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1.
The structure of nocturnal inversions in the first 300 m of the atmosphere is analyzed using observational data from the Boulder Atmospheric Observatory (BAO) from March through June 1981. The temperature profiles show more than one inversion layer 41% of the time during the observational period. The vertical distributions of wind speed and moisture also show evidence of stratification during these multiple-layer events. The relation between the radiative cooling rate in time and height, including moisture, and the vertical structure of the multiple layers is calculated. The vertical distribution of eddy kinetic energy and the turbulent vertical fluxes of heat and momentum are also calculated. Turbulent structure in the elevated inversion layers is more complicated than that in the single-layer, stable nocturnal boundary layer. The total heat budget for a multiple-layer case is calculated, and turbulent cooling is found to be negligible relative to radiative cooling and to horizontal advection and/or horizontal divergence of heat flux.  相似文献   

2.
Temperature inversions are a common feature of the Arctic wintertime boundary layer. They have important impacts on both radiative and turbulent heat fluxes and partly determine local climate-change feedbacks. Understanding the spread in inversion strength modelled by current global climate models is therefore an important step in better understanding Arctic climate and its present and future changes. Here, we show how the formation of Arctic air masses leads to the emergence of a cloudy and a clear state of the Arctic winter boundary layer. In the cloudy state, cloud liquid water is present, little to no surface radiative cooling occurs and inversions are elevated and relatively weak, whereas surface radiative cooling leads to strong surface-based temperature inversions in the clear state. Comparing model output to observations, we find that most climate models lack a realistic representation of the cloudy state. An idealised single-column model experiment of the formation of Arctic air reveals that this bias is linked to inadequate mixed-phase cloud microphysics, whereas turbulent and conductive heat fluxes control the strength of inversions within the clear state.  相似文献   

3.
Summary Past investigations of the summertime heat budget over the Tibetan Plateau have not included detailed estimates of radiative cooling (Q R ) nor have they carefully considered the effects of cloudiness on this term. The various attempts to eyaluate different combinations of heat and moisture budget torms and to understand the sources of energy to the summer armospheric heat source over the plateau are not in agreement, partly because of remaining discrepancies in the radiative and turbulent flux components, and partly because until recently, the conventional data sets needed for independently estimating the total heating and moistening terms have been incomplete. The uncertainties in the radiative term have also led to difficulties in assessing the uncertainties in the other budget terms, since no study to date has assembled a complete enough data set to allow a unified calculation of all budget quantities or to obtain budget closure. Recently published results of Yanai and his colleagues involving apparent heat source calculations for the plateau region based on a much improved FGGE data set, have motivated the examination of whether more detailed radiative calculations can help resolve past discrepancies in the budget terms on a monthly time scale. This study uses a continuous time series of 22-km resolution INSAT geosynchronous satellite measurements and ECMWF profile analyses in conjunction with medium spectral resolution radiative transfer models to estimate the slicrtwave and longwave components of the radiative cooling term and the role of cloudiness on these components for the 1988 summer period. The calculations reveal both meridional and zonal structure in radiative divergence across the plateau associated with the substantial gradients of cloudiness and aridity that dominate the summertime plateau climatology. The calculations also indicate that the magnitudes of both cloud-induced shortwave heating and longwave cooling over the plateau are much greater than over low-elevation regions. Moreover, since cloud-induced longwave cooling exceeds cloud-induced shortwave heating, the bulk effect of clouds is to radiatively cool the plateau atmosphere. The high resolution calculations are reduced to monthly averaged budget quantities for analyzing whether existing discrepancies in the plateau heat budget can be resolved. Although there is no means to rigorously verify the accuracies and representativeness of the individual budget terms, the new radiative estimates combined with the most reliable current estimates of total heating and turbulent fluxes, produce near closure (within 4%) of the plateau heat budget for the June to August period.With 10 Figures  相似文献   

4.
辐射参数化的变化对模式中期和月预报的影响   总被引：2，自引：3，他引：2       下载免费PDF全文
在T106中期数值预报模式和T63气候模式中进行了两种辐射过程参数化的对比试验。目前在这两个模式中用作对照积分的辐射方案(OPE)是ECMWF的早期业务方案，而新的辐射方案(NEW)是ECMWF 1989年5月2日成为业务的版本。试验结果表明，OPE高估了短波水的吸收，导致太大的短波大气吸收和地表太小的向下短波辐射；OPE还低估了长波辐射冷却和大气顶的向外长波辐射(OLR)；NEW增加了地表有效辐射能量和对流层总的冷却，产生了较大的湍流热通量，对流活动加强，降水量明显增加；NEW还使平流层温度偏暖得到矫正。业务平行试验的统计检验表明，NEW对4～7天中期预报有较显著的改进。  相似文献   

