Latitudinal variation of tropospheric temperature lapse rate

Summary The latitudinal variation of tropospheric temperature lapse rate is examined on the basis of recently published data for the northern hemisphere. In January, the lapse rate systematically decreases from 6.4°C/km. for the equator to 3.6°C/km. for 70°N; in July, the lapse rate is nearly constant in respect of latitude between the equator and 50°N, but decreases from the latter poleward. The average January–July lapse rate decreases systematically from 6.3°C/km. for the equator to 4.7°C/km. for 70°N. This variation is closely described by the equation =6.25–2 sin⁴, where is the lapse rate, in °C/km, for latitude . For the north pole, the equation gives a lapse rate of 4.25°C/km.

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Zusammenfassung Die Änderung des vertikalen Gradienten der Troposphärentemperatur mit der geographischen Breite wird auf Grund neuen Zahlenmaterials für die Nordhemisphäre untersucht. Im Januar nimmt der Temperaturgradient allmählich von 6.4°C/km am Äquator bis zu 3.6°C/km in 70°N Breite ab; im Juli ist der Temperaturgradient in den Breiten zwischen dem Äquator und 50°N Breite fast gleich und nimmt von hier aus polwärts ab. Der mittlere Temperaturgradient für Januar–Juli nimmt allmählich von 6.3°C/km am Äquator bis auf 4.7°C/km in 70°N Breite ab. Die Breitenabhängigkeit läßt sich gut durch die Gleichung =6.25–2 sin⁴ darstellen, in der der Temperaturgradient in °C/km in der Breite ist. Für den Nordpol ergibt die Gleichung einen Temperaturgradienten von 4.25°C/km.

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Résumé La variation latitudinale du gradient thermique vertical de la température troposphérique est examinée en se basant sur des données publiées récemment d'une valeur de 6.4°C/km pour l'équateur à 3.6°C/km pour 70°N; en juillet le gradient est presque constant pour les latitudes situées entre l'équateur et 50°N, mais il diminue d'ici vers le pôle. Le gradient moyen janvier/juillet diminue systématiquement de 6.3°C/km pour l'équateur à 4.7°C/km pour 70°N. Cette variation peut être représentée par l'équation =6.25–2 sin⁴ où est le gradient en °C/km pour la latitude . Pour le pôle nord, l'équation donne un gradient de 4.25°C/km.

     

Transport-Dissipation Analytical Solutions to the E-∈Turbulence Model and their Role in Predictions of the Neutral ABL

E- turbulence model predictions of the neutralatmospheric boundary layer (NABL) are reinvestigated to determine thecause for turbulence overpredictions found in previous applications. Analytical solutions to the coupled E and equations for the case of steady balance between transport and dissipation terms, the dominant balance just below the NABL top, are derived. It is found that analytical turbulence profiles laminarizeat a finite height only for values of closure parameter ratio c ₂ /_e equal toor slightly greater than one, with laminarization as z for greater . The point = 2 is additionally foundthat where analytical turbulent length scale (l) profilesmade a transition from ones ofdecreasing ( < 2) to increasing ( > 2)values with height. Numerically predicted profiles near the NABL topare consistent with analytical findings. The height-increasingvalues of l predicted throughout the NABL with standard values ofclosure parameters thus appear a consequence of 2.5(> 2), implied by these values (c ₂ = 1.92, _{= 1.3}, _{e = 1}). Comparison of numericalpredictions with DNS data shows that turbulence overpredictions obtained with standard-valued parameters are rectifiedby resetting and _e to 1.1 and 1.6, respectively, giving, with c ₂ = 1.92, 1.3, and laminarization of the NABL's cappingtransport-dissipation region at a finite height.     

Convective Profile Constants Revisited

This paper examines the interpolation betweenBusinger–Dyer (Kansas-type) formulae,_u = (1 -1 6 )^-1/4 and_t = (1 - 16 )^-1/2, and free convection forms. Based on matching constraints, the constants, a_u and a_t, in the convective flux-gradient relations, _u = (1 - a_u )^-1/3 and _t = (1 - a_t )^-1/3, are determined. It isshown that a_u and a_t cannot be completely independent if convective forms are blended with theKansas formulae. In other words, these relationships already carryinformation about a_u and a_t. This follows because the Kansas relations cover a wide stability range (up to = - 2), which includes a lower part of the convective sublayer (about 0.1 < - < 2). Thus, there is a subrange where both Kansas and convective formulae are valid. Matching Kansas formulae and free convection relations within thesubrange 0.1 < - < 2 and independently smoothing ofthe blending function are used to determine a_u and a_t. The values a_u = 10 for velocity and a_t = 34for scalars (temperature and humidity) give a good fit. This new approacheliminates the need for additional independent model constants and yields a`smooth' blending between Kansas and free-convection profileforms in the COARE bulk algorithm.     

