The breakup of marine boundary-layer clouds over an inhomogeneous sea surface temperature field

This numerical study examines the breakup of marine atmospheric boundary-layer (MABL) clouds through various physical processes over an inhomogeneous sea surface temperature (SST) field. Three regimes are identified under which the cloud layer will break up. (A) advection of drier air into the MABL for the California case. (B) daytime absorption of solar radiation, occurring most easily over the cold water. (C) mesoscale fluctuations in the flow, producing holes in the cloud layer.The budget study of these three situations concludes that large-scale subsidence, solar radiation, local mesoscale advection, and inhomogeneous surface fluxes cannot be neglected in modeling cloud breakup. This study also confirms the belief that the mixing process alone induced by evaporative entrainment is generally insufficient to predict the breakup of the cloud layer.Sections of this paper are based on an extended abstract by the author and Dr. Steven Stage for the Ninth Symposium on Turbulence and Diffusion held at Riso, Denmark, 1990.     

Aircraft measurements of boundary-layer turbulence over the central Equatorial Pacific Ocean

We analyze gust probe measurements obtained in the convective boundary layer over the central equatorial Pacific as part of the Equatorial Pacific Ocean Climate Studies (EPOCS) program. From the lowest level flights, the bulk transfer coefficients are found to be 1.1 × 10^-3 and 1.4 × 10^-3 for latent and sensible heat fluxes, respectively. Vertical profiles of water vapor density, potential temperature and wind velocity are obtained as are the profiles of the fluxes of latent and sensible heat and momentum. From the extrapolated profiles, we obtain surface fluxes of 120 W m^-2 and 13 W m^-2 for latent and sensible heat, respectively, and 0.11 N m^-2 for momentum. The 10 m drag coefficient is 1.5 × 10^-3. Two convergence boxes, north and south of the ITCZ, are analyzed. Enhanced convergence is found in the northern trades relative to the southern trades. The advective acceleration is found to be comparable in magnitude to the other terms in the horizontal equations of motion.     

Numerical model simulations of boundary-layer dynamics during winter conditions

Summary  A mesoscale numerical model, incorporating a land-surface scheme based on Deardorffs’ approach, is used to study the diurnal variation of the boundary layer structure and surface fluxes during four consecutive days with air temperatures well below zero, snow covered ground and changing synoptic forcing. Model results are evaluated against in-situ measurements performed during the WINTEX field campaign held in Sodankyl?, Northern Finland in March 1997. The results show that the land-surface parameterization employed in the mesoscale model is not able to reproduce the magnitude of the daytime sensible heat fluxes and especially the pronounced maximum observed in the afternoon. Additional model simulations indicate that this drawback is to a large extent removed by the implementation of a shading factor in the original Deardorff scheme. The shading factor, as discussed in Gryning et al. (2001), accounts for the fact that in areas with sparse vegetation and low solar angles, both typical for the northern boreal forests in wintertime, absorption of direct solar radiation is due to an apparent vegetation cover which is much greater than the actual one (defined as the portion of the ground covered by vegetation projected vertically). Moreover, the observed asymmetry in the diurnal variation of the sensible heat flux indicates that there might be a significant heat storage in the vegetation. The implementation of an objective heat storage scheme in the mesoscale model explains part of the observed diurnal variation of the sensible heat flux. Received November 12, 1999 Revised October 4, 2000     

Warm-season temperatures since 1600 BC reconstructed from Tasmanian tree rings and their relationship to large-scale sea surface temperature anomalies