5.
A case study of warm air advection over the Arctic marginalsea-ice zone is presented, based on aircraft observations with direct flux measurements carriedout in early spring, 1998. A shallow atmospheric boundary layer (ABL) was observed, which wasgradually cooling with distance downwind of the ice edge. This process was mainly connected with astrong stable stratification and downward turbulent heat fluxes of about 10–20 W m-2, but wasalso due to radiative cooling. Two mesoscale models, one hydrostatic and the other non-hydrostatic,having different turbulence closures, were applied. Despite these fundamental differences betweenthe models, the results of both agreed well with the observed data. Various closure assumptions had amore crucial influence on the results than the differences between the models.Such an assumption was, for example,the parameterization of the surface roughness for momentum (z0) and heat (zT). This stronglyaffected the wind and temperature fields not only close to the surface but also within and abovethe temperature inversion layer. The best results were achieved using a formulation for z0 that took intoaccount the form drag effect of sea-ice ridges together withzT = 0.1z0. The stability within theelevated inversion strongly depended on the minimum eddy diffusivity Kmin. A simple ad hocparameterization seems applicable, where Kmin is calculated as 0.005 timesthe neutral eddy diffusivity. Although the longwave radiative cooling was largest within the ABL, theapplication of a radiation scheme was less important there than above the ABL. This was related to theinteraction of the turbulent and radiative fluxes. To reproduce the strong inversion, it wasnecessary to use vertical and horizontal resolutions higher than those applied in most regional andlarge-scale atmospheric models.  相似文献   

6.
邢如楠 《气象学报》1983,41(2):211-218
用一个包括了湍流热交换、凝结、蒸发、辐射冷却、云量对辐射调节及海水上翻等各种物理过程在内的简单海-气耦合模式,进行了长期的数值积分。结果表明,热带海温异常通过热力过程影响大气的垂直环流,这种效应首先是局地的,以后这种影响会扩展到中纬度,而反过来,大气又对海洋产生动力影响,使海温发生变化。在只考虑上层海洋的情况下,试验表明在这一耦合系统中,存在着短期气候振荡,并且在赤道中太平洋和赤道东太平洋之间大尺度海表温度异常存在着落后相关。  相似文献   

7.
Large-eddy simulation in the GABLS3 intercomparison is concerned with the developed stable boundary layer (SBL) and the ensuing morning transition. The impact of radiative transfer on simulations of this case is assessed. By the time of the reversal of the surface buoyancy flux, a modest reduction of the lapse rate in the developed SBL is apparent in simulations that include longwave radiation. Subsequently, with radiation, the developing mixed layer grows significantly more quickly, so that four hours after the transition the mixed layer is roughly 40 % deeper; the resulting profiles of potential temperature and specific humidity are in better agreement with observations. The inclusion of radiation does not substantively alter the shape of turbulent spectra, but it does indirectly reduce the variance of temperature fluctuations in the mixed layer. The deepening of the mixed layer is interpreted as a response to the reduction of the strength of the capping inversion, resulting from cumulative radiative cooling in the residual layer and around the top of the former SBL. Sensitivity studies are performed to separate the two effects. Solar radiative heating of the atmosphere has a smaller impact on the development of the mixed layer than does longwave radiative cooling and slightly reduces its rate of growth, compared to simulations including longwave radiation alone. These simulations demonstrate that nocturnal radiative processes have an important effect on the morning transition and that they should be considered in future large-eddy simulations of the transition.  相似文献   

8.
A series of surveys was carried out with an instrumented vehicle along the A361 in south-west England between 20 January and 4 April 2007, primarily during clear calm nights in order to observe the formation of pools of cold air in valleys. High resolution simulations were performed for three of these cases using the Met Office Unified Model (MetUM®), which was shown to successfully reproduce many of the features of the observed along-route variations in air temperature. Analysis of the observations and the results of the simulations suggest that the cooling of air within these moderate valleys was the result of sheltering, which reduces the turbulent flux of heat down from aloft. Valleys that experience a large degree of cooling are those that become fully decoupled, allowing the near-surface air within the valley to cool rapidly in response to the surface longwave radiative cooling. The behaviour of the valley cooling intensity is shown to be similar to that observed in idealised simulations of the sheltering mechanism (Vosper and Brown, 2008, Boundary-Layer Meteorol 127:429–448).  相似文献   