La forme rotationnelle des équations de la dynamique atmosphérique et les invariants du mouvement de l'air

Résumé Après avoir rappelé quelques notions de calcul tensoriel et les équations de la dynamique atmosphérique en coordonnées généralisées, l'auteur établit l'équation fondamentale de la rotationnelle absolue de l'air. Diverses formes de cette équation sont mentionnées. La première de ces formes est celle d'un bilan qui permet de définir le transport et le taux de production de la rotationnelle absolue de l'air. De la forme particulière de ce transport, il résulte que ce bilan se réduit à un bilan dans un espace à deux dimensions. Deux exemples illustrent les formules générales, le premier, en coordonnées sphériques (, ,r), le second, en coordonnées (, , ) où est un scalaire quelconque (pression atmosphérique, température potentielle, ...). Une autre forme de l'équation fondamentale est celle qui donne le taux d'accroissement individuel d'une composante de la rotationnelle absolue. Cette forme conduit à l'équation d'Ertel. En terminant, l'auteur généralise l'invariant d'Ertel-Rossby.

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Summary Having recalled some results of the tensor analysis and the equations of atmospheric motion in general coordinates, the writer establishes the fundamental equation of the absolute vorticity. Different forms of this equation are mentioned. One of these is the equation of balance from which it is possible to deduce the definition of transport and production of vorticity. It is shown that the equation of balance of the absolute vorticity may be reduced to an equation of balance in two-dimensional space. Two examples illustrate the general equations and formulas: the first example, in spherical coordinates (, ,r), the second one in the coordinates (, , ) where is an arbitrary single-valued scalar quantity (pressure, potentiel temperature, ...). Another form of the absolute vorticity equation expresses the individual change of an arbitrary component of the absolute vorticity. This form leads toErtel's equation. Finally, the writer generalizes theErtel-Rossby invariant.

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Zusammenfassung Nach einer Rekapitulation einiger Resultate der Tensorrechnung und der atmosphärischen Bewegungsgleichungen in generalisierten Koordinaten wird die absolute Wirbelgleichung aufgestellt und es werden verschiedene Formen dieser Gleichung erwähnt. Eine derselben ist die Bilanzformel, aus der die Definition des Wirbeltransports und der Anteil der Wirbelbildung abgeleitet werden können. Es wird gezeigt, daß die Bilanzgleichung des absoluten Wirbels auf eine Bilanzgleichung im zweidimensionalen Raum reduziert werden kann. Die allgemeinen Formeln werden durch zwei Beispiele erläutert: das erste in Kugelkoordinaten (, ,r), das zweite in den Koordinaten (, , ), wo eine beliebige skalare Größe (luftdruck, potentielle Temperatur usw.) darstellt. Eine andere Form der absoluten Wirbelgleichung gibt die individuelle Zunahme einer beliebigen Komponente der absoluten Wirbelstärke wieder; diese Form führt zurErtelschen Gleichung. Zum Schlusse wird dieErtel-Rossbysche Invariante verallgemeinert.

     

Les bilans énergétiques approchés en variables λ, ϕ,p ett

Résumé L'auteur montre comment il faut déduire les formes approchées des bilans énergétiques des mouvements atmosphériques en variables , ,p ett de leurs formes générales exactes. Il a tenu compte de la variabilité de la pression à la surface du globe.

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Summary It is shown how the approximate energy equations of atmospheric motions expressed with the independent variables , ,p andt must bederived from their exact general form. The variability of surface pressure has been taken into account.

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Zusammenfassung Es wird gezeigt, wie sich Näherungsformeln der Energiebilanzen der atmosphärischen Bewegungsvorgänge durch die unabhängigen Variablen , ,p undt von ihrer exakten Formulierung ableiten lassen; dabei wird der Veränderlichkeit des Luftdrucks an der Erdoberfläche Rechnung getragen.