We describe an improved tree-ring reconstruction of mean warm-season (November–April) temperatures for Tasmania from Huon pine. This record extends back to 1600 BC and is based on a tree-ring chronology that was processed to retain as much low-frequency variance as possible. The resulting reconstruction explains 46.6% of the variance and verifies significantly when compared to withheld instrumental data. Cross-spectral analysis of actual and estimated temperatures over the 1886–1991 common period indicates that most of the unexplained variance is at periods < 12 years in length. At periods > 12 years, the squared coherency ranges between 0.6–0.8, and the cross-spectral gain indicates that the amplitude of the reconstruction is a nearly unbiased estimate of the true temperature amplitude. Therefore, this reconstruction should be especially useful for studying multi-decadal temperature variability in the Tasmanian sector of the Southern Hemisphere over the past 3592 years. To this end, we examined the time evolution of low-frequency temperature amplitude fluctuations and found evidence for a 35% amplitude reduction after AD 100 that persisted until about AD 1900. Since that time, the low-frequency temperature amplitude has systematically increased. We also show how this reconstruction is related to large-scale sea surface temperatures (SST) in the Indian Ocean and eastward to the dateline. Pointwise correlations between the Tasmanian record and SSTs reveal a relationship that extends across the southern Indian Ocean and towards the Arabian Sea. This pattern is largely determined by inter-decadal temperature variability, with correlations in this > 10-year bandwidth commonly exceeding 0.6 over most of the southern Indian and southwestern Pacific sectors. A rotated empirical orthogonal function analysis reveals that the pattern of pointwise correlations found between the temperature reconstruction and SSTs is largely explained by the linear combination of three orthogonal modes of SST variability. Received: 12 January 1999 / Accepted: 31 July 1999     

Estimation of surface stress over a forest from sodar measurements and its use to parameterize the stable boundary-layer height

Sodar observations from three nights of the HAPEX-MOBILHY experiment have been used to compute covariances between single measurements of the three velocity components. From these, estimates of a low frequency friction velocityu _* are derived which show better correlation with observed values of the stable boundary layer (SBL) height,h, than directu _*-measurements by an ultrasonic anemometer. On the contrary, interdiurnal variability ofh is better correlated with directu _*-measurements. These findings should be mainly due to the problem of different spectral and spatial representativity of the twou _*-values.     

星载SAR测量海洋流场研究进展

多普勒信息是合成孔径雷达（SAR）进行海洋动力遥感的一个重要参数,自从20世纪80年代末就已经开始利用该参数反演海表面动力过程的相关研究.本文重点论述了星载SAR多普勒信息反演海表面流及其应用研究,主要包括SAR干涉测量方法获取海洋流场信息的主要技术手段及研究进展;多普勒频移方法反演表层海洋流场的发展历程及在上层海洋现象（包括地转流、准地转流以及中小尺度过程）中的应用研究;相关的国外重要数据融合计划和研发计划.目前国内在海洋流场的遥感方面还处于起步阶段,重点关注于反演理论和方法,而将反演信息应用于上层海洋动力的研究,还十分缺乏.希望通过本文的综述,为后续在该方向的研究提供借鉴.     

Interannual to decadal summer drought variability over Europe and its relationship to global sea surface temperature

Interannual to decadal variability of European summer drought and its relationship with global sea surface temperature (SST) is investigated using the newly developed self calibrated Palmer drought severity index (scPDSI) and global sea surface temperature (SST) field for the period 1901–2002. A European drought severity index defined as the average of scPDSI over entire Europe shows quasiperiodic variations in the 2.5–5 year band as well as at 12–13 years suggesting a possible potential predictability of averaged drought conditions over Europe. A Canonical Correlation Analysis between summer scPDSI anomalies over Europe and global SST anomalies reveals the existence of three modes of coupled summer drought scPDSI patterns and winter global SST anomalies. The first scPDSI-SST coupled mode represents the long-term trends in the data which manifest in SST as warming over all oceans. The associated long-term trend in scPDSI suggests increasing drought conditions over the central part of Europe. The second mode is related to the inter-annual ENSO and decadal PDO influence on the European climate and the third one captures mainly the drought pattern associated to Atlantic Multidecadal Oscillation. The lag relationships between winter SST and summer drought conditions established in this study can provide a valuable skill for the prediction of drought conditions over Europe on interannual to decadal time scales.     

