hPa到地表之间的低层大气以及50 hPa到100 hPa的高层大气,最大差别可达0.03 K/d。因此,新方案有助于改善全球气候模式中普遍存在的赤道平流层中下层的温度冷偏差现象。     

The role of shallow convection in the water and energy cycles of the atmosphere

The Canadian Centre for Climate Modelling and Analysis atmospheric general circulation model (AGCM4) is used to study the role of shallow convection in the hydrologic and energy cycles of the atmosphere. Sensitivity tests with AGCM4 show a marked effect of the parameterization of shallow convection in the model. In particular, including the parameterization of shallow convection produces considerably enhanced vertical mixing and decreased stratiform cloud amounts in the lower subtropical atmosphere over the oceans. The differences in simulated stratiform cloud amounts are associated with a change in the globally averaged outgoing shortwave radiative flux at the top of the atmosphere of about 11 W m⁻². Additionally, precipitation rates are considerably reduced for stratiform clouds and enhanced for convective clouds in the subtropics, if the parameterization of shallow convection is included in the model. Additional tests show that the simulated responses in cloud amounts and precipitation to the treatment of shallow convection are robust. Additional simulations with modified closures for deep convection and other changes to the treatment of convection in the model still lead to similar responses of the model results.     

土壤热异常对地表能量平衡影响初探

将来自土壤深部的热通量引入off line的陆面过程模式 (NCAR—LSM ) ,通过长达 2a的数值试验对比分析了它对各层次土壤温度和地表能量平衡的影响。　　在土壤底部引入 5W /m2 的热通量使底层土壤显著升温 ,但升温随着接近表层而迅速衰减。积分 3个月后 ,由地下进入地表的热流量增幅可达 1W/m2 以上 ,并持续增大到 5W /m2 ,地表最大升温约 0 .5K ,同时地表感热、蒸发潜热及长波辐射通量均有 1W /m2 左右的正异常 ;若将土壤热传导系数放大一个量级以加速热量交换 ,则地表升温提高到 1K以上 ,长波辐射增加 3W /m2 以上 ,超过了气溶胶全球平均的辐射效应。结果表明 :一定量值的土壤热异常对地表能量平衡和短期气候变化 (10 -1～ 10 1a)有着不可忽略的影响。同时 ,深入的资料分析、完善的陆面过程模式以及它与大气模式的耦合试验也是亟待进行的相关工作。     

基于FY-3/IRAS利用非线性模式反演OLR

FY-3系列卫星星载IRAS仪器设有26个通道,其中20个通道用于探测地球大气在红外波段的热辐射,通道辐射率代表了地球大气系统在大气顶的向外辐射光谱信息,与总波段的射出长波辐射（OLR）通量相关性高。该文基于逐线辐射传输模式计算软件LBLRTM对全球2521条大气廓线的大气顶射出辐射率模拟数据,计算了每条廓线的OLR和FY-3B/IRAS,FY-3C/IRAS通道辐射率,用统计回归方法建立了利用IRAS的多通道辐射率计算OLR的非线性理论回归模式;应用模式和FY-3B/IRAS,FY-3C/IRAS的L1级数据,处理得到2016年4月1-30日的全球日平均、月平均OLR格点产品。与Aqua/CERES,Terra/CERES仪器宽波段观测OLR产品对比表明:对于水平分辨率为1°×1°的全球月平均OLR格点产品,均方根误差为2.22 W·m-2,相关系数为0.9982 W·m-2,平均偏差为-0.2 W·m-2,表明FY-3/IRAS仪器定标及反演模式均达到较高水平。文中还回顾了历史上不同气象卫星的多种OLR反演算法模式,并对不同模式精度进行了比较。     

Tropical sea surface temperature: An interactive one-dimensional atmosphere-ocean model

Because the atmosphere and ocean are interacting systems, it is inappropriate to specify sea surface temperature when dealing with the atmosphere, or atmospheric anemometer level temperature and moisture when dealing with the ocean. All of these quantities should be determined interactively in terms of the external forcing: the solar constant.In the tropics, it is shown that the (cumulus) convective processes may be described by a one-dimensional cloud model. The near-surface ocean may similarly be described by a one-dimensional mixed-layer model. The coupling is achieved through a sea surface flux budget combined with the flux parameterizations implied by Monin-Obukhov similarity theory.The coupled one-dimensional atmosphere-ocean model is applied to the equilibrium situation in which all temperatures reach a steady state. Since the ocean, lacking an internal heating or cooling mechanism, can only be heated or cooled through sensibleheat fluxes through the sea surface, in equilibrium these fluxes must vanish. The atmosphere, however, maintains a stable lapse rate by balancing cumulonimbus heating against net radiative cooling. All water precipitated from cumulonimbus clouds must have evaporated from sea surface. It is shown that this equilibrium system is closed and determinable solely in terms of the solar constant.For various values of the solar constant, the sea surface temperature, the flux of latent and sensible heat from the surface, the height of the tropopause, mixed layer, and trade inversion layer, and generally, the entire vertical structure of the tropical atmosphere and near-surface ocean can be determined. The equilibrium sea surface temperature is shown to be relatively insensitive to changes in the solar constant, additional solar flux being compensated mainly by additional evaporation. Finally, the usefulness and limitations of the model are pointed out.     

