Observations Of Nocturnal Drainage Flow In A Shallow Gully

The formation of cold air drainage flows in a shallow gully is studied during CASES-99 (Cooperative Atmosphere-Surface Exchange Study). Fast and slow response wind and temperature measurements were obtained on an instrumented 10-m tower located in the gully and from a network of thermistors and two-dimensional sonic anemometers, situated across the gully. Gully flow formed on clear nights even with significant synoptic flow. Large variations in surface temperature developed within an hour after sunset and in situ cooling was the dominant factor in wind sheltered locations. The depth of the drainage flow and the height of the down-gully wind speed maximum were found to be largest when the external wind speed above the gully flow is less than 2 m s^-1. The shallow drainage current is restricted to a depth of a few metres, and is deepest when the stratification is stronger and the external flow is weaker. During the night the drainage flow breaks down, sometimes on several occasions, due to intermittent turbulence and downward fluxes of heat and momentum. The near surface temperature may increase by 6 ° C in less than 30 min due to the vertical convergence of downward heat flux. The mixing events are related to acceleration of the flow above the gully flow and decreased Richardson number. These warming events also lead to warming of the near surface soil and reduction of the upward soil heat flux. To examine the relative importance of different physical mechanisms that could contribute to the rapid warming, and to characterize the turbulence generated during the intermittent turbulent periods, the sensible heat budget is analyzed and the behaviour of different turbulent parameters is discussed.     

Mixed-layer heat advection and entrainment during the sea breeze

A model is developed to simulate the potential temperature and the height of the mixed layer under advection conditions. It includes analytic expressions for the effects of mixed-layer conditions upwind of the interface between two different surfaces on the development of the mixed layer downwind from the interface. Model performance is evaluated against tethersonde data obtained on two summer days during sea breeze flow in Vancouver, Canada. It is found that the mixed-layer height and temperature over the ocean has a small but noticeable effect on the development of the mixed layer observed 10 km inland from the coast. For these two clear days, the subsidence velocity at the inversion base capping the mixed layer is estimated to be about 30 mm s^–1 from late morning to late afternoon. When the effects of subsidence are included in the model, the mixed-layer height is considerably underpredicted, while the prediction for the mean potential temperature in the mixed layer is considerably improved. Good predictions for both height and temperature can be obtained when values for the heat entrainment ratio,c, 0.44 and 0.68 for these two days respectively for the period from 1000 to 1300 LAT, were used. These values are estimated using an equation including the additional effects on heat entrainment due to the mechanical mixing caused by wind shear at the top of the mixed layer and surface friction. The contribution of wind shear to entrainment was equal to, or greater than, that from buoyant convection resulting from the surface heat flux. Strong wind shear occurred near the top of the mixed layer between the lower level inland flow and the return flow aloft in the sea breeze circulation.Symbols c entrainment parameter for sensible heat - c _p specific heat of air at constant pressure, 1010 J kg^–1 K^–1 - d ₁ the thickness of velocity shear at the mixed-layer top, m - Q _H surface sensible heat flux, W m^–2 - u _m mean mixed-layer wind speed, m s^–1 - u _* friction velocity at the surface, m s^–1 - w subsidence velocity, m s^–1 - W subsidence warming,^oC s^–1 - w _e entrainment velocity, m s^–1 - w _* convection velocity in the mixed layer, m s^–1 - x downwind horizontal distance from the water-land interface, m - y dummy variable forx, m - Z height above the surface, m - Z _i height of capping inversion, m - Z _m mixed-layer depth, i.e.,Z _i–Z_s, m - Z _s height of the surface layer, m - lapse rate of potential temperature aboveZ _i, K m^–1 - potential temperature step atZ _i, K - u _h velocity step change at the mixed-layer top - _m mean mixed-layer potential temperature, K     

A numerical modeling study of the marine boundary layer over the Gulf Stream during cold air advection