9.
Extended sheets of stratocumulus (Sc) in the upper part of the atmospheric boundary layer (ABL) often occur under appropriate meteorological conditions. These cloud decks are important both in climate studies and in weather forecasting. We review the current knowledge of the turbulent structure of the ABL capped by a cloud deck, in the light of recent observations and model studies. The most important physical processes determining this structure are longwave radiative cooling at cloud top, shortwave radiative wanning by absorption in the cloud, surface buoyancy flux, and wind shear in the ABL. As a result, turbulence can cause entrainment against the buoyancy jump at cloud top. In cases where only longwave radiative fluxes and surface buoyancy fluxes are important, the turbulent structure is relatively well understood. When shortwave radiative fluxes and/or wind shear are also important, the resulting turbulent structure may change considerably. A decoupling of the cloud from the sub-cloud layer or of the top of the cloud from the rest of the ABL is then regularly observed. In no cases are the details of the entrainment at cloud top understood well enough to derive a relatively simple formulation that is consistent with observations. Cloud-top entrainment instability may lead to the break-up of a cloud deck (but also to cloud deepening). The role of mesoscale circulations in determining fractional cloudiness is not yet well understood.  相似文献   

10.
On calm clear nights, air at a height of a few decimetres above bare soil can be cooler than the surface by several degrees in what we shall call the Ramdas layer (Ramdas and Atmanathan, 1932). The authors have recently offered a logical explanation for such a lifted temperature minimum, together with a detailed numerical model. In this paper, we provide physical insight into the phenomenon by a detailed discussion of the energy budget in four typical cases, including one with a lifted minimum. It is shown that the net cooling rate near ground is the small difference between two dominant terms, representing respectively radiative upflux from the ground and from the air layers just above ground. The delicate energy balance that leads to the lifted minimum is upset by turbulent transport, by surface emissivity approaching unity, or by high ground cooling rates. The rapid variation of the flux emissivity of humid air is shown to dominate radiative transport near the ground.  相似文献   

11.
Simulation of the Effect of an Increase in Methane on Air Temperature   总被引：2，自引：0，他引：2  
The infrared radiative effect of methane was analyzed using the 2D, interactive chemical dynamical radiative SOCRATES model of the National Center for Atmospheric Research. Then, a sensitivity experi ment, with the methane volume mixing ratio increased by 10%, was carried out to study the influence of an increase of methane on air temperature. The results showed that methane has a heating effect through the infrared radiative process in the troposphere and a cooling effect in the stratosphere. However, the cooling effect of the methane is much smaller than that of water vapor in the stratosphere and is negligible in the mesosphere. The simulation results also showed that when methane concentration is increased by 10%, the air temperature lowers in the stratosphere and mesosphere and increases in the troposphere. The cooling can reach 0.2 K at the stratopause and can vary from 0.2-0.4 K in the mesosphere, and the temperature rise varies by around 0.001-0.002 K in the troposphere. The cooling results from the increase of the infrared radiative cooling rate caused by increased water vapor and O3 concentration, which are stimulated by the increase in methane in most of the stratosphere. The infrared radiation cooling of methane itself is minor. The depletion of O3 stimulated by the methane increase results indirectly in a decrease in the rate of so- lar radiation heating, producing cooling in the stratopause and mesosphere. The tropospheric warming is mainly caused by the increase of methane, which produces infrared radiative heating. The increase in H2O and O3 caused by the methane increase also contributes to a rise in temperature in the troposphere.  相似文献   

