Ozone in the Marine Boundary Layer at Cape Grim: Model Simulation

A photochemical box model has been used to simulate the mixing ratio ofozone under conditions reflecting those encountered in the marine boundarylayer at Cape Grim, Tasmania, where a decade-long record of ozone mixingratio is available. The model is based on the proposition that ozone loss byphotolysis, atmospheric reaction with hydroperoxy and hydroxyl radicals, andsurface deposition is balanced by ozone gain via entrainment from the lowerfree troposphere with a small additional source in summer from photolysis ofnitrogen dioxide. This model simulates very well the observed ozone records,reproducing both the small diurnal cycle in ozone mixing ratio observedduring the summer months, and the factor of two seasonal ozone cycle showinga distinct winter maximum and summer minimum. The model result confirms thatunder the low-NO_x conditions of the clean marine boundarylayer net photochemical loss of ozone occurs at all times of year.     

Development of the Convective Boundary Layer Capping with a Thick Neutral Layer in Badanjilin: Observations and Simulations

In this study,the development of a convective boundary layer (CBL) in the Badanjilin region was investigated by comparing the observation data of two cases.A deep neutral layer capped a CBL that occurred on 30 August 2009.This case was divided into five sublayers from the surface to higher atmospheric elevations:surface layer,mixed layer,inversion layer,neutral layer,and sub-inversion layer.The development process of the CBL was divided into three stages:S1,S2,and S3.This case was quite different from the development of the three-layer CBL observed on 31 August 2009 because the mixed layer of the five-layer CBL (CBL5) eroded the neutral layer during S2.The specific initial structure of the CBL5 was correlated to the synoptic background of atmosphere during nighttime.The three-stage development process of the CBL5 was confirmed by six simulations using National Center for Atmospheric Research (USA) large-eddy simulation (NCAR-LES),and some of its characteristics are presented in detail.     

地形和边界层摩擦对登陆热带气旋路径和强度影响的研究

采用准地转的正压模式, 研究了无非绝热加热时地形和边界层摩擦对登陆热带气旋路径和强度的影响.结果表明: 地形作用对登陆热带气旋西北移动路径的影响比较明显, 而对登陆热带气旋强度的影响不明显; 边界层摩擦可以通过改变热带气旋X方向上和Y方向上的移动速度以及改变热带气旋水平环流结构对登陆热带气旋西北移动路径产生一定的影响, 边界层摩擦对登陆热带气旋强度的影响非常明显, 其中摩擦是造成登陆热带气旋强度迅速减弱的一个重要因素.     

DMS and SO2 Measurements in the Tropical Marine Boundary Layer

Dimethyl sulfide (DMS) and sulfur dioxide (SO₂) mixing ratios were measured in the boundary layer on Oahu, Hawaii in April and May 2000. Average DMS and SO₂ levels were 22 ± 7 (n = 488) pmol/mol and 23 ± 7 (n = 471) pmol/mol respectively. Anti-correlated DMS and SO₂ diurnal cycles, consistent with DMS + OH oxidation were observed on most days. Photochemical box model simulations suggest that the yield of SO₂ and total SO₂ sink are ∼85% and ∼2 × 10⁴ molec cm^{− 3} s^{− 1} respectively. On several days the rate of decrease in DMS and increase in SO₂ levels in the early morning were larger that predicted by the model. Dynamical and chemical causes for the anomalous early morning data are explored.     

Evaluation of the Ocean Feedback on Height Characteristics of the Tropical Cyclone Boundary Layer

