Impact of swell on simulations using a regional atmospheric climate model

When long, fast swell waves travel in approximately the same direction as the wind, the surface stress is reduced compared with under wind-sea conditions. Using measurements from the Östergarnsholm site in the Baltic Sea, new expressions of the roughness length were developed for wind sea and swell. These new expressions were implemented in the RCA3 regional climate model covering Europe. A 3-year simulation and two case studies using the wavefield from the ECMWF reanalysis (ERA-40) were analysed using the improved formulations. Wind-following swell led to a significant reduction of mean wind stress and heat fluxes. The mean surface layer wind speed was redistributed horizontally and the marine boundary layer cooled and dried slightly. This cooling was most pronounced over North Sea and the Norwegian Sea (almost 0.2 °C annually on average) whereas the drying was most pronounced over the Mediterranean Sea (almost 0.4 g kg⁻¹). Somewhat less convective precipitation and low-level cloudiness over the sea areas were also indicated, in particular over the Mediterranean Sea. The impact on the atmosphere, however, is significantly locally greater in time and space.     

全球海洋模式中CFC-11吸收对次网格尺度混合参数化的敏感性

以描述中尺度涡旋对示踪物的输送作用为目的的湍流混合方案GM90经证明对海洋模式的模拟能力较以前的湍流混合方案有较大的提高.该方案涉及到两个主要参数:等密度面扩散系数(A_I)和等密度面厚度扩散系数(A_ith).该文的目的就是利用中国科学院大气物理研究所(IAP)全球海洋环流模式L30T63研究以上两个系数取值大小对主动示踪物(温盐)以及被动示踪物(CFC-11)海洋分布的影响.实验结果表明这两个系数的取值可明显改变大洋温盐垂直分布以及海洋对CFC-11的吸收,且两个系数在其中起到的作用有很大的差异.从几个剖面的分析结果可知,总的来说,AI的增加使得CFC-11主要储存区的模拟结果更接近观测资料,而A_ith的增大使得模拟结果变差.     

Active and passive ocean regimes in a low‐order climate model

A low‐order climate model is studied which combines the Lorenz‐84 model for the atmosphere on a fast time scale and a box model for the ocean on a slow time scale. In this climate model, the ocean is forced strongly by the atmosphere. The feedback to the atmosphere is weak. The behaviour of the model is studied as a function of the feedback parameters. We find regions in parameter space with dominant atmospheric dynamics, i.e., a passive ocean, as well as regions with an active ocean, where the oceanic feedback is essential for the qualitative dynamics. The ocean is passive if the coupled system is fully chaotic. This is illustrated by comparing the Kaplan–Yorke dimension and the correlation dimension of the chaotic attractor to the values found in the uncoupled Lorenz‐84 model. The active ocean behaviour occurs at parameter values between fully chaotic and stable periodic motion. Here, intermittency is observed. By means of bifurcation analysis of periodic orbits, the intermittent behaviour, and the rôle played by the ocean model, is clarified. A comparison of power spectra in the active ocean regime and the passive ocean regime clearly shows an increase of energy in the low frequency modes of the atmospheric variables. The results are discussed in terms of itinerancy and quasi‐stationary states observed in realistic atmosphere and climate models.     