12.
The nocturnal atmospheric boundary layer (ABL) poses several challenges to standard turbulence and dispersion models, since the stable stratification imposed by the radiative cooling of the ground modifies the flow turbulence in ways that are not yet completely understood. In the present work we perform direct numerical simulation of a turbulent open channel flow with a constant (cooling) heat flux imposed at the ground. This configuration provides a very simplified model for the surface layer at night. As a result of the ground cooling, the Reynolds stresses and the turbulent fluctuations near the ground re-adjust on times of the order of L/u τ , where L is the Obukhov length scale and u τ is the friction velocity. For relatively weak cooling turbulence survives, but when ReL=Lut/n <~100{Re_L=Lu_\tau/\nu \lesssim 100} turbulence collapses, a situation that is also observed in the ABL. This criterion, which can be locally measured in the field, is justified in terms of the scale separation between the largest and smallest structures of the dynamic sublayer.  相似文献   

13.
The radiative forcings and feedbacks that determine Earth’s climate sensitivity are typically defined at the top-of-atmosphere (TOA) or tropopause, yet climate sensitivity itself refers to a change in temperature at the surface. In this paper, we describe how TOA radiative perturbations translate into surface temperature changes. It is shown using first principles that radiation changes at the TOA can be equated with the change in energy stored by the oceans and land surface. This ocean and land heat uptake in turn involves an adjustment of the surface radiative and non-radiative energy fluxes, with the latter being comprised of the turbulent exchange of latent and sensible heat between the surface and atmosphere. We employ the radiative kernel technique to decompose TOA radiative feedbacks in the IPCC Fourth Assessment Report climate models into components associated with changes in radiative heating of the atmosphere and of the surface. (We consider the equilibrium response of atmosphere-mixed layer ocean models subjected to an instantaneous doubling of atmospheric CO2). It is shown that most feedbacks, i.e., the temperature, water vapor and cloud feedbacks, (as well as CO2 forcing) affect primarily the turbulent energy exchange at the surface rather than the radiative energy exchange. Specifically, the temperature feedback increases the surface turbulent (radiative) energy loss by 2.87 W m?2 K?1 (0.60 W m?2 K?1) in the multimodel mean; the water vapor feedback decreases the surface turbulent energy loss by 1.07 W m?2 K?1 and increases the surface radiative heating by 0.89 W m?2 K?1; and the cloud feedback decreases both the turbulent energy loss and the radiative heating at the surface by 0.43 and 0.24 W m?2 K?1, respectively. Since changes to the surface turbulent energy exchange are dominated in the global mean sense by changes in surface evaporation, these results serve to highlight the fundamental importance of the global water cycle to Earth’s climate sensitivity.  相似文献   

14.
The impact of enhanced turbulent mixing induced by radiative cooling at the top of the stratocumulus-topped boundary layer (STBL) on numerical weather prediction is examined. An additional term involving top-down turbulent mixing via in-cloud radiative cooling is applied to the Yonsei University (YSU) planetary boundary layer (PBL) parameterization scheme using a top-down diffusivity profile and cloud-top entrainment. The modified scheme is evaluated in an advection fog case over the Yellow Sea of Korea using the Weather Research and Forecasting (WRF) model and in global medium-range forecasts using the Global/Regional Integrated Model system (GRIMs). In the fog case simulation, consideration of the additional top-down mixing parameterization in the YSU PBL simulates less formation and more rapid dispersion of the fog. As a result, the modified scheme simulates a drier and warmer boundary layer and a moister and cooler layer above the PBL. The modified algorithm also improves surface temperature prediction over the Yellow Sea accompanying early dissipation of the fog. In the global medium-range forecast experiment, the modified scheme simulates overall enhanced PBL mixing over the STBL in the tropics and subtropical ocean, showing drier and warmer regions near the surface and moister and cooler regions above the PBL, resulting in prediction of reduced low level cloud amount and increased downward shortwave radiation at the surface. The modified scheme appears to improve systematic bias in temperature and humidity in the lower troposphere compared to the control simulation.  相似文献   

15.
Results derived from simultaneous measurements of turbulent heat flux and radiation convergence in the daytime convective boundary layer are presented. It is found that the effects of long-wave radiation result in a warming near the surface and cooling at higher levels, in good agreement with infra-red radiative transfer models. Heating rates, roughly 30% of those produced by turbulence, are observed as a result of the absorption of short-wave radiation in the lowest 1000 m of the atmosphere.  相似文献   

16.
The linear hydrodynamic instability of the spatially periodic system of updrafts and downdrafts in a stably stratified atmosphere is studied. Such formulation of the problem is used to simulate the cloud systems observed in the atmosphere, when the zones of updrafts correspond to clouds and those of downdrafts correspond to intercloud intervals. A rather unexpected conclusion was made that the consideration of turbulent viscosity and thermal conductivity of the atmosphere as well as of radiative cooling leads to the instability at the Richardson numbers exceeding the critical value of 1/8 for the nondissipative case.  相似文献   

