Equilibrium Evaporation and the Convective Boundary Layer

A theory is developed for surface energy exchanges in well-mixed, partlyopen systems, embracing fully open and fully closed systems as limits.Conservation equations for entropy and water vapour are converted intoan exact rate equation for the potential saturation deficit D in a well-mixed, partly open region. The main contributions to changes in D arise from (1) the flux of D at the surface, dependent on a conductance g_q that is a weighted sum of the bulk aerodynamic and surface conductances; and (2) the exchange flux of D with the external environment by entrainment or advection, dependent on a conductance g_e that is identifiable with the entrainment velocity when the partly open region is a growing convective boundary layer (CBL). The system is fully open when g_e/g_q , and fully closed when g_e/g_q 0. The equations determine the steady state surface energy balance (SEB) in a partly open system, the associated steady-state deficit, and the settling time scale needed to reach the steady state. The general result for the steady-state SEB corresponds to the equations of conventional combination theory for the SEB of a vegetated surface, with the surface-layer deficit replaced by the external deficit and with g_q replaced by the series sum (g_q ^-1 + g_e ^-1)^-1. In the fully open limit D is entirely externally prescribed, while in the fully closed limit, D is internally determined and the SEB approaches thermodynamic equilibrium energy partition. In the case of the CBL, the conductances g_q and g_e are themselves functions of D through short-term feedbacks, induced by entrainment in the case of g_e and by both physiological and aerodynamic (thermal stability) processes in the case of g_q. The effects of these feedbacks are evaluated. It is found that a steady-state CBL is physically achievable only over surfaces with at least moderate moisture availability; that entrainment has a significant accelerating effect on equilibration; that the settling time scale is well approximated by h/(g_q + g_e), where h is the CBL depth; and that this scale is short enough to allow a steady state to evolve within a semi-diurnal time scale only when h is around 500 m or less.     

A closure to derive a three-dimensional well-mixed trajectory-model for non-Gaussian, inhomogeneous turbulence

A three-dimensional Lagrangian stochastic (LS) model to evaluate pollutant dispersion in the atmospheric boundary layer has been developed. The model satisfies the well-mixed criterion of Thomson and allows for inhomogeneous, skew turbulence. Making use of the spherical reference frame, one of the possible solutions has been obtained. A skewed joint probability density function (PDF), which reproduces the given velocity moments (means, variances, skewness and covariances), has been built-up by a linear combination of eight Gaussian PDFs. In order to verify consistency with the well-mixed criterion, the long term results have been compared with the theoretical behaviour. A comparison between our model and Thomson's published algorithms was also carried out. By comparing wind-tunnel data and numerical predictions, a further validation of our LS model has been obtained. From an analysis of the numerical results, we can state that our model is able to evaluate dispersion in the case of complex flows where the application of previous models is unsuccessful.     

Nonlinear Dynamics in Atmospheric Chemistry Rate Equations

Nonlinear terms in the gas-phase rate equations of atmospheric trace constituents give rise to unexpected oscillations in the concentrations. For a simplified chemical scheme of the troposphere that contains only the generic reaction paths, the underlying dynamical structure is investigated. It is shown that the source strengths of CO and NO are the controlling parameters of the system. A linear stability analysis reveals that the steady state solutions lose stability due to the occurrence of two Hopf bifurcations. Furthermore, it appears that the dynamical behaviour of the oscillatory solutions is dominated by O₃ and CO. Based on the two-variable system (CO–O₃) it is shown that the oscillatory solution involves an autocatalytic ozone production phase which is followed by a phase in which CO is oxidised quickly. A simple expression is presented from which the period of the oscillation can be obtained. The implications for the present troposphere are unclear, since other hydrocarbons are present and transport is taking place. Nevertheless, the system nicely shows the general nonlinear mechanisms that operate in the tropospheric chemistry equations.     