一个区域海洋——海冰——大气耦合模式中的北极气候偏差：年结果检验

The Coupling of three model components, WRF/PCE （polar climate extension version of weather research and forecasting model （WRF））, ROMS （regional ocean modeling system）, and CICE （community ice code）, has been implemented, and the regional atmosphere-ocean-sea ice coupled model named WRF/PCE- ROMS-CICE has been validated against ERA-interim reanalysis data sets for 1989. To better understand the reasons that generate model biases, the WRF/PCE-ROMS-CICE results were compared with those of its components, the WRF/PCE and the ROMS-CICE. There are cold biases in surface air temperature （SAT） over the Arctic Ocean, which contribute to the sea ice concentration （SIC） and sea surface temperature （SST） biases in the results of the WRF/PCE-ROMS-CICE. The cold SAT biases also appear in results of the atmo- spheric component with a mild temperature in winter and similar temperature in summer. Compared to results from the WRF/PCE, due to influences of different distributions of the SIC and the SST and inclusion of interactions of air-sea-sea ice in the WRF/PCE-ROMS-CICE, the simulated SAT has new features. These influences also lead to apparent differences at higher levels of the atmosphere, which can be thought as responses to biases in the SST and sea ice extent. There are similar atmospheric responses in feature of distribution to sea ice biases at 700 and 500 hPa, and the strength of responses weakens when the pressure decreases in January. The atmospheric responses in July reach up to 200 hPa. There are surplus sea ice ex- tents in the Greenland Sea, the Barents Sea, the Davis Strait and the Chukchi Sea in winter and in the Beau- fort Sea, the Chukchi Sea, the East Siberian Sea and the Laptev Sea in summer in the ROMS-CICE. These differences in the SIC distribution can all be explained by those in the SST distributions. These features in the simulated SST and SIC from ROMS-CICE also appear in the WRF/PCE-ROMS-CICE. It is shown that the performance of the WRF/PCE-ROMS-CICE is determined to a l     

Open‐ocean convection and polynya formation in a large‐scale ice‐ocean model

The physical processes responsible for the formation in a large‐scale ice–ocean model of an offshore polynya near the Greenwich meridian in the Southern Ocean are analysed. In this area, the brine release during ice formation in autumn is sufficient to destabilise the water column and trigger convection. This incorporates relatively warm water into the surface layer which, in a first step, slows down ice formation. In a second step, it gives rise to ice melting until the total disappearance of the ice at the end of September. Two elements are crucial for the polynya opening. The first one is a strong ice‐transport divergence in fall induced by south‐easterly winds, which enhances the amount of local ice formation and thus of brine release. The second is an inflow of relatively warm water at depth originating from the Antarctic Circumpolar Current, that sustains the intense vertical heat flux in the ocean during convection. The simulated polynya occurs in a region where such features have been frequently observed. Nevertheless, the model polynya is too wide and persistent. In addition, it develops each year, contrary to observations. The use of a climatological forcing with no interannual variability is the major cause of these deficiencies, the simulated too low density in the deep Southern Ocean and the coarse resolution of the model playing also a role. A passive tracer released in the polynya area indicates that the water mass produced there contributes significantly to the renewal of deep water in the Weddell Gyre and that it is a major component of the Antarctic Bottom Water (AABW) inflow into the model Atlantic.     

Ideal shocks in 2‐layer flow Part II: Under a passive layer

In this second part of the study, ideal shock theory in two‐layer stratified flow is extended to include a third passive layer (i.e., a two and a half layer system). With the presence of a passive layer, two linear wave modes and "viscous tail modes" exist, complicating the solubility conditions and uniqueness proofs for two layer shocks. It is found however, that shocks can be unambiguously classified as external or internal based on the states of criticality that they connect. The steepening condition, while still necessary, provides a less restrictive constraint than it did with a rigid lid. Thus, we have to rely more on solutions to the full viscous shock equations to establish shock existence. The detailed structure, momentum exchange, and Bernoulli loss in a viscous shock are examined using an analytical weak shock solution and a set of numerical solutions for shocks with finite amplitudes. A shock regime diagram ( F ₁ by F ₂) is constructed based on the numerical integration of the full viscous shock equations. For strong external jumps, a cusp region (i.e., in the sense of catastrophe theory) is identified on the regime diagram. For pre‐shock states within the cusp, three end states are possible and two of these are realizable. The cusp has several physical implications. It indicates that an equal distribution of dissipation between the two layers in shocks is mathematically possible but physically inaccessible. It also allows hysteresis in time varying flows, and promotes the occurrence of double shocks (i.e., closely spaced shocks of different character). The results are compared with classical shock solutions and a set of time dependent numerical experiments.     