17.
重庆雾的二维非定常数值模拟   总被引：26，自引：5，他引：21  
张利民  李子华 《大气科学》1993,17(6):750-755
重庆是著名的“雾都”.本文建立了一个复杂地形下详细考虑长波辐射冷却、地表热量平衡、雾水沉降、水汽凝结和蒸发、湍流交换系数等的二维非定常雾模式,以实测资料为初始场,研究了一次重庆雾的形成和发展过程.对江河、热岛、辐射、地形和山城的影响,模式分别进行了数值试验.  相似文献   

18.
Based on coupled modelling evidence we argue that topographically-induced modifications of the large-scale atmospheric circulation during the last glacial maximum may have led to a reduction of the westerlies, and a slowdown of the Pacific subtropical gyre as well as to an intensification of the Pacific subtropical cell. These oceanic circulation changes generate an eastern North Pacific warming, an associated cooling in the Kuroshio area, as well as a cooling of the tropical oceans, respectively. The tropical cooling pattern resembles a permanent La Niña state which in turn forces atmospheric teleconnection patterns that lead to an enhancement of the subtropical warming by reduced latent and sensible cooling of the ocean. In addition, the radiative cooling due to atmospheric CO2 and water vapor reductions imposes a cooling tendency in the tropics and subtropics, thereby intensifying the permanent La Niña conditions. The remote North Pacific response results in a warming tendency of the eastern North Pacific which may level off the effect of the local radiative cooling. Hence, a delicate balance between oceanic circulation changes, remotely induced atmospheric flux anomalies as well local radiative cooling is established which controls the tropical and North Pacific temperature anomalies during the last glacial maximum. Furthermore, we discuss how the aftermath of a Heinrich event may have affected glacial temperatures in the Pacific Ocean.  相似文献   

19.
The effects of doubled carbon dioxide on rainfall responses to radiative processes of water clouds are investigated in this study. The two groups of two-dimensional cloud-resolving model sensitivity experiments in pre-summer heavy rainfall around the summer solstice and tropical rainfall around the winter solstice are conducted and their averages over 5 days and model domain are analyzed. In the presence of radiative effects of ice clouds, doubled carbon dioxide changes pre-summer rainfall from the decrease associated with the enhanced atmospheric cooling to the increase associated with the enhanced infrared cooling as a result of the exclusion of radiative effects of water clouds. Doubled carbon dioxide leads to the reduction in tropical rainfall caused by the removal of radiative effects of water clouds through the suppressed infrared cooling. In the absence of radiative effects of ice clouds, doubled carbon dioxide changes pre-summer rainfall from the increase associated with the increased atmospheric warming to the decrease associated with the weakened release of latent heat caused by the elimination of radiative effects of water clouds. The exclusion of radiative effects of water clouds increases tropical rainfall through the strengthened infrared cooling, which is insensitive to the change in carbon dioxide.  相似文献   

20.
To analyze the mechanism by which water vapor increase leads to cooling in the stratosphere, the effects of water-vapor increases on temperature in the stratosphere were simulated using the two-dimensional, interactive chemical dynamical radiative model (SOCRATES) of NCAR. The results indicate that increases in stratospheric water vapor lead to stratospheric cooling, with the extent of cooling increasing with height, and that cooling in the middle stratosphere is stronger in Arctic regions. Analysis of the radiation process showed that infrared radiative cooling by water vapor is a pivotal factor in middle-lower stratospheric cooling. However, in the upper stratosphere (above 45 km), infrared radiation is not a factor in cooling; there, cooling is caused by the decreased solar radiative heating rate resulting from ozone decrease due to increased stratospheric water vapor. Dynamical cooling is important in the middle-upper stratosphere, and dynamical feedback to temperature change is more distinct in the Northern Hemisphere middle-high latitudes than in other regions and signiffcantly affects temperature and ozone in winter over Arctic regions. Increasing stratospheric water vapor will strengthen ozone depletion through the chemical process. However, ozone will increase in the middle stratosphere. The change in ozone due to increasing water vapor has an important effect on the stratospheric temperature change.  相似文献   

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