中国区域夏季地表气温与陆面过程耦合强度的分布特征

鉴于陆气相互作用对极端高温事件的重要作用,利用ERA5-Land陆面再分析资料,分析了基于敏感度的中国区域夏季地表气温与土壤湿度、土壤温度耦合强度的多时间尺度分布特征。研究结果表明,基于不同耦合过程指标定义的陆气耦合强度在中国区域呈不同的空间分布特征,其中基于潜热通量的陆面变量—地表气温耦合的“热点”区域主要分布在中国的西北地区和长江流域,基于感热通量的陆面变量—地表气温耦合的“热点”区域则主要分布在河套—内蒙古地区、新疆西南部地区,以及长江以南部分区域。这说明夏季地表气温对陆面变量异常的敏感度的区域差异,与不同区域陆面异常影响地表气温的主导过程密切相关。此外,陆气耦合强度的强弱还随时间尺度而变化,其中月—季节尺度的陆气耦合强度要明显弱于日、候及旬尺度的耦合强度;就日、候及旬时间尺度而言,基于潜热交换过程的陆面变量—地表气温的耦合强度在全国大部分地区随时间尺度的增加而减弱,基于感热通量的陆气耦合强度在南方大部分地区也随时间尺度的增加逐渐减弱,但在北方大部分地区则表现为随时间尺度增加逐渐增强。相比较于表层土壤湿度与地表气温的耦合强度,次表层土壤湿度与地表气温的耦合强度在中国西北地区明显减弱。     

Exchange of heat and momentum between the atmosphere and the ocean: a minimal model of decadal oscillations

 A model of the large-scale interaction between the troposphere and the upper ocean, wind-driven circulation is formulated. Simplified parametrizations, built upon the conservation of global heat and momentum, relate the atmospheric eddy heat and momentum fluxes to the zonally averaged oceanic and atmospheric temperatures. The formulation shows that the wind-driven circulation influences the winds by controlling the strength of the oceanic northward heat transport, and thus the atmospheric northward heat transport and temperature distribution. Because the ocean takes decades to adjust to changes in the winds, the coupled system equilibrates into a state which is periodic in time, rather than steady. The period is linearly proportional to the transit time of long Rossby waves across the basin, and thus is of the order of decades for large-scale basins. Received: 15 December 1998 / Accepted: 29 October 1999     

Short-term temporal variability of atmospheric surface pressure and wind speed in the Canadian Arctic

In this study, the spatial differences and interannual fluctuations in temporal variability of surface pressure and wind speed on different timescales at 12 locations in the Canadian Arctic are documented. Temporal variability is defined as the mean-squared value of time tendencies smoothed by running means over different intervals. It is shown that variability on timescales of up to 1 month is itself highly variable, both in space and time. Due to the significant impacts from the immediate geographical environment, for surface wind speed, these variations show no spatial pattern on a continental scale, and only a few persistent trends over periods of more than 10 years. Also, spatial and temporal anomalies do not significantly depend on timescale. Contrary to this, spatial and temporal variations in the variability of surface pressure and its changes with time show well-defined regional similarities, as well as a strong spatial and temporal dependence on timescale. As a result, variability of surface pressure on timescales between 1 and 3 days increases from the northeast region of the domain towards the southwest. On longer timescales, this spatial gradient is reversed. The consistent spatial pattern across the study domain suggests that variability of surface pressure is primarily governed by large-scale atmospheric processes, and is to a large extent independent of the exact geographical setting.     

Dynamic responses of atmospheric carbon dioxide concentration to global temperature changes between 1850 and 2010

Changes in Earth's temperature have significant impacts on the global carbon cycle that vary at different time scales, yet to quantify such impacts with a simple scheme is traditionally deemed difficult. Here, we show that, by incorporating a temperature sensitivity parameter(1.64 ppm yr~(-1) ?C~(-1)) into a simple linear carbon-cycle model, we can accurately characterize the dynamic responses of atmospheric carbon dioxide(CO_2) concentration to anthropogenic carbon emissions and global temperature changes between 1850 and 2010(r~2 0.96 and the root-mean-square error 1 ppm for the period from 1960onward). Analytical analysis also indicates that the multiplication of the parameter with the response time of the atmospheric carbon reservoir(～12 year) approximates the long-term temperature sensitivity of global atmospheric CO_2concentration(～15 ppm?C~(-1)), generally consistent with previous estimates based on reconstructed CO_2 and climate records over the Little Ice Age. Our results suggest that recent increases in global surface temperatures, which accelerate the release of carbon from the surface reservoirs into the atmosphere, have partially offset surface carbon uptakes enhanced by the elevated atmospheric CO_2 concentration and slowed the net rate of atmospheric CO_2 sequestration by global land and oceans by ～30%since the 1960 s. The linear modeling framework outlined in this paper thus provides a useful tool to diagnose the observed atmospheric CO_2 dynamics and monitor their future changes.     