北极海冰密集度预报对大气强迫敏感性的个例研究

A regional Arctic configuration of the Massachusetts Institute of Technology general circulation model （MIT-gcm） is used as the coupled ice-ocean model for forecasting sea ice conditions in the Arctic Ocean at the Na-tional Marine Environmental Forecasting Center of China （NMEFC）, and the numerical weather prediction from the National Center for Environmental Prediction Global Forecast System （NCEP GFS） is used as the atmospheric forcing. To improve the sea ice forecasting, a recently developed Polar Weather Research and Forecasting model （Polar WRF） model prediction is also tested as the atmospheric forcing. Their forecasting performances are evaluated with two different satellite-derived sea ice concentration products as initializa-tions： （1） the Special Sensor Microwave Imager/Sounder （SSMIS） and （2） the Advanced Microwave Scanning Radiometer for EOS （AMSR-E）. Three synoptic cases, which represent the typical atmospheric circulations over the Arctic Ocean in summer 2010, are selected to carry out the Arctic sea ice numerical forecasting experiments. The evaluations suggest that the forecasts of sea ice concentrations using the Polar WRF atmo-spheric forcing show some improvements as compared with that of the NCEP GFS.     

北冰洋浮冰区湍流通量观测试验及参数化研究

利用2008年8月21～29日我国第3次北极考察期间在北冰洋海区(84°27′N,143°37′W～85°13′N,147°20′W)冰站观测的湍流资料及相关资料,对海冰近地层湍流通量及其特征参数进行了研究.结果表明:观测期间浮冰近地层始终存在逆温和逆湿层.这与我们以前(1999年在75°N和2003年在78°N)的观...     

气候模式和再分析数据中北冰洋中层水再现能力的评估

The simulations of the Arctic Intermediate Water in four datasets of climate models and reanalyses, CCSM3, CCSM4, SODA and GLORYS, are analyzed and evaluated. The climatological core temperatures and depths in both CCSM models exhibit deviations over 0.5°C and 200 m from the PHC. SODA reanalysis reproduces relatively reasonable spatial patterns of core temperature and depth, while GLORYS, another reanalysis, shows a remarkable cooling and deepening drift compared with the result at the beginning of the dataset especially in the Eurasian Basin(about 2°C). The heat contents at the depth of intermediate water in the CCSM models are overestimated with large positive errors nearly twice of that in the PHC. To the contrary, the GLORYS in 2009 show a negative error with a similar magnitude, which means the characteristic of the water mass is totally lost. The circulations in the two reanalyses at the depth of intermediate water are more energetic and realistic than those in the CCSMs, which is attributed to the horizontal eddy-permitting resolution. The velocity fields and the transports in the Fram Strait are also investigated. The necessity of finer horizontal resolution is concluded again. The northward volume transports are much larger in the two reanalyses, although they are still weak comparing with mooring observations. Finally, an investigation of the impact of assimilation is done with an evidence of the heat input from assimilation. It is thought to be a reason for the good performance in the SODA, while the GLORYS drifts dramatically without assimilation data in the Arctic Ocean.     

Sea ice volume and age: Sensitivity to physical parameterizations and thickness resolution in the CICE sea ice model

New dynamics parameterizations in Version 5 of the Los Alamos Sea Ice Model, CICE, feature an anisotropic rheology and variable drag coefficients. This study investigates their effect on Arctic sea ice volume and age simulations, along with the effects of several pre-existing model options: a parameter that represents the mean cumulative area of ice participating in ridging, the resolution of the ice thickness distribution, and the resolution of the vertical temperature and salinity profiles.By increasing shear stress between floes, the anisotropic rheology slows the ice motion, producing a thicker, older ice pack. The inclusion of variable drag coefficients, which depend on modeled roughness elements such as deformed ice and melt pond edges, leads to thinner ice and a more realistic simulation of sea ice age. Several feedback processes act to enhance differences among the runs. Notably, if less open water is produced mechanically through ice deformational processes, the simulated ice thins relative to runs with more mechanically produced open water. Thermodynamic processes can have opposing effects on ice age and volume; for instance, growth of new ice increases the volume while decreasing the age of the pack. Therefore, age data provides additional information useful for differentiating among process parameterization effects and sensitivities to other model parameters.Resolution of thicker ice types is crucial for proper modeling of sea ice volume, because the volume of ice in the thicker ice categories determines the total ice volume. Model thickness categories tend to focus resolution for thinner ice; this paper demonstrates that 5 ice thickness categories are not enough to accurately resolve the ice thickness distribution for simulations of ice volume.     