Impact of a sea breeze on the boundary-layer dynamics and the atmospheric stratification in a coastal area of the North Sea

In-situ sodar and lidar measurements were coupled with numerical simulations for studying a sea-breeze event in a flat coastal area of the North Sea. The study’s aims included the recognition of the dynamics of a sea-breeze structure, and its effects on the lower troposphere stratification and the three-dimensional (3D) pollutant distribution. A sea breeze was observed with ground-based remote sensing instruments and analysed by means of numerical simulations using the 3D non-hydrostatic atmospheric model Meso-NH. The vertical structure of the lower troposphere was experimentally determined from the lidar and sodar measurements, while numerical simulations focused on the propagation of the sea breeze inland. The sea-breeze front, the headwind, the thermal internal boundary layer, the gravity current and the sea-breeze circulation were observed and analysed. The development of a late stratification was also observed by the lidar and simulated by the model, suggesting the formation of a stable multilayered structure. The transport of passive tracers inside the sea breeze and their redistribution above the gravity current was simulated too. Numerical modelling showed that local pollutants may travel backward to the sea above the gravity current at relatively low speed due to the shearing between the landward gravity current and the seaward synoptic wind. Such dynamic conditions may enhance an accumulation of pollutants above coastal industrial areas.     

一种基于TMI观测结果的海表温度反演算法

基于星载微波仪器观测结果反演海表温度,能很好地克服云对反演结果的干扰,实现对海表温度全天候的监测.文中利用热带测雨卫星所搭载的微波成像仪的观测结果,建立了一种新的适用于非降水条件下的海表温度反演算法.作为一种半经验统计算法,它以辐射传输方程为基础,通过理论模拟计算,建立海表温度与微波成像仪多通道亮温之间的关系,较好地反...     

Correlation of optical propagation near the sea surface with bulk meteorological measurements

A time-dependent integrated dynamical boundary-layer model is used to study various features of the nocturnal low-level jet (LLJ). The basic concept is that of Thorpe and Guymer (1977). It is extended by entrainment processes, advection and an equation for the upper height of the turbulent layer. Applications show the role of the energy terms and how the LLJ changes considerably under synoptic forcing processes of variable geostrophic wind.     

A dynamical analysis of the drag conditions at the sea surface

When applied to a sea surface, shortcomings are noted for the ordinary classification of drag conditions at rigid underlying surfaces according to the Reynolds roughness number Re _s . It is shown that in the case of mobile underlying surfaces, it would be more natural to use the dynamical classification of drag conditions according to the order of magnitude of the ratio ( = /) of the momentum flux toward the waves ( _w) to the viscous momentum flux through the surface ( _w). The relevant estimates of for the main stages of development of the wind waves indicate that the observed values of the drag coefficient of the sea surface correspond to the case of underdeveloped roughness.     

Atmospheric boundary-layer measurements of concentration statistics from isolated and multiple sources

Atmospheric tracer dispersion experiments have been carried out to measure the statistical characteristics (variance, frequency distribution, spectrum) of the concentration downwind of a pair of partly overlapping plumes. By releasing different tracer substances from each source, it was possible to identify the contributions of the two sources at a given measurement point, both separately and jointly, and thus to compare and interpret the joint statistics in terms of those from the individual sources.Statistics for the individual sources agree well with, and support, existing wind tunnel and theoretical results. Nondimensionalization of the data using the mean concentration and the lateral width of the plume as concentration and length scales successfully removes much of the variation due to changes in atmospheric and surface conditions.Measurements of the correlation between the concentration contributions from separated sources are consistent with recent wind tunnel measurements. Entirely new measurements of the frequency distribution of the combined concentration from a pair of sources show that in many situations, high concentrations relative to the mean occur much less frequently than for an isolated source. Generally the extent of the reduction in frequency of occurrence is inversely related to the degree of correlation between concentrations from the two sources.     

Recent trends in vegetation dynamics in the African Sahel and their relationship to climate

Contrary to assertions of widespread irreversible desertification in the African Sahel, a recent increase in seasonal greenness over large areas of the Sahel has been observed, which has been interpreted as a recovery from the great Sahelian droughts. This research investigates temporal and spatial patterns of vegetation greenness and rainfall variability in the African Sahel and their interrelationships based on analyses of Normalized Difference Vegetation Index (NDVI) time series for the period 1982–2003 and gridded satellite rainfall estimates. While rainfall emerges as the dominant causative factor for the increase in vegetation greenness, there is evidence of another causative factor, hypothetically a human-induced change superimposed on the climate trend.     