In this study, the interaction between the tropical cyclone（TC） and the underlying ocean is reproduced by using a coupled atmosphere-ocean model. Based on the simulation results, characteristics of the TC boundary layer depth are investigated in terms of three commonly used definitions, i.e., the height of the mixed layer depth（HVTH）, the height of the maximum tangential winds（HTAN）, and the inflow layer depth（HRAD）. The symmetric height of the boundary layer is shown to be cut down by the ocean response, with the decrease of HVTH slightly smaller than that of HTAN and HRAD. The ocean feedback also leads to evident changes in asymmetric features of the boundary layer depth. The HVTH in the right rear of the TC is significantly diminished due to presence of the cold wake, while the changes of HVTH in other regions are rather small. The decreased surface virtual potential temperature by the cold wake is identified to be dominant in the asymmetric changes in HVTH. The impacts of ocean response on the asymmetric distributions of HTAN are nonetheless not distinct, which is attributed to the highly axisymmetric property of tangential winds. The HRAD possesses remarkable asymmetric features and the inflow layer does not exist in all regions, an indication of the inadequacy of the definition based on symmetric inflow layer depth. Under influences of the cold wake, the peak inflow area rotates counterclockwise distinctly. As a consequence, the HRAD becomes deeper in the east while shallower in the west of the TC.     

An Intercomparison of Large-Eddy Simulations of the Stable Boundary Layer

Results are presented from the first intercomparison of large-eddy simulation (LES) models for the stable boundary layer (SBL), as part of the Global Energy and Water Cycle Experiment Atmospheric Boundary Layer Study initiative. A moderately stable case is used, based on Arctic observations. All models produce successful simulations, in as much as they generate resolved turbulence and reflect many of the results from local scaling theory and observations. Simulations performed at 1-m and 2-m resolution show only small changes in the mean profiles compared to coarser resolutions. Also, sensitivity to subgrid models for individual models highlights their importance in SBL simulation at moderate resolution (6.25 m). Stability functions are derived from the LES using typical mixing lengths used in numerical weather prediction (NWP) and climate models. The functions have smaller values than those used in NWP. There is also support for the use of K-profile similarity in parametrizations. Thus, the results provide improved understanding and motivate future developments of the parametrization of the SBL.     

Iodine Chemistry and its Role in Halogen Activation and Ozone Loss in the Marine Boundary Layer: A Model Study

A detailed set of reactions treating the gas and aqueous phase chemistry of the most important iodine species in the marine boundary layer (MBL) has been added to a box model which describes Br and Cl chemistry in the MBL. While Br and Cl originate from seasalt, the I compounds are largely derived photochemically from several biogenic alkyl iodides, in particular CH2I2, CH2ClI, C2H5I, C3H7I, or CH3I which are released from the sea. Their photodissociation produces some inorganic iodine gases which can rapidly react in the gas and aqueous phase with other halogen compounds. Scavenging of the iodine species HI, HOI, INO2, and IONO2 by aerosol particles is not a permanent sink as assumed in previous modeling studies. Aqueous-phase chemical reactions can produce the compounds IBr, ICl, and I2, which will be released back into the gas phase due to their low solubility. Our study, although highly theoretical, suggests that almost all particulate iodine is in the chemical form of IO-3. Other aqueous-phase species are only temporary reservoirs and can be re-activated to yield gas phase iodine. Assuming release rates of the organic iodine compounds which yield atmospheric concentrations similar to some measurements, we calculate significant concentrations of reactive halogen gases. The addition of iodine chemistry to our reaction scheme has the effect of accelerating photochemical Br and Cl release from the seasalt. This causes an enhancement in ozone destruction rates in the MBL over that arising from the well established reactions O(1D) + H2O 2OH, HO2 + O3 OH + 2O2, and OH + O3 HO2 + O2. The given reaction scheme accounts for the formation of particulate iodine which is preferably accumulated in the smaller sulfate aerosol particles.     

Dynamic Impact of the Vertical Shear of Gradient Wind on the Tropical Cyclone Boundary Layer Wind Field