两种对流参数化方案对辐射能量收支的影响研究

利用中国科学院大气物理研究所大气科学和地球流体力学数值模拟国家重点实验室的格点大气环流模式(GAMIL)1.0版设计了两组数值模拟实验来研究两种不同的对流参数化方案对辐射能量收支的影响.这两种对流参数化方案分别是:Zhang and McFarlance/Hack方案(简称ZM)和Tiedtke/Nordeng方案(简称 TN).对应的数值模拟实验分别取名为EX-ZM和EX-TN.通过对实验结果的分析表明:在对流过程中,EX-ZM允许深对流和浅对流同时发生,因此两种对流同时在模式低层消耗了更多的水汽,释放了更多的潜热,引起了更大的增温;EX_TN每次只允许一种对流发生,也就避免了不同类型的对流在同一层同时消耗水汽的现象.因此对流过后,EX-ZM的环境空气相对湿度较小,而EX-TN周围空气的相对湿度较大,有利于低云云量的生成和大尺度的凝结,因此EX-TN模拟的低云云量偏多,低层的云水含量偏高,模式低层的云光学厚度偏大,这就使得EX_TN中更多的太阳短波辐射通量被云反射掉,严重低估了模式对短波波段的辐射通量的模拟.此外,不同的对流参数化方案通过改变云的长波发射率和降水,进而影响了模式对长波波段的辐射通量、感热和潜热通量的模拟.     

Inverse modeling of energy transports and budgets of the atmosphere

The vertically integrated horizontal energy transports and the vertically integrated vertical energy flux divergence from ERA-40 and ISCCP are not in balance assuming a stationary climate as a time mean over several years. The reasons are the inherent uncertainties in each of the respective data sets. We therefore modify them using a variational approach with a discretization in spherical harmonics to obtain consistent values. The variational approach only modifies the smaller yet more uncertain divergent part of the flow, leaving the large rotational part untouched. From these consistent fields we can calculate posterior covariance matrices of the vertically integrated horizontal energy transport and the vertically integrated vertical energy flux divergence, providing a measure of the uncertainty of the previous calculation. We are able to use these posterior covariance matrices to give an estimate of the uncertainty of the zonally and vertically integrated meridional energy transport, which is about 0.25 PW in the tropics and 0.04 PW in high latitudes, as well as for the vertical energy flux divergence of the atmosphere, which ranges from 2.5 to 5 W/m² in the tropics to 15–17 W/m² in high latitudes.     

TropFlux: air-sea fluxes for the global tropical oceans—description and evaluation

In this paper, we evaluate several timely, daily air-sea heat flux products (NCEP, NCEP2, ERA-Interim and OAFlux/ISCCP) against observations and present the newly developed TropFlux product. This new product uses bias-corrected ERA-interim and ISCCP data as input parameters to compute air-sea fluxes from the COARE v3.0 algorithm. Wind speed is corrected for mesoscale gustiness. Surface net shortwave radiation is based on corrected ISCCP data. We extend the shortwave radiation time series by using “near real-time” SWR estimated from outgoing longwave radiation. All products reproduce consistent intraseasonal surface net heat flux variations associated with the Madden-Julian Oscillation in the Indian Ocean, but display more disparate interannual heat flux variations associated with El Ni?o in the eastern Pacific. They also exhibit marked differences in mean values and seasonal cycle. Comparison with global tropical moored buoy array data, I-COADS and fully independent mooring data sets shows that the two NCEP products display lowest correlation to mooring turbulent fluxes and significant biases. ERA-interim data captures well temporal variability, but with significant biases. OAFlux and TropFlux perform best. All products have issues in reproducing observed longwave radiation. Shortwave flux is much better captured by ISCCP data than by any of the re-analyses. Our “near real-time” shortwave radiation performs better than most re-analyses, but tends to underestimate variability over the cold tongues of the Atlantic and Pacific. Compared to independent mooring data, NCEP and NCEP2 net heat fluxes display ~0.78 correlation and >65?W?m^?2 rms-difference, ERA-I performs better (~0.86 correlation and ~48?W?m^?2) while OAFlux and TropFlux perform best (~0.9 correlation and ~43?W?m^?2). TropFlux hence provides a useful option for studying flux variability associated with ocean–atmosphere interactions, oceanic heat budgets and climate fluctuations in the tropics.     