Planetary boundary-layer turbulence studies from acoustic echo sounder and in-situ measurements

Clear-air plume and wave-like structures are revealed in the atmospheric boundary layer by combined acoustic remote sensing and meteorological tower measurements. The magnitude of turbulent production and dissipation plus properties of velocity and temperature spectra determined from the tower measurements are well correlated with phenomena indicated by the acoustic sounder. Interpretation of either set of records is greatly enhanced by the other. For example, the onset of a sudden burst of turbulent production from the tower measurements may correspond to plume passage or breaking of stable waves recorded by the acoustic echo sounder.     

Changing sea ice conditions and marine transportation activity in Canadian Arctic waters between 1990 and 2012

Declining sea ice area in the Canadian Arctic has gained significant attention with respect to the prospect of increased shipping activities. To investigate relationships between recent declines in sea ice area with Arctic maritime activity, trend and correlation analysis was performed on sea ice area data for total, first-year ice (FYI), and multi-year ice (MYI), and on a comprehensive shipping dataset of observed vessel transits through the Vessel Traffic Reporting Arctic Canada Traffic Zone (NORDREG zone) from 1990 to 2012. Links to surface air temperature (SAT) and the satellite derived melt season length were also investigated. Between 1990 and 2012, statistically significant increases in vessel traffic were observed within the NORDREG zone on monthly and annual time-scales coincident with declines in sea ice area (FYI, MYI, and total ice) during the shipping season and on a monthly basis. Similarly, the NORDREG zone is experiencing increased shoulder season shipping activity, alongside an increasing melt season length and warming surface air temperatures (SAT). Despite these trends, only weak correlations between the variables were identified, although a step increase in shipping activity is apparent following the former summer sea ice extent minimum in 2007. Other non-environmental factors have also likely contributed to the observed increase in Arctic shipping activity within the Canadian Arctic, such as tourism demand, community re-supply needs, and resource exploration trends.     

The relationship between sea surface temperature anomalies and atmospheric circulation in GCM experiments

Several 19-year integrations of the Hamburg version of the ECMWF/T21 general circulation model driven by the monthly mean sea surface temperature (SST) observed in 1970–1988 were examined to study extratropical response of the atmospheric circulation to SST anomalies in the Northern Hemisphere in winter. In the first 19-years run SST anomalies were prescribed globally (GAGO run), and in two others SST monthly variability was limited to extratropical regions (MOGA run) and to tropics (TOGA run), respectively. A canonical correlation analysis (CCA), which select from two time-dependent fields optimally correlated pairs of patterns, was applied to monthly anomalies of SST in the North Alantic and Pacific Oceans and monthly anomalies of sea level pressure and 500 hPa geopotential height in the Northern Hemisphere. In the GAGO run the best correlated atmospheric pattern is global and is characterized by north-south dipole structures of the same polarity in the North Atlantic and the North Pacific sectors. In the MOGA and TOGA experiments the atmospehric response is more local than in the GAGO run with main centers in the North Atlantic and North Pacific, respectively. The extratropical response in the GAGO run is not equal to the sum of the responses in the MOGA and TOGA runs. The artificial meridional SST gradients at 25°–30°N probably influence the results of the MOGA and TOGA runs. The atmopsheric modes found by the CCA were compared with the normal modes of the barotropic vorticity equation linearized about the 500 hPa. winter climate. The normal modes with smallest eigenvalues are similar to the model leading variability modes and canonical patterns of 500 hPa geopotential height. The corresponding eigenvectors of the adjoint operator, which represent an external forcing optimal for exciting normal modes, have a longitudinal structure with maxima in regions characterized by enhanced high frequency baroclinic activity over both oceans.     

Typical influences of moisture on profile measurements in the marine atmospheric surface layer

Under a wide variety of meteorological conditions in a coastal environment, 250 cases were studied to determine the typical influences of moisture in calculating the gradient Richardson number stability and the fluxes of heat, humidity, and momentum with the profile method. Although the stability calculation was altered in some cases up to 95% by the moisture correction, the typical influence in computing the fluxes was found to be less than 10% when the humidity flux was upward and less then 25% when the humidity flux was downward.