Timescales and dynamics of the formation of a thermocline

In a state of equilibrium, the constraint of a balanced heat budget for the ocean strongly influences the depth of the tropical thermocline because that depth controls the rate at which the ocean absorbs heat from the atmosphere. Thus, an increase in the oceanic heat loss in high latitudes results in a shoaling of the equatorial thermocline so that the heat gain also increases. How does the ocean adjust to such a new equilibrium state after an abrupt change in the heat flux in high latitudes? The adjustment of the wind-driven circulation of the upper ocean is shown to involve two timescales. The first is the familiar adiabatic wave-adjustment time associated with the horizontal redistribution of warm water above the thermocline in shallow water models. (This is essentially the time it takes Rossby and Kelvin waves to propagate from the disturbed extra-equatorial region to the equator.) The second adjustment-time is associated with the diabatic processes that come into play once the waves from higher latitudes modify the thermal structure in low latitudes and hence the flux of heat into the ocean; it is the timescale on which the ocean recovers a balanced heat budget. The identification of this timescale is the main result of this paper.Through a series of simulations of an idealized ocean basin, we identify the diabatic timescale and argue that it is determined by the strength of the upwelling and the intensity of the air–sea heatfluxes. By simulating the formation of a thermocline from isothermal conditions, we are able to relate this timescale to other relevant timescales such as that associated with diffusive processes and the adiabatic timescale invoked by Gu and Philander [Gu, D., Philander, S.G.H., 1997. Interdecadal climate fluctuations that depend on exchanges between the tropics and extra-topics. Science 275, 805–807].     

Tropospheric Box-Modelling and Analytical Studies of the Hydroxyl (OH) Radical and Related Species: Comparison with Observations

Calculated and observed hydroxyl (OH) fields are presented. Calculated OH was obtained in three ways using (1) a photochemical box-model (2) a simple OH steady state approach and (3) a variant on (2) – the multiple equation steady state approach which assumes steady state for OH, HO2 and RO2 and hence obtains three simultaneous, non linear, equations. All three methods used data collected in June 1995 during the Weybourne Atmospheric Observatory Summer Experiment (WAOSE'95). Julian Days 169, 178, 179 and 180 displayed especially good data capture and were consequently chosen for study. The two steady state methods are essentially driven purely by observations and derive OH from the ratio of the relevant source and sink terms. The box-model was constrained where possible to observations; remaining unmeasured volatile organic compounds (VOCs) were initialised to an arbitrary low value of 10 ppt. Agreement between theory and experiment was usually around 50% and often better than this value, especially on J169, though discrepancies of up to a factor of 3 were occasionally apparent. Despite the inherent scatter, neither the box-model nor the simple steady state method were found to consistently over-estimate OH (a common feature of many numerical approaches) although this did occur to a certain extent using the multiple equation steady state approach, probably due to breakdowns in the steady state approximation. More data spread was evident in the box-model approach compared with the other methods. An analysis of the major sources and sinks of OH is presented for the three methods of calculation. Calculated and observed peroxy radicals are also presented. Calculated peroxy radicals were generally lower than that observed at night yet higher, sometimes by up to a factor of 7, during the day. Possible explanations for this result are explored.     

MOPITT观测的CO分布规律及与瓦里关地面观测结果的比较

利用TERRA/MOPITT仪器测量的2000年3月—2004年5月的CO数据, 分析了CO的时空分布特征及其变化趋势, 并且与美国国家海洋大气管理局气候监测与诊断实验室 (CMDL/NOAA) 在瓦里关站的CO观测结果进行比较和验证。结果表明: CO的高值区在北半球主要位于东亚、西欧和北美, 而在南半球主要位于非洲中西部和南美洲的赤道地区; CO的分布随季节变化显著, 春季北半球的CO浓度最高, 而秋季南半球的CO浓度偏高; 东亚的CO高值区主要是位于中国东部沿海地区和日本列岛一带。对于北京和瓦里关CO的趋势分析明表:这两个地区的CO浓度在这4年内都是呈上升趋势。结合CMDL的观测资料与卫星观测结果进行比较和检验发现, 瓦里关站卫星观测结果和CMDL的结果在时间序列的变化趋势一致, 卫星柱总量的观测数据和CMDL数据的相关性非常好。     

Non-uniform interhemispheric temperature trends over the past 550 years

The warming trend over the last century in the northern hemisphere (NH) was interrupted by cooling from ad 1940 to 1975, a period during which the southern hemisphere experienced pronounced warming. The cause of these departures from steady warming at multidecadal timescales are unclear; the prevailing explanation is that they are driven by non-uniformity in external forcings but recent models suggest internal climate drivers may play a key role. Paleoclimate datasets can help provide a long-term perspective. Here we use tree-rings to reconstruct New Zealand mean annual temperature over the last 550 years and demonstrate that this has frequently cycled out-of-phase with NH mean annual temperature at a periodicity of around 30–60 years. Hence, observed multidecadal fluctuations around the recent warming trend have precedents in the past, strongly implicating natural climate variation as their cause. We consider the implications of these changes in understanding and modelling future climate change.     

