An experimental investigation of Arctic haze at Alert,N.W.T., March 1985

Abstract

Arctic haze has been attributed to industrial pollution released at mid‐latitudes. Our current understanding has been pieced together from routine meteorological data, ground‐based air chemistry observations and limited aircraft measurements. This study investigates the relationship between synoptic boundary‐layer meteorology and the composition of the near‐surface atmosphere during the polar sunrise at Alert, N.W.T. A secondary objective is to characterize the influence of local activity on the atmospheric composition at a site for a new baseline monitoring station and at a location where aerosol chemistry and grab‐flask samples for CO₂ have been made for many years. Detailed measurements of the vertical distribution of aerosols were obtained from an upward‐looking lidar to complement the ground‐based measurements. Meteorological profiles of the near‐surface boundary layer were obtained from both free‐flying and tethered balloons. Near‐surface measurements were made of aerosol physical and chemical properties, O₃, NO₂, NO/NO_x, Peroxyacetylnitrate (PAN) and hydrocarbons.

The study period was characterized by prolonged periods with strong surface inversions, which were broken up occasionally by intrusions of cold air into the warmer air aloft. Lidar observations showed that ice crystals often accompanied aerosols and were responsible for reducing visibility below 30–40 km. There was a strong correlation between aerosol mass in the diameter size range 0.15 to 1.5 μm and total SO₄ ⁼ . PAN found at concentrations of about 200 ppt(v) was the main carrier of atmospheric nitrogen. Aerosol trace elements were divisible into anthropogenic soil, mixed soil/anthropogenic sea salt and halogens. Vertical transport in the surface boundary layer, as regulated by the strength of the surface radiation inversion, may play an important role in influencing the chemical composition of the air at the ground. The location of the new baseline monitoring laboratory was found to be generally windier and warmer than the lower altitude weather station, and the influence of local activity was found to be minimal.     

A numerical investigation of a moderate coastal storm with intense precipitation

Abstract

In this study, the development of a moderate coastal storm with intense precipitation that occurred during 12–14 February 1993 is examined using a high‐resolution version of the Canadian Regional Finite‐element (RFE) model with more realistic physical representations. It is shown that the improved RFE model predicts well the coastal cyclogenesis events and also the distribution and intensity of heavy mixed precipitation (rain and snow) associated with the storm. It is found that the cyclogenesis takes place in response to the low‐level inshore advection of high‐θ_e air from the maritime boundary layer, and the approach of a mid‐level shortwave trough with a warm pool above that is previously associated with a decaying cyclone upstream. More rapid deepening of the cyclone ensues as intense precipitation falls along the warm and cold fronts near the cyclone centre.

Diagnosis of the control and sensitivity simulations reveals that the low‐level inshore warm advection and the propagation of the stratospheric warm pool contribute more significantly to the surface pressure falls during the incipient stage, whereas the mid‐level shortwave trough plays an important role in the cyclogenesis at later stages. Overall, latent heat release accounts for about 50% of the cyclone's total deepening, in agreement with the presence of a moderate baroclinic environment and the generation of intense precipitation.

The diabatic and kinematic structures near the rain‐snow boundary are examined to gain insight into the influence of melting snow on the cyclogenesis. It is shown that the improved RFE model reproduces well the rain‐snow boundary structures as previously observed. Moreover, a thermally indirect circulation (perturbation) can be seen in the vicinity of the rain‐snow boundary. It is found, however, that melting of snow tends to produce a weak negative or negligible impact on the cyclogenesis, as opposed to previous hypotheses.     