A two-dimensional mesoscale model has been developed to simulate the air flow over the Gulf Stream area where typically large gradients in surface temperature exist in the winter. Numerical simulations show that the magnitude and the maximum height of the mesoscale circulation that develops downwind of the Gulf Stream depends on both the initial geostrophic wind and the large-scale moisture. As expected, a highly convective Planetary Boundary Layer (PBL) develops over this area and it was found that the Gulf Stream plays an important role in generating the strong upward heat fluxes causing a farther seaward penetration as cold air advection takes place. Numerical results agree well with the observed surface fluxes of momentum and heat and the mesoscale variation of vertical velocities obtained using Doppler Radars for a typical cold air outbreak. Precipitation pattern predicted by the numerical model is also in agreement with the observations during the Genesis of Atlantic Lows Experiment (GALE).List of Symbols u east-west velocity [m s^–1] - v north-south velocity [m s^–1] - vertical velocity in coordinate [m s^–1] - w vertical velocity inz coordinate [m s^–1] - gq potential temperature [K] - q moisture [kg kg^–1] - scaled pressure [J kg^–1 K^–1] - U _g the east-south component of geostrophic wind [m s^–1] - V _g the north-south component of geostrophic wind [m s^–1] - vertical coordinate following terrain - x east-west spatial coordinate [m] - y north-south spatial coordinate [m] - z vertical spatial coordinate [m] - t time coordinate [s] - g gravity [m² s^–1] - E terrain height [m] - H total height considered in the model [m] - q _s saturated moisture [kg kg^–1] - p pressure [mb] - p ₀₀ reference pressure [mb] - P precipitation [kg m^–2] - vertical lapse rate for potential temperature [K km^–1] - L latent heat of condensation [J kg^–1] - C _p specific heat at constant pressure [J kg^–1 K^–1] - R gas constant for dry air [J kg^–1 K^–1] - R _v gas constant for water vapor [J kg^–1 K^–1] - f Coriolis parameter (2 sin ) [s^–1] - angular velocity of the earth [s^–1] - latitude [^o] - K _H horizontal eddy exchange coefficient [m² s^–1] - t integration time interval [s] - x grid interval distance inx coordinate [m] - y grid interval distance iny coordinate [m] - adjustable coefficient inK _H - subgrid momentum flux [m² s^–2] - subgrid potential temperature flux [m K s^–1] - subgrid moisture flux [m kg kg^–1 s^–1] - u _* friction velocity [m s^–1] - _* subgrid flux temperature [K] - q _* subgrid flux moisture [kg kg^–1] - w _* subgrid convective velocity [m s^–1] - z ₀ surface roughness [m] - L Monin stability length [m] - _s surface potential temperature [K] - k von Karman's constant (0.4) - v air kinematic viscosity coefficient [m² s^–1] - K _M subgrid vertical eddy exchange coefficient for momentum [m² s^–1] - K subgrid vertical eddy exchange coefficient for heat [m² s^–1] - K _q subgrid vertical eddy exchange coefficient for moisture [m² s^–1] - z _i the height of PBL [m] - h _s the height of surface layer [m]     

Fluxes of gases and particles above a deciduous forest in wintertime

Eddy-correlation measurements of the vertical fluxes of ozone, carbon dioxide, fine particles with diameter near 0.1 m, and particulate sulfur, as well as of momentum, heat and water vapor, have been taken above a tall leafless deciduous forest in wintertime. During the experimental period of one week, ozone deposition velocities varied from about 0.1 cm s^–1 at night to more than 0.4 cm s^-1 during the daytime, with the largest variations associated primarily with changes in solar irradiation. Most of the ozone removal took place in the upper canopy. Carbon dioxide fluxes were directed upward due to respiration and exhibited a strong dependence on air temperature and solar heating. The fluxes were approximately zero at air temperatures less than 5 °C and approached 0.8 mg m^–2 s^–1 when temperatures exceeded 15 °C during the daytime. Fine-particle deposition rates were large at times, with deposition velocities near 0.8 cm s^–1 when turbulence levels were high, but fluxes directed upward were found above the canopy when the surface beneath was covered with snow. Diffusional processes seemed to dominate fine-particle transfer across quasilaminar layers and subsequent deposition to the upper canopy. Deposition velocities for particulate sulfur were highly variable and averaged to a value small in magnitude as compared to similar measurements taken previously over a pine forest in summer.     