On the Formulation of Lagrangian Stochastic Models of Scalar Dispersion Within Plant Canopies

Lagrangian stochastic models, quadratic in velocity and satisfying the well-mixed condition for two-dimensional Gaussian turbulence, are used to make predictions of scalar dispersion within a model plant canopy. The non-uniqueness associated with satisfaction of the well-mixed condition is shown to be non-trivial (i.e. different models produce different predictions for scalar dispersion). The best agreement between measured and predicted mean concentrations of scalars is shown to be obtained with a small sub-class of optimal models. This sub-class of optimal models includes Thomson's model (J. Fluid Mech. 180, 529–556, 1987), the simplest model that satisfies the well-mixed condition for Gaussian turbulence, but does not include two other models identified recently as being in optimal agreement with the measured spread of tracers in a neutral boundary layer. It is therefore demonstrated that such models are not universal, i.e. applicable to a wide range of flows without readjustment of model parameters. Predictions for scalar dispersion in the model plant canopy are also obtained using the model of Flesch and Wilson (Boundary-Layer Meteorol. 61, 349–374, 1992). It is shown that, when used with a Gaussian velocity distribution or a maximum-missing-information velocity distribution, which accounts for the measured skewness and kurtosis of velocity statistics, the agreement between predictions obtained using the model of Flesch and Wilson and measurements is as good as that obtained using Thomson's model.     

Application of solar max ACRIM data to analysis of solar-driven climatic variability on Earth

Climatic change caused by solar variability has been proposed for at least a century, but could not be assessed reliably in the past because the uncertainty in solar irradiance measured from the Earth's surface is too large. Now satellite measurements by such instruments as the Active Cavity Radiometer Irradiance Monitor (ACRIM) permit a preliminary assessment. The satellite data exhibit irradiance variations over a spectrum of shorter timescales, but the first 5-yr overall trend indicates slightly decreasing luminosity. The global temperature response to monthly-mean ACRIM-measured fluctuations from 1980–1984 was computed from the NYU 1D transient climate model - which includes thermal inertia effects of the world oceans - starting from an assumed pre-existing steady state, and the results compared with observations of recent global temperature trends. The modeled surface temperature evolution exhibited a complex history-dependent behavior whose fluctuations were an order of magnitude smaller than observed, primarily owing to oceanic thermal damping. Thus solar variability appears unlikely to have been an important factor in global-scale climate change over this period. The possibility of using the measurements to develop simple correlations for irradiance with longer term solar activity observable from the surface, and therefore to analyze historical effects, was considered, but is not supported by the satellite data. However, we have used a model of solar irradiance variation with time (Schatten, 1988), covering the period 1976–1997 in order to assess our model's response to forcing whose fluctuation timescale is comparable to the thermal relaxation time of the upper ocean. Continuous monitoring of solar flux by space-based instruments over timescales of 20 yr or more, comparable to timescales for thermal relaxation of the oceans, and of the solar cycle itself, is probably needed to resolve issues of long-term solar variation effects on climate.Presently at Lamont-Doherty Geological Observatory of Columbia University, Palisades, NY 10964.     

Tropospheric adjustment to increasing CO2: its timescale and the role of land–sea contrast

Physical processes responsible for tropospheric adjustment to increasing carbon dioxide concentration are investigated using abrupt CO₂ quadrupling experiments of a general circulation model (GCM) called the model for interdisciplinary research on climate version 5 with several configurations including a coupled atmosphere–ocean GCM, atmospheric GCM, and aqua-planet model. A similar experiment was performed in weather forecast mode to explore timescales of the tropospheric adjustment. We found that the shortwave component of the cloud radiative effect (SWcld) reaches its equilibrium within 2 days of the abrupt CO₂ increase. The change in SWcld is positive, associated with reduced clouds in the lower troposphere due to warming and drying by instantaneous radiative forcing. A reduction in surface turbulent heat fluxes and increase of the near-surface stability result in shoaling of the marine boundary layer, which shifts the cloud layer downward. These changes are common to all experiments regardless of model configuration, indicating that the cloud adjustment is primarily independent of air–sea coupling and land–sea thermal contrast. The role of land in cloud adjustment is further examined by a series of idealized aqua-planet experiments, with a rectangular continent of varying width. Land surface warming from quadrupled CO₂ induces anomalous upward motion, which increases high cloud and associated negative SWcld over land. The geographic distribution of continents regulates the spatial pattern of the cloud adjustment. A larger continent produces more negative SWcld, which partly compensates for a positive SWcld over the ocean. The land-induced negative adjustment is a factor but not necessary requirement for the tropospheric adjustment.     