Marginal mixing theories

Abstract

Mixing near the sloping boundaries of oceans or lakes may be a significant mechanism of diapycnal transport. The basic physics of this is reviewed, with emphasis on the reduction of the effectiveness of the process due to both reduced stratification and the restratifying secondary circulation driven by buoyancy forces. This re stratification is shown to reduce the effectiveness of intermittent mixing events as well as steady mixing. It is argued that for boundary mixing to be effective in the abyssal ocean it must extend sufficiently far from the boundary that the stratification can be maintained; this may be true for breaking bottom‐reflected internal waves. The alongslope flow implied by steady‐state boundary mixing theories is downwelling‐favourable and has a magnitude related to the thickness and other properties of the boundary layer. Mixing near a boundary may thus tend to drive a downwelling‐favourable mean circulation in the interior. If the interior circulation is imposed by other forces, the bottom boundary layer may evolve to a steady state if the interior flow is downwelling‐favourable, but if it is upwelling‐favourable initially a steady state seems unlikely and the downwelling‐favourable alongslope flow induced by the boundary mixing will tend to diffuse slowly into the interior. The nature of the solution in all these cases is sensitive to the Burger number, N² sin² θ/f², where θ is the bottom slope, and to the eddy Prandtl number.     

Boundary‐layer influence on the development of a mesoscale rainstorm

Abstract

A major convective rainstorm slowly developed in the largely barotropic warm sector of an open‐wave cyclone. Meso‐beta‐scale boundary‐layer conditions played a significant role in the location of storm development, and provided favourable thermodynamic gradients and persistently convergent flow in preferred areas. Minimal storm movement and a strong association between the surface divergence field and cloud evolution permitted the estimation of that fraction of the moisture fed into the storm from the boundary layer that was returned to the ground as rain.     

A comparison of satellite winds and surface buoy winds in the northeast Pacific: Research note

Abstract

Cloud‐motion winds measured from organized and disorganized cumulus cloud fields are compared with winds measured at collocated buoys in the northeast Pacific Ocean. Findings suggest that an automated tracking algorithm using GOES satellite imagery can measure cloud‐level winds at these latitudes. Comparisons with buoy wind measurements show that the influence of boundary‐layer stability should be included in estimates of surface winds from cloud‐motion data.     

On the sensitivity of predictions of maritime cyclogenesis to convective precipitation and sea temperature

Abstract

The influences of surface fluxes and convective precipitation are investigated for two 36‐h periods of cyclogenesis over the northeastern Pacific Ocean. Three methods are tested of specifying the fraction of moisture supply that produces convective precipitation in a modified form of Kuo's (1974) parametrization scheme using an 8‐level primitive equations model.

When convection is included, precipitation amounts are greater and the cyclone deepening is better predicted than when convection is not included. Predicted cyclogenesis is very sensitive to sea temperature. As the low moves over warmer water, the effect of sensible heating is to increase the moisture convergence in the atmospheric boundary layer. This increases the precipitation rates and accelerates deepening. It is concluded that the CISK mechanism plays an important role in extratropical cyclogenesis.     

Prairie agroclimate boundary‐layer model: A simulation of the atmosphere/crop‐soil interface

Abstract

This paper describes a 1‐D agroclimatic model of the atmosphere/crop‐soil interface. Vertical profiles of wind, potential temperature and water vapour are constructed twice daily for the overnight‐low and maximum temperature times by combining 1200 and 0000 UTC upper‐air standard‐level grid‐point data with climatological observations. The vertical structure of the atmospheric boundary layer has a surface constant‐flux layer that is usually topped by a mixed layer by day but not at night. The crop‐soil boundary layer consists of a shallow top‐zone and a growing root‐zone. Vegetation cover and root depth depend upon crop type and phenological stage. Water‐balance accounting tracks the moisture contents of both the top‐ and root‐zones. Evapotranspiration or the vertical flux of water vapour in the atmospheric boundary layer is tied to the evolution of the crop‐soil boundary layer.

The model was calibrated using field data from the Regional Evaporation Study's primary site in an agricultural area of central Saskatchewan. The evolution of 1991's wheat‐soil boundary layer from the crop's heading to ripe stages was then successfully simulated at two additional sites in the same geographical area.     