Sulfur dioxide plume dispersion and subsequent absorption by coniferous forests: Field measurements and model calculations

A Forest SO₂ Absorption Model (ForSAM) was developed to simulate (1) SO₂ plume dispersion from an emission source, (2) subsequent SO₂ absorption by coniferous forests growing downwind from the source. There are three modules: (1) a buoyancy module, (2) a dispersion module, and (3) a foliar absorption module. These modules were used to calculate hourly abovecanopy SO₂ concentrations and in-canopy deposition velocities, as well as daily amounts of SO₂ absorbed by the forest canopy for downwind distances to 42 km. Model performance testing was done with meteorological data (including ambient SO₂ concentrations) collected at various locations downwind from a coal-burning power generator at Grand Lake in central New Brunswick, Canada. Annual SO₂ emissions from this facility amounted to about 30,000 tonnes. Calculated SO₂ concentrations were similar to those obtained in the field. Calculated SO₂ deposition velocities generally agreed with published values.Notation c air parcel cooling parameter (non-dimensional) - E foliar absorption quotient (non-dimensional) - f areal fraction of foliage free from water (non-dimensional) - f _w SO₂ content of air parcel - h height of the surface layer (m) - H height of the convective mixing layer (m) - H _stack stack height (m) - k time level - k _drag coefficient of drag on the air parcel (non-dimensional) - K _z eddy viscosity coefficient for SO₂ (m²·s^–1) - L Monin-Obukhov length scale (m) - L _A single-sided leaf area index (LAI) - n degree-of-sky cloudiness (non-dimensional) - N number of parcels released with every puff (non-dimensional) - PAR photosynthetically active radiation (W m^–2) - Q emission rate (kg s^–2) - r _b diffusive boundary-layer resistance (s m^–1) - r _c canopy resistance (s m^–1) - r _cuticle cuticular resistance (s m^–1) - r _m mesophyllic resistance (s m^–1) - r _s stomatal resistance (s m^–1) - r _exit smokestack exit radius (m) - R normally distributed random variable with mean of zero and variance of t (s) - u _* frictional velocity scale, (m s^–1) - v lateral wind vector (m s^–1) - v _d SO₂ dry deposition velocity (m s^–1) - VCD water vapour deficit (mb) - z _can mean tree height (m) - Z zenith position of the sun (deg) - environmental lapse rate (°C m^–1) - dry adiabatic lapse rate (0.00986°C m^–1) - von Kármán's constant (0.04) - _B vertical velocities initiated by buoyancy (m s^–1) - canopy extinction coefficient (non-dimensional) - ()a denotes ambient conditions - ()can denotes conditions at the top of the forest canopy - ()h denotes conditions at the top of the surface layer - ()H denotes conditions at the top of the mixed layer - ()s denotes conditions at the canopy surface - ()p denotes conditions of the air parcels     

Numerical study of a downslope windstorm in Northwestern Greece

This paper describes the leeside wind storm of 25–26 March 1998, the most intense wind storm of the last decade in Northwestern Greece. This wind storm produced wind gusts of  30 m s^− 1 that resulted in tree uprooting, roof damaging, electric power network disruption and flooding in the lake-side areas of Ioannina city in Northwestern Greece. With the aim to identify the role of Mountain Mitsikeli near the city of Ioannina on the windstorm and to investigate the physical mechanisms responsible for such orographically induced weather events, numerical simulations with MM5 model have been performed. The model results showed that a resolution of 2-km resolution is necessary in order to reproduce the localized character of the wind storm. The analysis revealed that a synergistic combination of the cross-barrier northeasterly flow, the stable layer above the mountain top and the presence of a critical level, led to the intensification of the lee side winds during the studied wind event. Sensitivity experiments with modified topography, further supported the important role of mountain Mitsikeli that stands as an isolated obstacle, on the modification of the wind field during the observed windstorm.     