This work studies the impact of the vertical shear of gradient wind （VSGW） in the free atmosphere on the tropical cyclone boundary layer （TCBL）. A new TCBL model is established, which relies on five- force balance including the pressure gradient force, Coriolis force, centrifugal force, turbulent friction, and inertial deviation force. This model is then employed to idealize tropical cyclones （TCs） produced by DeMaria＇s model, under different VSGW conditions （non-VSGW, positive VSGW, negative VSGW, and VSGW increase/decrease along the radial direction）. The results show that the free-atmosphere VSGW is particularly important to the intensity of TC. For negative VSGW, the total horizontal velocity in the TCBL is somewhat suppressed. However, with the maximum radial inflow displaced upward and outward, the radial velocity notably intensifies. Consequently, the convergence is enhanced throughout the TCBL, giving rise to a stronger vertical pumping at the TCBL top. In contrast, for positive VSGW, the radial inflow is significantly suppressed, even with divergent outflow in the middle-upper TCBL. For varying VSGW along the radial direction, the results indicate that the sign and value of VSGW is more important than its radial distribution, and the negative VSGW induces stronger convergence and Ekman pumping in the TCBL. which favors the formation and intensification of TC.     

A Scale-Dependent Dynamic Model for Scalar Transport in Large-Eddy Simulations of the Atmospheric Boundary Layer

An important challenge in large-eddy simulationsof the atmospheric boundarylayer is the specification of the subgrid-scale(SGS) model coefficient(s)and, in particular, how to account for factorssuch as position in the flow,grid/filter scale and atmospheric stability.A dynamic SGS model (thatassumes scale invariance of the coefficients)is implemented in simulationsof a neutral boundary layer with a constantand uniform surface flux of apassive scalar. Results from our simulationsshow evidence that the lumpedcoefficient in the eddy-diffusion modelcomputed with the dynamic proceduredepends on scale. This scale dependence isstronger near the surface, and itis more important for the scalar than for thevelocity field (Smagorinskycoefficient) due to the stronger anisotropicbehaviour of scalars. Based onthese results, a new scale-dependent dynamicmodel is developed for theeddy-diffusion lumped coefficient. The newmodel, which is similar to theone proposed earlierfor the Smagorinsky coefficient,is fully dynamic, thus not requiring anyparameter specification or tuning.Simulations with the scale-dependent dynamicmodel yield the expected trendsof the coefficients as functions of positionand filter/grid scale.Furthermore, in the surface layer the newmodel gives improved predictionsof mean profiles and turbulence spectra ascompared with the traditionalscale-invariant dynamic model.     

Effect of the Vertical Diffusion of Moisture in the Planetary Boundary Layer on an Idealized Tropical Cyclone

Previous numerical studies have focused on the combined effect of momentum and scalar eddy diffusivity on the intensity and structure of tropical cyclones. The separate impact of eddy diffusivity estimated by planetary boundary layer(PBL) parameterization on the tropical cyclones has not yet been systematically examined. We have examined the impacts of eddy diffusion of moisture on idealized tropical cyclones using the Advanced Research Weather Research and Forecasting model with the Yonsei University PBL scheme. Our results show nonlinear effects of moisture eddy diffusivity on the simulation of idealized tropical cyclones. Increasing the eddy diffusion of moisture increases the moisture content of the PBL, with three different effects on tropical cyclones:(1) an decrease in the depth of the PBL;(2) an increase in convection in the inner rain band and eyewall; and(3) drying of the lowest region of the PBL and then increasing the surface latent heat flux. These three processes have different effects on the intensity and structure of the tropical cyclone through various physical mechanisms. The increased surface latent heat flux is mainly responsible for the decrease in pressure. Results show that moisture eddy diffusivity has clear effects on the pressure in tropical cyclones, but contributes little to the intensity of wind. This largely influences the wind–pressure relationship, which is crucial in tropical cyclones simulation. These results improve our understanding of moisture eddy diffusivity in the PBL and its influence on tropical cyclones, and provides guidance for interpreting the variation of moisture in the PBL for tropical cyclone simulations.     