Atmospheric moisture budget estimates of regional evapotranspiration from RES‐91

Abstract

Diurnal changes in the local atmospheric moisture budget over the Canadian Prairies are computed using sequential radiosonde soundings from the 1991 Regional Evaporation Study (RES‐91). Previous attempts to estimate evapotranspiration with radiosonde data have used either similarity theory or a moisture budget, but have been confined to the boundary layer in either case. These studies, as well as semi‐empiric operational techniques which use surface‐based data, exclude the effects of moisture advection and energy exchanges between the boundary layer and the free atmosphere, assuming negligible effects on evapotranspiration. The moisture budget method adopted here includes horizontal advection explicitly, and treats vertical fluxes implicitly through a total tropospheric moisture budget.

Comparison of the evapotranspiration estimates with those of other techniques are positive only when results are averaged over several days to weeks. While the advection estimates are a major source of error for the “daily” estimates in this particular study, it is shown that neither advection nor moisture flux through the boundary layer can be ignored in estimating daily evapotranspiration, regardless of the technique used. The results also suggest that evapotranspiration is more variable on a daily basis than other techniques have indicated. With an improved synoptic database now available for advection estimates, the moisture budget technique may provide an excellent ground‐truth method for fine‐tuning techniques for remote sensing of evapotranspiration, and could lead to improved parametrization schemes for both NWP models and GCMs.     

The life cycle of the intense IOP‐14 storm during CASPII. Part I: Analysis and simulations

Abstract

In this study, a 5‐day life‐cycle of the IOP‐14 storm during CASP II is examined using conventional observations and numerical simulations with a mesoscale version of the Canadian Regional Finite‐Element (RFE) model. Observational analysis reveals that the IOP‐14 storm forms from a lee trough, occurring along a strong baroclinic zone with an intense frontogenetic deformation, that interacts with an upper‐level travelling short‐wave trough across the Canadian Rockies. Then the storm experiences a slow, but nearly steady, growth while traversing the North American continent. It deepens explosively as it moves into the Atlantic Ocean. It appears that i) the enhanced large‐scale baroclinicity due to land‐sea temperature contrasts, ii) the tremendous latent heat release due to the transport of high‐θ_e air from the marine boundary layer, Hi) the decrease of surface drag and iv) the favourable westward tilt of the low with an amplifying trough all contribute to the explosive deepening of the storm.

Two consecutive simulations covering a total of 102 h during the storm development are carried out with a grid size of 50 km. The RFE model reproduces very well the formation of the surface low on the lee side of the Rockies, the track and deepening rates, the explosive development and decay of the storm, and various mesoscale phenomena (e.g., a “bent‐back” warm front, a “T‐bone” thermal pattern, a cold frontal “fracture”, an upper‐level “eye” and warm‐core structures), as verified by conventional observations, satellite imagery, flight‐level and dropsonde data from a research aircraft. It is found from potential vorticity (PV) analysis that the storm reaches its peak intensity as the upper‐level dry PV anomaly, the low‐level moist PV anomaly and surface thermal warmth are vertically superposed. PV inversions reveal that these anomalies contribute about 60%, 30% and 10%, respectively, to the 900‐hPa negative height perturbation. It is shown that the warm‐core structure near the cyclone centre is produced by advection of warmer air ahead of the cold front, rather than by adiabatic warming associated with subsidence.     

Airborne surveys of the atmospheric boundary layer above the marginal ice zone on the Newfoundland shelf

Abstract

Airborne measurements in the atmospheric boundary layer (ABL) above the marginal ice zone (MIZ) on the Newfoundland Shelf reveal strong lateral variations in mean wind, temperature and the vertical fluxes of heat and momentum under conditions of cold, off‐ice wind. Flux measurements in (and near) the surface layer indicate that the neutral 10‐m drag coefficient depends on ice concentration, ranging from 2 × 10^‐3 at 10% coverage to 5 × 10^‐3 at 90%. Furthermore, cross‐ice‐edge transects consistently show increasing wind speed, temperature and heat flux in the off‐ice direction, but the momentum flux may either increase or decrease, depending on the relative importance of surface buoyancy flux and roughness. For the conditions encountered in this experiment, it appears surface wave maturity does not have a significant influence on the drag coefficient in fetch‐limited regimes near the ice edge.     