Observations of the Relation Between Upslope Flows and the Convective Boundary Layer in Steep Terrain

Slope flow mechanisms are crucial for the transport of air pollutants in complex terrain. Previous observations in sloping terrain showed upslope flows filling the entire convective boundary layer (CBL) and reducing air pollution concentrations by venting air pollutants out of the CBL into the free atmosphere. During the Pacific 2001 Air Quality Field Study in the Lower Fraser Valley, British Columbia, Canada, we observed slope flows during weak synoptic winds, clear skies, and strong daytime solar heating. With a Doppler sodar we measured the three wind components at the foot of a slope having an average angle of 19° and a ridge height of 780 m. We operated a scanning lidar system and a tethersonde at a nearby site on the adjacent plain to measure backscatter of particulate matter, temperature, wind speed, wind direction, and specific humidity. Strong daytime upslope flows of up to 6 m s⁻¹ through a depth of up to 500 m occurred in the lower CBL, but with often equally strong and deep return flows in the upper part of the CBL. The mass transport of upslope flow and return flow approximately balanced over a 4-h morning period, suggesting a closed slope-flow circulation within the CBL. These observations showed that air pollutants can remain trapped within a CBL rather than being vented from the CBL into the free atmosphere.     

Influence of Transport over a Mountain Ridge on the Chemical Composition of Marine Aerosols during the ACE-2 Hillcloud Experiment

Aerosol chemical composition and trace gas measurements were made at twolocations on the northeastern peninsula of Tenerife during the ACE-2HILLCLOUD experiment, between 28 June and 23 July 1997. Measurementswere made of coarse (#gt;2.5 m aerodynamic diameter) and fine (#lt; 2.5m) aerosol Cl^–, NO₃ ^–,SO₄ ^2–, non-sea saltSO₄ ^2– (NSSS),CH₃SO₃ ^– (MSA) andNH₄ ⁺, and gas phase dimethylsulphide (DMS), HCl,HNO₃, SO₂, CH₃COOH, HCOOH andNH₃. Size distributions were measured using a cascadeimpactor. Results show that in marine air masses NSSS and MSA wereformed via DMS oxidation, with additional NSSS present in air massescontaining a continental component. Using a Eulerian box model approachfor aerosols transported between upwind and downwind sites, a mean NSSSproduction rate of 4.36 × 10^–4 gm^–3 s^–1 was calculated for daytimeclear sky periods (highest insolation), with values for cloudy periodsduring daytime and nighttime of 3.55 × 10^–4 and2.40 × 10^–4 g m^–3s^–1, respectively. The corresponding rates for MSA were6.23 × 10^–6, 8.49 × 10^–6and 6.95 × 10^–6 g m^–3s^–1, respectively. Molar concentration ratios forMSA/NSSS were 8.7% (1.8–18.2%) and 1.9%(1.3–3.5%) in clean and polluted air masses, respectively.Reactions occurring within clouds appeared to have a greater influenceon rates of MSA production, than of NSSS, while conversely daytime gasphase reactions were more important for NSSS. For MSA, nighttimein-cloud oxidation rates exceeded rates of daytime gas phase productionvia OH oxidation of DMS. NSSS, MSA and ammonium had trimodal sizedistributions, with modes at 0.3, 4.0 and >10.0 m (NSSS andNH₄ ⁺), and 0.3, 1.5 and 4.0 m (MSA). Nosignificant production of other aerosol species was observed, with theexception of ammonium, which was formed at variable rates dependent onneutralisation of the aerosol with ammonia released from spatiallynon-uniform surface sources. Seasalt components were mainly present incoarse particles, although sub-micrometre chloride was also measured.Losses by deposition exceeded calculated expectations for all species,and were highest for the seasalt fraction and nitrate.     