从熵原理得出的稳定层云的云滴谱方程

通过对层云的云滴大小的分布观测,揭示了其滴谱常里单峰偏态分布。这种谱的经常出现,表明它是一种优势谱。它的稳定性,从统计物理角度看也表示云体处于一种平衡态,而平衡态则对应某种熵达到极大值。 本文指出的熵是云滴表面自由能所对应的熵。文中用熵极大原理导出了稳定层云的云滴谱方程(公式11),並且用资料证实了这一理论与事实相符。     

Assessing climate persistence from climatic ‘noise’

Changes in climate are generally defined as differences between means. The autocorrelation usually encountered in geophysical series and the implied persistence, or reduced number of independent observations, tend to render significance tests of mean differences inconclusive until a considerable amount of data has accumulated, by which time a climatic change could be well under way. A more efficient alternative approach is provided by small-sample variances. Under slightly idealized conditions their frequency distribution is of the chi-square type with a number of degrees of freedom that measures the persistence in the series and can be monitored on a short time scale with a sequential sampling procedure.Also at homein CIRES, Campus Box 449, University of Colorado, Boulder, CO 80309, U.S.A.     

On the moments approximation method for constructing a Lagrangian Stochastic model

The exact Eulerian velocity probability density function (pdf) of a turbulent field is generally unknown, and one normally has available only partial information in the form of low order moments. We compare two alternative Lagrangian Stochastic (LS) approaches formed from this partial information, (i) the moments approximation approach (Kaplan and Dinar, 1993); and (ii) the well-mixed model (Thomson, 1987) that corresponds to the maximum missing information pdf formed from the available information. We show that the moments approximation model does not in general satisfy the well-mixed constraint, and can give an inferior prediction of dispersion.     

Climate effects on atmospheric carbon dioxide over the last century

The buildup of atmospheric CO₂ since 1958 is surprisingly well explained by the simple premise that 57% of the industrial emissions (fossil fuel burning and cement manufacture) has remained airborne. This premise accounts well for the rise both before and after 1980 despite a decrease in the growth rate of fossil fuel CO₂ emissions, which occurred at that time, and by itself should have caused the airborne fraction to decrease. In contrast, the buildup prior to 1958 was not simply proportional to cumulative fossil fuel emissions, and notably included a period during the 1940s when CO₂ growth stalled despite continued fossil fuel emissions. Here we show that the constancy of the airborne fraction since 1958 can be in part explained by decadal variations in global land air temperature, which caused a warming-induced release of CO₂ from the land biosphere to the atmosphere. We also show that the 1940s plateau may be related to these decadal temperature variations. Furthermore, we show that there is a close connection between the phenomenology producing CO₂ variability on multidecadal and El Niño timescales.     

Considering the influence of sequestration duration and carbon saturation on estimates of soil carbon capacity

Rates of soil C sequestration have previously been estimated for a number of different land management activities, and these estimates continue to improve as more data become available. The time over which active sequestration occurs may be referred to as the sequestration duration. Integrating soil C sequestration rates with durations provides estimates of potential change in soil C capacity and more accurate estimates of the potential to sequester C. In agronomic systems, changing from conventional plow tillage to no-till can increase soil C by an estimated 16±3%, whereas increasing rotation intensity can increase soil C by an estimated 6±3%. The increase in soil C following a change in rotation intensity, however, may occur over a slightly longer period (26 yr) than that for tillage cessation (21 yr). Sequestration strategies for grasslands have, on average, longer sequestration durations (33 yr) than for croplands. Estimates for sequestration rates and durations are mean values and can differ greatly between individual sites and management practices. As the annual sequestration rate declines over the sequestration duration period, soil C approaches a new steady state. Sequestration duration is synonymous with the time to which soil C steady state is reached. However, soils could potentially sequester additional C following additional changes in management until the maximum soil C capacity, or soil C saturation, is achieved. Carbon saturation of the soil mineral fraction is not well understood, nor is it readily evident. We provide evidence of soil C saturation and we discuss how the steady state C level and the level of soil C saturation together influence the rate and duration of C sequestration associated with changes in land management.     

A model of fair-weather cumulus convection

By assuming that cumulus clouds grow from patches of air that extend from the well-mixed layer bear the surface, a model of fair-weather cumulus convection is developed. The model predicts the structure of the well-mixed layer and the cloud layer; in particular, cloud cover is estimated as a function of time. The model results are compared with laboratory and field observations.