The position of the lowest levels in the boundary layer of atmospheric circulation models

Abstract

The error associated with the position of the lowest wind level in atmospheric boundary‐layer modelling is studied in connection with vertical resolutions typical of parametrization schemes for atmospheric circulation models. The test case is the neutrally stratified steady‐state boundary layer. Finite‐difference and finite‐element schemes of two types are used: in one case the lowest wind level is centred with respect to the surface and the lowest internal level where the shear stress is calculated, in the other case the lowest wind level is set very close to the surface (5 m). It is found that schemes of the latter type underestimate the friction force at the lowest level and consequently overestimate the wind and the surface stress. This error is largest at low resolution since it is due to the uncentring of the lowest wind level with respect to the stress levels. The error in schemes of the former type is different, and is associated with the determination of the surface stress from a wind at a height that may exceed the extent of the surface layer. For all neutral cases, this error can be made small by adding a corrective term to the traditional logarithmic formulation. This paper shows that considerably more error is generated by uncentred differencing than by deepening the surface layer.     

On the similarity of the characteristics of turbulence in an unstable boundary layer

The statistics of turbulence, such as the standard deviation of fluctuating velocities, in an unstable atmospheric boundary layer are assumed to be characterized by the combination of three specific lengths, Monin-Obukhov length L, observation height z and the height of mixing layer h. Unlike Monin-Obukhov similarity, even near the ground the effect of h is taken into account. According to observation, the length scale of the vertical velocity is proportional to z at least near the ground, but the lateral component depends mostly on h alone. The length scale of the longitudinal component depends on z and h.     

Etude de la couche limite convective sahelienne en presence de brumes seches (Expérience ECLATS)

The ECLATS experiment was conducted in order to investigate the influence of radiative processes on the dynamics of the atmospheric boundary layer during its diurnal evolution. This experiment was carried out over Niger, near Niamey, by measuring continuously the energy balance at ground level and by using an instrumented aircraft for turbulence, radiative fluxes and aerosol measurements in the boundary layer during dusty conditions (brumes sèches). This paper is restricted to an analysis of the turbulent structure in the homogeneous and stationary convective boundary layer. The turbulence moments for kinetic energy and the spectral characteristics of the vertical velocity are discussed. These results are compared with a set of data obtained for clear convective boundary layers. The differences observed are quite important and seem, at least in part, due to radiative processes (infrared radiative divergence in the surface layer and absorption of solar radiation in the boundary layer).     

Effect of Roughness on Surface Boundary Conditions for Large-Eddy Simulation

An important parameterization in large-eddy simulations (LESs) of high- Reynolds-number boundary layers, such as the atmospheric boundary layer, is the specification of the surface boundary condition. Typical boundary conditions compute the fluctuating surface shear stress as a function of the resolved (filtered) velocity at the lowest grid points based on similarity theory. However, these approaches are questionable because they use instantaneous (filtered) variables, while similarity theory is only valid for mean quantities. Three of these formulations are implemented in simulations of a neutral atmospheric boundary layer with different aerodynamic surface roughness. Our results show unrealistic influence of surface roughness on the mean profile, variance and spectra of the resolved velocity near the ground, in contradiction of similarity theory. In addition to similarity-based surface boundary conditions, a recent model developed from an a priori experimental study is tested and it is shown to yield more realistic independence of the results to changes in surface roughness. The optimum value of the model parameter found in our simulations matches well the value reported in the a priori wind-tunnel study.     