Air-sea bulk transfer coefficients in diabatic conditions

On the basis of recent data for the roughness Reynolds number of the sea surface, and using the Owen-Thomson theory on the transfers of heat and mass between a rough surface and the flow above it, the bulk transfer coefficients of the sea surface have been estimated. For a reference height of 10 m, the neutral-lapse transfer coefficient for water vapor is larger by only a few percent than that for sensible heat. When the wind speed at the 10-m height is u ₁₀>3 m s^–1, the coefficient for sensible heat C _H is larger by about 10% than that for momentum C _D . For u ₁₀<5 m s^–1, however, the value of C _D exceeds the value of C _H , and for u ₁₀=15 m s^–1 it is shown that C _H 0.8C _D . It may be also proposed that 10³ C _D =1.11 to 1.70, 10³ C _E =1.18 to 1.30, and 10³ C _H =1.15 to 1.26 for a range of u ₁₀=4 to 20 m s^–1. A plot of diabatic transfer coefficients versus wind speed is obtained by using a parameter of the sea-air temperature difference. For practical purposes, the coefficients are approximated by empirical formulae.     

The prediction of dust erosion by wind: An interactive model

We have devised a partial differential equation for the prediction of dust concentration in a thin layer near the ground. In this equation, erosion (detachment), transport, deposition and source are parameterised in terms of known quantities. The interaction between a wind prediction model in the boundary layer and this equation affects the evolution of the dust concentration at the top of the surface layer. Numerical integrations are carried out for various values of source strength, ambient wind and particle size. Comparison with available data shows that the results appear very reasonable and that the model should be subjected to further development and testing.Notation (x, y, z, t) space co-ordinates and time (cm,t) - u, v components of horizontal wind speed (cm s^–1) - u _g, v_g components of the geostrophic wind (cm s^–1) - V=(u²+v²)^1/2 (cm s^–1) - (û v)= 1/(h – k) _k ^h(u, v)dz(cm s^–1) - V _* friction velocity (cm s^–1) - z ₀ roughness length (cm) - k ₁ von Karman constant =0.4 - V _d deposition velocity (cm s^–1) - V _g gravitational settling velocity (cm s^–1) - h height of inversion (cm) - k height of surface layer (cm) - potential temperature (°K) - _gr potential temperature at ground (°K) - _K potential temperature at top of surface layer (°K) - P pressure (mb) - P ₀ sfc pressure (mb) - C _p/C_v - (t)= /z lapse rate of potential temperature (°K cm^–1) - A(z) variation of wind with height in transition layer - B(z) variation of wind with height in transition layer - Cd drag coefficient - C _HO transfer coefficient for sensible heat - C dust concentration (g m^–3) - C _K dust concentration at top of surface layer (g m^–3) - D(z) variation with height of dust concentration - u, v, w turbulent fluctuations of the three velocity components (cm s^–1) - A ₁ constant coefficient of proportionality for heat flux =0.2 - Ri Richardson number - g gravitational acceleration =980 cm s^–2 - Re Reynolds number = - D _s thickness of laminar sub-layer (cm) - v molecular kinematic viscosity of air - coefficient of proportionality in source term - dummy variable - t time step (sec) - n time index in numerical equations On sabbatical leave at University of Aberdeen, Department of Engineering, September 1989–February 1990.     