RegCM4对中国东部区域气候模拟的辐射收支分析

利用卫星和再分析数据,评估了区域气候模式Reg CM4对中国东部地区辐射收支的基本模拟能力,重点关注地表净短波(SNS)、地表净长波(SNL)、大气顶净短波(TNS)、大气顶净长波(TNL)4个辐射分量。结果表明:1)短波辐射的误差值在夏季较大,而长波辐射的误差值在冬季较大。但各辐射分量模拟误差的空间分布在冬、夏季都有较好的一致性。2)对于地表辐射通量,SNS表现为正偏差(向下净短波偏多),在各分量中误差最大,区域平均误差值近50 W/m2;SNL表现为负偏差(向上净长波偏多);对于大气顶辐射通量,TNS和TNL分别表现为"北负南正"的误差分布和整体正偏差。3)利用空间相关和散点线性回归方法对4个辐射分量的模拟误差进行归因分析,发现在云量、地表反照率、地表温度三个直接影响因子中,云量模拟误差的贡献最大,中国东部地区云量模拟显著偏少。     

Measured And Simulated Latent And Sensible Heat Fluxes At Two Marine Sites In The Baltic Sea

In this study, turbulent heat flux data from two sites within the Baltic Sea are compared with estimates from two models. The main focus is on the latent heat flux. The measuring sites are located on small islands close to the islands of Bornholm and Gotland. Both sites have a wide wind direction sector with undisturbed over-water fetch. Mean parameters and direct fluxes were measured on masts during May to December 1998.The two models used in this study are the regional-scale atmospheric model HIRLAM and the ocean model PROBE-Baltic. It is shown that both models overestimate the sensible and latent heat fluxes. The overestimation can, to a large extent, be explained by errors in the air-water temperature and humidity differences. From comparing observed and modelled data, the estimated 8-month mean errors in temperature and humidity are up to 1 °C and 1 g kg^-1, respectively. The mean errors in the sensible and latent heat fluxes for the same period are approximately 15 and 30 W m^-2, respectively.Bulk transfer coefficients used for calculating heat and humidity fluxes at the surface were shown to agree rather well with the measurements, at least for the unstable data. For stable stratification, the scatter in data is generally large, and it appears that the bulk formulation chosen overestimates turbulent heat fluxes.     

Anomalies in the tropics associated with the heavy rainfall in east asia during the summer of 1998

1.IntroductionInthesummerof1998,theYangtzeRiverbasin,includingNenjiangRiverValleyinNortheastChinasufferedaseverelarge--scalefloodonlynexttothatinthesummerof1954inthiscentury.Thefloodcausedapproximatelythedeathof3000individualsandthedirecteconomicdamageof250billionRMByuans(Yan,1998).Thisextremedisasterpromptedaseriesofimmediatestudiesonit(e.g.,Iluangetal.,1998;Taoetal.,1998).TheevolutionoftheEastAsiansupsmermonsoonshowsagreatvariabilityfromyeartoOThisstudywassupportedbytheNationalNatura…     

基于OLR资料的青藏高原地区对流活动研究

本文利用1980～2019年美国NOAA系列卫星观测的向外长波辐射(OLR)月平均资料和欧洲中心ERA5月平均地表热通量资料,研究青藏高原(以下简称高原)地区OLR与对流活动的时空分布及其演变特征,以及地表热通量与高原夏季对流活动之间的关系.结果表明:高原地区平均OLR强度由高原周边地区向中部递减,高原东部OLR低于西...     

A solely radiance-based spectral angular distribution model and its application in deriving clear-sky spectral fluxes over tropical oceans

The radiation budget at the top of the atmosphere plays a critical role in climate research. Compared to the broadband flux, the spectrally resolved outgoing longwave radiation or flux(OLR), with rich atmospheric information in different bands,has obvious advantages in the evaluation of GCMs. Unlike methods that need auxiliary measurements and information, here we take atmospheric infrared sounder(AIRS) observations as an example to build a self-consistent algorithm by an angular distribution model(ADM), based solely on radiance observations, to estimate clear-sky spectrally resolved fluxes over tropical oceans. As the key step for such an ADM, scene type estimations are obtained from radiance and brightness temperature in selected AIRS channels. Then, broadband OLR as well as synthetic spectral fluxes are derived by the spectral ADM and validated using both synthetic spectra and CERES(Clouds and the Earth's Radiant Energy System) observations. In most situations, the mean OLR differences between the spectral ADM products and the CERES observations are within ±2 W m~(-2), which is less than 1% of the typical mean clear-sky OLR over tropical oceans. The whole algorithm described in this study can be easily extended to other similar hyperspectral radiance measurements.