An observational‐data based evapotranspiration function for general circulation models

Abstract

In this paper Nappo’ s (1975) formulation of the moisture availability function was used to derive the ß function of the form employed in the evapotranspiration calculations of various GCMs (Carson, 1981). An inverse calculation using the planetary boundary‐layer parameterizations of the GLAS general circulation model was made to derive this function. For this purpose two ground temperatures, namely those of saturated and naturally dry ground, were prognostically carried in a 47‐day integration with the model. The form of the calculated ß function is different from those reviewed by Carson. An example of global evapotranspiration obtained with the derived ß function is shown. Two separate five‐day simulations, one using ß as derived here and the other using Nappo's (1975) M function as a ß function, are compared. Large differences in the calculated evapotranspiration occur in dry regions.     

南京及周边黑碳气溶胶对臭氧影响的观测分析和数值模拟

利用南京地面站点2016-2017年黑碳气溶胶(Black Carbon,BC)和臭氧(O3)逐小时观测资料,对比分析了不同季节BC与近地面O3的关系.结果 表明,高BC(高于平均值)影响下的O3质量浓度值明显比低BC(低于平均值)影响下的O3质量浓度值低,这种抑制作用在秋冬季明显高于春夏季,且BC与O3的负相关性在秋...     

Properties of unstable waves in the lower St Lawrence Estuary

Abstract

The lower St Lawrence Estuary is an interesting case amongst estuaries in that it is wide enough to accommodate the development of mesoscale unstable waves and eddies. These features are generated by the runoff‐driven jet along this body's south shore. We present data yielding estimates of the length, time and velocity scales of these unstable disturbances. To relate these quantities to the dynamics we employ a 2‐layer quasigeos‐trophic instability model featuring realistic lateral shear. All model runs show short time and length scales, e‐folding periods of less than 10 days and wavelengths less than 50 km.     

A vertical profile of Ozone concentration in the atmosphericboundary layer over central Taiwan

Summary  In the central region of Taiwan, ozone episodes occur most often during autumn. Two field experiments were conducted during the autumns of 1998 and 1999 to analyze the vertical profile of the boundary layer and determine its effects on ozone concentration over the region. The vertical virtual potential temperature and wind profiles were derived from tethersonde data. The NO_x, NMHC and O₃ concentration vertical profiles were monitored up to a height of 500 meters using black-covered Teflon tedler sampling bags. During the experimental periods, nighttime terrestrial long wave radiation could cause the inversion height to reach 500 meters by the following morning. It was shown that these types of synoptic structures suppress the vertical diffusion of NO_x, NMHC and O₃. During the daytime, measurements indicate that pollutants were well mixed in the upper portion of the mixing layer. At night, the ground level ozone concentration was on the decrease but increased with altitude to a height of 500 m. The NO_x decreased with altitude whereas the NMHC showed no significant variations. Received April 13, 2000 Revised July 24, 2000     

Aircraft-Based Measurements Of Turbulence Structures In The Katabatic Flow Over Greenland

Turbulence structures in the katabatic flow in the stable boundary layer (SBL) over the ice sheet are studied for two case studies with high wind speeds during the aircraft-based experiment KABEG (Katabatic wind and boundary layer front experiment around Greenland) in the area of southern Greenland. The aircraft data allow the direct determination of turbulence structures in the katabatic flow. For the first time, this allows the study of the turbulence structure in the katabatic wind system over the whole boundary layer and over a horizontal scale of 80 km.The katabatic flow is associated with a low-level jet (LLJ), with maximum wind speeds up to 25 m s^-1. Turbulent kinetic energy (TKE) and the magnitude of the turbulent fluxes show a strong decrease below the LLJ. Sensible heat fluxes at the lowest level have values down to -25 W m^-2. Latent heat fluxes are small in general, but evaporation values of up to +13 W m^-2 are also measured. Turbulence spectra show a well-defined inertial subrange and a clear spectral gap around 250-m wavelength. While turbulence intensity decreases monotonously with height above the LLJ for the upper part of the slope, high spectral intensities are also present at upper levels close to the ice edge. Normalized fluxes and variances generally follow power-law profiles in the SBL.Terms of the TKE budget are computed from the aircraft data. The TKE destruction by the negative buoyancy is found to be very small, and the dissipation rate exceeds the dynamical production.