Sea surface winds derived from cloud motion vectors over the tropical Atlantic

The relationship between satellite-derived low-level cloud motion, surface wind and geostrophic wind vectors is examined using GATE data. In the trades, surface wind speeds can be derived from cloud motion vectors by the linear relation: V = 0.62 V _s + 1.9 m s^–1 with a mean scatter of ±1.3 m s^–1. The correlation coefficient between surface and satellite wind speed is 0.25. Considering baroclinicity, i.e., the influence of the thermal wind, the correlation coefficient does not increase, because of the uncertainty of the thermal wind vectors. The ratios of surface to geostrophic wind speed and surface to satellite wind speed are 0.7 and 0.8, respectively, with a statistical uncertainty of ±0.3. Calculations of the ratio of surface to geostrophic wind speed on the basis of the resistance law yield V/V _g = 0.8 ± 0.2, in agreement with experimental results. The mean angle difference between the surface and the satellite wind vectors amounts to - 18 °, taking into account baroclinicity. This value is in good agreement with the mean ageostrophic angle - 25 °.     

Boundary-layer observations over water and arctic sea-ice during on-ice air flow

Observations made on 8 and 9 May 1988 by aircraft and two ships in and around the marginal ice zone of the Fram Strait during on-ice air flow under cloudy and cloud-free conditions are presented.The thermodynamic modification of the air mass moving from the open water to the ice over horizontal distances of 100–300 km is only a few tenth of a degree for temperature and a few tenth of a gram per kilogram for specific humidity. This is due to the small temperature differences between sea and ice surfaces. During the day, the ice surface is even warmer than the sea surface. The stably stratified 200–400 m deep boundary layer is often topped by a moisture inversion leading to downward fluxes of sensible as well as latent heat.The radiation and energy balance at the surface are measured as functions of ice cover, cloud cover and sun elevation angle. The net radiationR _Nis the dominating term of the energy budget. During the day, the difference ofR _Nbetween clear and overcast sky is only a few W/m² over ice, but 100–200 W/m² over water. During the night,R _Nover ice is more sensitive to cloud cover.The kinematic structure is characterized by strong shears of the longitudinal and the transversal wind component. The profile of the latter one shows an inflection point near the top of the boundary layer. Dynamically-driven roll circulations are numerically separated from the mean flow. The secondary flow patterns have wavelengths of about 1 km and contribute substantially to the total variances and covariances.     

The role of gravitophotophoresis for stratospheric and mesospheric particulates

Gravitophotophoresis, a type of photophoresis related to the direction of gravity, is examined in view of its possible importance to some aerosols in stratosphere and mesosphere. particles of various materials from about 1 to 100 m in size show levitation by photophoretic forces under laboratory simulation of irradiation by the sun at air densities of the middle atmosphere. Minimum air densities for levitation are about 2–3 g m^–3 with mineral and metallic powders, about 0.08 g m^–3 with carbonaceous powders. The fraction of rising particles can be about 0.01 to 1%. Velocities of ascent are in the range of 0.001 to 0.01 m s^–1 at a pressure of several mbar. The magnitude and the mechanical character of the force of gravitophotophoresis can be explained if it is identified with a radiometer force caused by a difference of the accommodation coefficient on the surface. It is suggested that gravitophotophoresis can be important to the residence time and the maximum altitude of carbonaceous and mineral particles, such as volcanic ash or products of meteorite impact, and to the presence of microorgnisms in the middle atmosphere.     

Observed generation of an atmospheric gravity wave by shear instability in the mean flow of the planetary boundary layer

Observations of a single boundary-layer event — the generation of an atmospheric gravity wave by an unstable shear flow at Haswell, Colorado on November 12, 1971 — are briefly described and discussed. The observations were made using: (a) an acoustic echo sounder, (b) anemometers mounted at two fixed levels on a 150-m tower, (c) an anemometer and a thermometer mounted on a movable carriage on the tower, and (d) a microbarograph array, including one microbarograph mounted atop the tower. The wave phase velocity (–3.5–4.0 m s^–1) was found to equal the wind velocity in the middle of the shear flow, as assumed by other authors. The wave-associated vertical fluxes of momentum and energy measured just above the wave critical layer were estimated to be –5 dyn cm^–2 and –800 erg cm^–2 s^–1, respectively. These are large values. The annual average vertical flux of momentum at temperate and high latitudes is –0.25 dyn cm^–2, while the average kinetic energy dissipation rate in a unit column of atmosphere is –5 × 10³ erg cm^–2 s^–1. If the region of wave generation was itself propagating horizontally, its propagation velocity was large compared with the horizontal phase speed of the small-scale waves generated. Wave generation appeared to occur over an area large compared with the size of the microbarograph array (i.e., 2 km).     