     

A statistical-dynamical downscaling procedure for global climate simulations

Summary A statistical-dynamical downscaling procedure for global climate simulations is described. The procedure is based on the assumption that any regional climate is associated with a specific frequency distribution of classified large-scale weather situations. The frequency distributions are derived from multi-year episodes of low resolution global climate simulations. Highly resolved regional distributions of wind and temperature are calculated with a regional model for each class of large-scale weather situation. They are statistically evaluated by weighting them with the according climate-specific frequency. The procedure is exemplarily applied to the Alpine region for a global climate simulation of the present January climate.List of Symbols west-east mesh size in geographic coordinates south-north mesh size in geographic coordinates N number of large-scale weather classes n number of regional-scale event classes p pressure P probability Ø large-scale event regional-scale event q _v specific humidity potential temperature u west-east wind component v south-north wind componentAbbreviations AGL above ground level - LT local time - UTC universal time coordinated With 13 Figures     

A study of the sensitivity of bare soil evaporation schemes to soil surface wetness,using the coupled soil moisture and surface temperature prediction model,BARESOIL

Summary The performance of evaporation schemes with and approach and their combination within resistance representation of evaporation from bare soil surface is discussed. For this purpose nine schemes, based on different functions of or , on the ratio of the volumetric soil moisture content and its saturated value are used.The quality of the chosen schemes has been evaluated using the results of time integration by the coupled soil moisture and surface temperature prediction model, BARESOIL, using in situ data. A sensitivity analysis was made using two sets of data derived from the volumetric soil moisture content of the top soil layer. One with values below the wilting point (0.17 m³m^–3) and the second with values above 0.20m³m^–3. Data sets were obtained at the experimental site Rimski anevi, Yugoslavia, from the bare surface of a chernozem soil.With 4 Figures     

Bioaccumulation of Cobalt in Parmelia sulcata

Six locations across mainland Portugal were selected for exposing Parmelia sulcata, for a one-year period (8 months for one site), with simultaneous measurement of total (dry + wet) deposition (one-month periods). The exposed lichens and the total (dry + wet) deposition were analysed for cobalt contents by INAA (instrumental neutron activation analysis) and ICP-MS (inductively coupled plasma mass spectroscopy), respectively. The designated wet deposition was evaluated through the collected water volume; the designated dry deposition was assessed after the (dried) residual mass of the wet deposition. An excellent agreement between Co contents in exposed lichens and the cumulative (1) Co contents in the dry deposition, (2) dry deposition, and (3) wet deposition has been found for the locations with alternate drought and precipitation months, high dry deposition, and high Co contents in the latter. Continuous rainfall was found to hinder the Co accumulation in the lichen due to its release from the lichen and/or lower Co contents in the dry deposition. At three locations, P. sulcata Co contents, after subtraction of the background (before exposure), equalled or exceeded the Co contents in the cumulative dry deposition at the end of the exposure time. The optimal exposure period for this species likely depends on the exposure conditions.     

Arctic hazes in summer over Greenland and the North American Arctic: II. Nature and concentrations of accumulation-mode and giant particles

Airborne measurements made during August 1985 over Greenland and its environs show that both accumulation-mode (0.1 m D2.0 m) and giant (D2 m) particles were present in relatively high concentrations in arctic haze layers and that the accumulation-mode particles dominated light scattering. Particles with diameters (D) between 1 and 4 m consisted predominately of mixed materials, small and dense inclusions, and probably organic compounds containing sulfur. Many of the particles from 0.1 to 1 m in diameter were also of mixed composition, with sulfuric acid, ammonium sulfate and organics probably the dominant constituents.     

On some new statistical characteristics of the wind “Koshava”

Summary Koshava is a gusty wind of moderate to strong intensity, blowing from the south-eastern direction, over the area of the Republic of Serbia. It is caused by the interaction between the synoptic circulation and the orography of the Carpathian and the Balkan mountains. The Koshava wind can damage buildings, factories and industrial plants or city infrastructure. Therefore it is important to estimate its gust and the gustiness factor on the basis of the measured data.This paper discusses a statistical analysis of wind data in the maximum influence area of the Koshava wind in the periods of maximum duration of Koshava. The focus of the paper is the examination of urban and suburban effects on Koshava wind and the correlation between the instantaneous maximum wind speed and the hourly mean wind speed. The best fitting with various empirical distributions is proposed.With 10 Figures     

SODAR-based estimation of TKE and momentum flux profiles in the atmospheric boundary layer: Test of a parameterization model