BCC_CSM1.1(m) 模式对热带太平洋潜热通量的评估

利用1979—2005年OAFlux (Objectively Analyzed air-sea Fluxes) 观测资料以及CMIP5的15个耦合模式的模拟结果,评估了BCC_CSM1.1(m) 模式对热带太平洋年平均潜热通量气候态和变化趋势的模拟能力,并分析造成趋势偏差的可能原因。结果表明:BCC_CSM1.1(m) 模式模拟热带太平洋年平均潜热通量气候态在各纬度上差异较大, 其中在赤道的模拟能力较佳,而在10°N和8°S附近模拟偏差较大;BCC_CSM1.1(m) 模式对热带太平洋年平均潜热通量趋势的模拟能力一般,造成趋势偏差的主要原因是该模式低估了风速对潜热通量的局地贡献以及它对风速的非局地贡献的模拟存在较大偏差。此外,该模式未能较好地模拟出风速对全球变暖响应。因此,BCC_CSM1.1(m) 模式对热带太平洋年平均潜热通量趋势模拟的改进需加强其对风速模拟的改进。     

Observations of Solitary Waves in the Stable Atmospheric Boundary Layer

Large amplitude, isolated, wave-like phenomena have been observed in the lowest 40 m of the strongly stably stratified atmospheric boundary layer overlying a coastal Antarctic ice shelf. The waves only occur when prevailing wind speeds are low. They always propagate from over the land, with phase speeds exceeding the local mean wind speed. They have wavelengths of the order of 200 m. Several examples are described and a summary of the statistical properties of these waves events is presented.     

Oceanic Origin of A Recent La Nina-Like Trend in the Tropical Pacific

Global ocean temperature has been rising since the late 1970s at a speed unprecedented during the past century of recordkeeping.This accelerated warming has profound impacts not only on the marine ecosystem and oceanic carbon uptake but also on the global water cycle and climate.During this rapid warming period,the tropical Pacific displays a pronounced La Nin a-like trend,characterized by an intensification of west-east SST gradient and of atmospheric zonal overturning circulation,namely the Walker circulation.This La Nin a-like trend differs from the El Nin o-like trend in warm climate projected by most climate models,and cannot be explained by responses of the global water cycle to warm climate.The results of this study indicate that the intensification of the zonal SST gradient and the Walker circulation are associated with recent strengthening of the upper-ocean meridional overturning circulation.     

用大涡模式研究对流边界层湍流

利用三维对流边界层的大涡模拟模式研究对流边界层湍流统计特征量,模拟结果与前人的同类模拟工作及实验观测结果相比较,得到了一致的结论.       

Observations of Atmospheric Solitary Waves in the Urban Boundary Layer

The simultaneous operation of a three-axis Doppler sodar system in the centralurban area of Rome and two similar systems in the suburban area, forming atriangle about 20 km on each side, provided evidence of solitary-type wavesin the urban boundary layer. Three events, each lasting from a few minutes toabout 30 min, and ranging in depth from the minimum range of the sodar (39 m) to over 500 m, are reported here. Two events were recognizable onall three sodar records while the third event could be observed at the urbanlocation only. Time-height acoustic echo intensity records showed no-echoregions within the wave indicating transport of trapped recirculating air.This is typical of large amplitude solitary waves. The time series plots ofsodar-derived vertical wind velocity revealed a maximum peak-to-peakvariation of about 5 m s^-1 during periods of wave-associated disturbance.The vertical velocity is found to increase with height up to the top of the closedcirculation within the wave and decreases further above. The normalisedamplitude-wavelength relationship for the two events indicates that theobserved waves are close to a strongly nonlinear regime.     

Observations of the Daytime Boundary Layer in Deep Alpine Valleys

Mixing depth structure and its evolution have been diagnosed from radar wind profiler data in the Chamonix and the Maurienne valleys (France) during summer 2003. The behaviour of refractive index structure parameter C _n ² peaks coupled with the vertical velocity variance σ _w ² was used to estimate the height of the mixed layer. Tethersonde vertical profiles were carried out to investigate the lower layers of the atmosphere in the range of approximately 400–500 m above ground level. The tethersonde device was especially useful to study the reversal of the valley wind system during the morning transition period. Specific features such as wind reversal and the convective mixed layer up to approximately the altitude of the surrounding mountains were documented. The wind reversal was observed to be much more sudden in the Maurienne valley than in the Chamonix valley     

Length Scales of Scalar Diffusion in the Convective Boundary Layer: Laboratory Observations