Mesoscale nocturnal jetlike winds within the planetary boundary layer over a flat,open coast

Mesoscale nocturnal jetlike winds have been observed over a flat, open coast. They occur within the planetary boundary layer between 100 and 600 m. At times the wind shear may reach 15 m s^-1 per 100 m. Unlike the common low-level jet that occurs most often at the top of the nocturnal inversion and only with a wind from the southerly quadrant, this second kind of jet exists between nocturnal ground-based inversion layers formed by the cool pool, or mesohigh, and the elevated mesoscale inversion layer over the coast. It occurs mostly when light % MathType!MTEF!2!1!+-% feaafiart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9% vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x% fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaaiikaiabgs% MiJkaaiwdacqGHsislcaaI2aGaaeyBaiaabccacaqGZbWaaWbaaSqa% beaacqGHsislcaaIXaaaaOGaaiykaaaa!3FCF!\[( \leqslant 5 - 6{\text{m s}}^{ - 1} )\] geostrophic winds blow from land to sea and when the air temperature over adjacent seas is more than 5 °C warmer than that over the coast. This phenomenon may be explained by combined Venturi and gravity-wind effects existing in a region from just above the area a few kilometres offshore to 100–600 m in height approximately 40–50 km inland because this region is sandwiched between the aforementioned two inversion layers.     

On the stable stratification of the nocturnal lower troposphere inferred from lidar observations over Pune,India

The nocturnal structure of the lower troposphere is studied using aerosol profile data (50–2800 m AGL) obtained with a bistatic, continuous wave, Argon ion lidar system during October 1986–September 1989 at Pune (18°32 N, 73°51 E, 559m AMSL), India. The top of the nocturnal groundbased inversion is taken as the height above ground where the negative vertical gradient in aerosol concentration first reaches a maximum. During the post-sunset period over this station, this height is as low as 160m and frequently lies around 550m. Greater heights are observed in pre-monsoon months and smaller ones during the southwest monsoon season. Positive vertical gradients in aerosol concentration, indicative of stable/elevated layers, appear frequently around 750m. Temporal variations of aerosol concentration gradients in two adjacent air layers, 920–1000m and 100–1100m, provide evidence that stability increases downward in the early night hours.     

The UHF wind profiler at Vienna airport – data quality control and comparisons to rawinsonde data

Summary A 1290MHz wind profiler (Radian Lap-3000), at present one of three operational wind profilers in Austria, is operated at Vienna airport. In spite of quality assurance procedures as consensus averaging included in the data evaluation process from profiler raw data, some spurious peaks of wind speed and unrealistic changes of the wind vector in time or height occur in the wind measurements. This is especially true for sampling intervals of only 5 minutes which are used to resolve the temporal evolution of summer thunderstorms and frontal passages. Averaging periods of only a few minutes are rather the lower limit apt for wind profiler observations and result in a low data availability of 28%, whereas about 55% of data (relative to the maximum height range according to the parameter setting) are available for 10 to 30 minutes profiles.Approaches to a posteriori quality control using checks for automatic error detection are proposed and tested on a one and a half year data-set: Flagging data when the three-dimensional wind divergence exceeds a predefined limit (0.5s^–1) is in most cases successful in combination with thresholds for wind speed (2 times the median of the daily data-set) or wind shear (0.2s^–1).The wind profiler data is compared to wind profiles from the next radiosonde station where soundings are launched 4 times a day at Hohe Warte, approx. 20km northwest, at the hill-side of the Viennese Woods. Deviations of about 1m s^–1 in wind speed are found between the observations of the two systems. Differences between the wind profiles within the boundary layer can be explained by local differences in the wind regime observed at the airport and the radiosounding – blocking effects of the Viennese Woods during south-easterly flow. Comparing the profiler data to radiosoundings on a monthly basis gives a tool to monitor the profiler performance.     