How soon will climate records of the 20th century be broken according to climate model simulations?

What will happen to local record values of temperature and precipitation in a world with ongoing global warming? Here we first examine how many of the observed local temperature maxima of 1901–2006 occurred in the years 2001–2006 and compare the observations with model simulations. Then we study whether, and how soon, the models simulate the climate records of the 20th century to be broken in the ongoing 21st century.
In 27% of our analysis area, the highest annual mean temperatures of the whole period 1901–2006 were observed in 2001–2006. For the 22 climate models in our study, this fraction varies from 17% to 70%, with a multimodel mean of 40%. In simulations based on the SRES A1B emissions scenario, the highest annual mean temperature of the 20th century is exceeded on average in 99% of the global area by the year 2080. The same number for the highest (lowest) annual precipitation total is 60% (43%). Monthly and seasonal temperature and precipitation records are also analysed, and the geographical distributions of record value occurrence are related to the distributions of time mean climate change and magnitude of interannual variability.     

A time‐split, forward‐backward numerical model for solving a nonhydrostatic and compressible system of equations

In this paper, we present a numerical procedure for solving a 2‐dimensional, compressible, and nonhydrostatic system of equations. A forward‐backward integration scheme is applied to treat high‐frequency and internal gravity waves explicitly. The numerical procedure is shown to be neutral in time as long as a Courant–Friedrichs–Lewy criterion is met. Compared to the leap‐frog‐scheme most models use, this method involves only two time steps, which requires less memory and is also free from unstable computational modes. Hence, a time‐filter is not needed. Advection and diffusion terms are calculated with a time step longer than sound‐wave related terms, so that extensive computer time can be saved. In addition, a new numerical procedure for the free‐slip bottom boundary condition is developed to avoid using inaccurate one‐sided finite difference of pressure in the surface horizontal momentum equation when the terrain effect is considered. We have demonstrated the accuracy and stability of this new model in both linear and nonlinear situations. In linear mountain wave simulations, the model results match the corresponding analytical solution very closely for all three cases presented in this paper. The analytical streamlines for uniform flow over a narrow mountain range were obtained through numerical integration of Queney's mathematical solution. It was found Queney's original diagram is not very accurate. The diagram had to be redrawn before it was used to verify our model results. For nonlinear tests, we simulated the famous 1972 Boulder windstorm and a bubble convection in an isentropic enviroment. Although there are no analytical solutions for the two nonlinear tests, the model results are shown to be very robust in terms of spatial resolution, lateral boundary conditions, and the use of the time-split scheme.     

Ideal shocks in 2‐layer flow Part I: Under a rigid lid

Previous work on the classical problem of shocks in a 2‐layer density‐stratified fluid used either a parameterized momentum exchange or an assumed Bernoulli loss. We propose a new theory based on a set of viscous model equations. We define an idealized shock in two‐layer density stratified flow under a rigid lid as a jump or drop of the interface in which (1) the force balance remains nearly hydrostatic in the shock, (2) there is no exchange of momentum between the two layers except by pressure forces on the sloping interface, and (3) dissipative processes can be treated with a constant viscosity. We proceed in two steps. First, we derive a necessary condition for shock existence based on a requirement for wave steepening. Second, we formulate and solve a set of viscous model equations. Some results are the following: Shocks require strong layer asymmetry; one layer must be much faster and/or shallower than the other layer. The linearized equations describing the shock tails provide boundary conditions and a proof of shock uniqueness. It is possible to derive an analytical solution for weak shocks if the steepening condition is met. The weak shock solutions provide closed form expressions for the Bernoulli loss in each layer. Bernoulli losses are strongly concentrated in the expanding layer as the relative layer depth change is much larger in that layer. Bernoulli losses are independent of layer viscosity. A sudden cessation of shock existence is found for strong shocks when the possible end state migrates into the supercritical regime. Surprisingly, the new ideal shock theory compares well with a 2‐D, time‐dependent shallow water model (SWM) with a flux formulation, but with no viscous formulation. Both the Bernoulli drop and shock cessation condition agree quantitatively.