A laboratory study of scalar diffusion in the convective boundary layer has found results that are consistent with a 1999 large-eddy simulation (LES) study by Jonker, Duynkerke and Cuijpers. For bottom-up and top-down scalars (introduced as ‘infinite’ area sources of passive tracer at the surface and inversion, respectively) the dominant length scale was found to be much larger than the length scale for density fluctuations, the latter being equal to the boundary-layer depth h. The variance of the normalized passive scalar grew continuously with time and its magnitude was about 3–5 times larger for the top-down case than for the bottom-up case. The vertical profiles of the normalized passive scalar variance were found to be approximately constant through the convective boundary layer (CBL) with a value of about 3–8c_*² for bottom-up and 10–50c_*² for top-down diffusion. Finally, there was some evidence of a minimum in the variance and dominant length scale for scalar flux ratios (top-down to bottom-up flux) close to −0.5. All these convection tank results confirm the LES results and support the hypothesis that there is a distinct difference in behaviour between the dynamic and passive variables in the CBL.     

Comparison of Measured Ozone Production Efficiencies in the Marine Boundary Layer at Two European Coastal Sites under Different Pollution Regimes

Ozone production efficiencies (E_N), which can be defined as the netnumber of ozone molecules produced per molecule of NO_xoxidised, have been calculated from measurements taken during three intensive field campaigns (one in the spring, EASE 96, and two in the summer, EASE 97 and TIGER 95), at two European coastal sites (Mace Head, Ireland (EASE) and Weybourne, Norfolk (TIGER)) impacted by polluted air masses originating from both the U.K. and continental Europe, as well as relatively clean oceanic air masses from the Arctic and Atlantic. From a detailed wind sector analysis of the EASE 96 and 97 data it is clear that two general types of pollution regime were encountered at Mace Head. The calculated ozone production efficiency in clean oceanic air masses was approximately 65, which contrasted to more polluted air, from the U.K. and the continental European plume, where the efficiency decreased to between 4 and 6. The latter values of E_Nagree well with literature measurements conducted downwind of various urban centres in the U.S. and Europe, which are summarised in a wide-ranging review table. The E_N value calculated for clean oceanic air is effectivelyan upper limit, owing to the relatively rapid deposition of HNO₃ tothe ocean. Consideration of the variation of E_N with NO_x forthe three campaigns suggests that ozone production efficiency is relatively insensitive to both geographical location and season. The measuredE_N values are also compared with values derived from steady-state expressions. An observed anti-correlation between E_N and measured ozone tendencyis briefly discussed.     

A Homogeneous Langevin Equation Model, Part ii: Simulation of Dispersion in the Convective Boundary Layer

We present a Lagrangian stochastic model of vertical dispersion in the convective boundary layer (CBL). This model is based on a generalized Langevin equation that uses the simplifying assumption that the skewed vertical velocity probability distribution is spatially homogeneous. This approach has been shown to account for two key properties of CBL turbulence associated with large-scale coherent turbulent structures: skewed vertical velocity distributions and long velocity correlation time. A 'linear-skewed' form of the generalized Langevin equation is used, which has a linear (in velocity) deterministic acceleration and a skewed random acceleration. 'Reflection' boundary conditions for selecting a new velocity for a particle that encounters a boundary were investigated, including alternatives to the standard assumption that the magnitudes of the particle incident and reflected velocities are positively correlated. Model simulations were tested using cases for which exact, analytic statistical properties of particle velocity and position are known, i.e., well-mixed spatial and velocity distributions. Simulations of laboratory experiments of CBL dispersion show that (1) the homogeneous linear-skewed Langevin equation model (as well as an alternative 'nonlinear-Gaussian' Langevin equation model) can simulate the important aspects of dispersion in the CBL, and (2) a negatively-correlated-speed reflection boundary condition simulates the observed dispersion of material near the surface in the CBL significantly better than alternative reflection boundary conditions. The homogeneous linear-skewed Langevin equation model has the advantage that it is computationally more efficient than the homogeneous nonlinear-Gaussian Langevin equation model, and considerably more efficient than inhomogeneous Langevin equation models.