Coastal CCN measurements at Mace Head with enhanced concentrations in strong winds

Surface measurements of cloud condensation nuclei (CCN) number concentration (cm⁻³) are presented for unmodified marine air and for polluted air at Mace Head, for the years 1994 and 1995. The CCN number concentration active at 0.5% supersaturation is found to be approximately log-normal for marine and polluted air at the site. Values of geometric mean, median and arithmetic mean of CCN number concentration (cm⁻³) for marine air are in the range 124–135, 140–150 and 130–157 for the two years of data. Analysis of CCN number concentration for high wind speed, U, up to 20 m s⁻¹ show enhanced CCN production for U in excess of about 10–12 m s⁻¹. Approximately 7% increase in CCN per 1 m s⁻¹ increase in wind speed is found, up to 17 m s⁻¹. A relationship of the form log₁₀CCN=a+bU is obtained for the periods March 1994 and January, February 1995 for marine air yielding values a of 1.70; 1.90 and b of 0.035 for both periods.     

Meteorological results of MONSOON-88 expedition (pre-monsoon period)

Mean atmospheric circulation, moisture budget and net heat exchange were studied during a pre-monsoon period (18th March to 3rd May, 1988), making use of the data collected on board Akademik Korolev in the central equatorial and southern Arabian Sea region. The net heat exchange (R _n ) is found to be about 20 W m^–2 for a small area (0–4° N; 55–60° E), 50% less than the dimatological value. The mean value of net radiation (140 W m^–2) is less than the climatological value, which was due to higher cloud amount. The higher SST enhanced both the latent and sensible heat fluxes.The mean atmospheric circulation obtained from the upper air data is quite convincing. The mean exchange coefficient (C _e ) estimated from the moisture budget is about 1.0 × 10^–3 for a wind speed of 4 m s^–1. This value is slightly lower than that obtained by the usual methods.National Institute of Oceanography, RC, 52-Kirlampudi layout, Visakhapatnam — 530 023.India Meteorological Department, Gauhati.     

Surface-layer scintillation measurements of daytime sensible heat and momentum fluxes

Line-averaged measurements of the structure parameter of refractive index (C _n ² ) were made using a semiconductor laser diode scintillometer above two markedly different surfaces during hours of positive net radiation. The underlying vegetation comprised in the first instance a horizontally homogeneous, pasture sward well-supplied with water, and in the second experiment, a sparse thyme canopy in a semi-arid environment. Atmospheric stability ranged between near neutral and strongly unstable (–20). The temperature structure parameterC _T ² computed from the optical measurements over four decades from 0.001 to 2 K² m^–2/3 agreed to within 5% of those determined from temperature spectra in the inertial sub-range of frequencies. Spectra were obtained from a single fine thermocouple sensor positioned near the midway position of the 100m optical path and at the beam propagation height (1.5m).With the inclusion of cup anemometer measurements, rule-of-thumb assumptions about surface roughness, and Monin-Obukhov similarity theory, path-averaged optical scintillations allow calculation of surface fluxes of sensible heat and momentum via a simple iterative procedure. Excellent agreement was obtained between these fluxes and those measured directly by eddy correlation. For sensible heat, agreement was on average close to perfect over a measured range of 0 to 500 W m^–2 with a residual standard deviation of 30 W m^–2. Friction velocities agreed within 2% over the range 0–0.9 m s^–1 (residual standard deviation of 0.06 m s^–1). The results markedly increase the range of validation obtained in previous field experiments. The potential of this scintillation technique and its theoretical foundation are briefly discussed.