Summary A simple parameterization for the estimation of turbulent kinetic energy (TKE) and momentum flux profiles under near-neutral stratification based on sodar measurements of the vertical velocity variance has been tested using data from the LINEX-2000 experiment. Measurements included operation of a phased-array Doppler sodar DSDPA.90 and of a sonic anemometer USA-1 mounted at a meteorological tower at a height of 90m. Good agreement has been found between the TKE and momentum flux values derived from the sonic and sodar data (with correlation coefficients r>0.90 and a slope of the regression lines of about 1.01.1) suggesting the possible use of sodar measurements of _w ² to derive turbulence parameter profiles above the tower range.     

Bistability of the thermohaline circulation identified through comprehensive 2-parameter sweeps of an efficient climate model

The effect of changes in zonal and meridional atmospheric moisture transports on Atlantic overturning is investigated. Zonal transports are considered in terms of net moisture export from the Atlantic sector. Meridional transports are related to the vigour of the global hydrological cycle. The equilibrium thermohaline circulation (THC) simulated with an efficient climate model is strongly dependent on two key parameters that control these transports: an anomaly in the specified Atlantic–Pacific moisture flux (F_a) and atmospheric moisture diffusivity (K_q). In a large ensemble of spinup experiments, the values of F_a and K_q are varied by small increments across wide ranges, to identify sharp transitions of equilibrium THC strength in a 2-parameter space (between Conveyor On and Off states). Final states from this ensemble of simulations are then used as the initial states for further such ensembles. Large differences in THC strength between ensembles, for identical combinations of F_a and K_q, reveal the co-existence of two stable THC states (Conveyor On and Off)—i.e. a bistable regime. In further sensitivity experiments, the model is forced with small, temporary freshwater perturbations to the mid-latitude North Atlantic, to establish the minimum perturbation necessary for irreversible THC collapse in this bistable regime. A threshold is identified in terms of the forcing duration required. The model THC, in a Conveyor On state, irreversibly collapses to a Conveyor Off state under additional freshwater forcing of just 0.1 Sv applied for around 100 years. The irreversible collapse is primarily due to a positive feedback associated with suppressed convection and reduced surface heat loss in the sinking region. Increased atmosphere-to-ocean freshwater flux, under a collapsed Conveyor, plays a secondary role.     

The behaviour of aerosol optical depth during clear summer days in Athens,Greece

Summary The integral aerosol optical depths (k ) at the hour of 08:20 Local Standard Time (LST), are compared with those calculated previously at 11:20 and 14:20 LST, for clear days during summer in Athens over the period 1962–1988. The mean values at 08:20 LST were consistently lower than the values at 11:20 and 14:20 LST. The influence of the vertical wind profile on the values ofk was also investigated. A comparison was made of the wind profiles at 02:00 and 14:00 LST, for days in which the 11:20 and 14:20 LST values ofk were 0.200 andk 0.350, respectively. The corresponding bulk wind shear s was also found for the period 1980–1988. The most significant results occurred with the first category of days. The resultant wind velocities from the surface to the 900 hPa level, in each hour were higher by 2–4 m·s^–1 with respect to the corresponding values for the second category. At 02:00 LST the bulk wind shear showed a considerable difference (1.8) between the two categories of days in the surface to 700 hPa layer at 02:00 LST. Finally, the associated weather conditions that appear to initiate a period of low values ofk (k 0.200) at 11:20 and 14:20 LST were examined for the period 1980–1988. Fifteen such cases were identified and it was found that they all occurred after the passage of weak cold fronts.With 6 Figures     

A mechanism of Alpine lee cyclogenesis as revealed by a quasigeostrophic variational filter

Summary Mechanisms associated with Alpine lee cyclogenesis during the early phase of their generation are investigated using a variational quasigeostrophic filter technique. It was possible to extract the quasigeostrophic signal from the available analyzed real data set.The results presented here are for the 11–12 March 1982, an example of so-called orographically induced lee cyclogenesis. Non-quasigeostrophic fields, calculated as a difference between observations and the quasigeostrophic fields, show significant magnitudes indicating the possible importance of non-quasigeostrophic processes. A dipole structure in the residual geopotential field was observed, similar to the results of numerical model experiments. Also, a strong upper-level non-quasigeostrophic divergence was found in the Alpine region 24 hours prior to lee cyclogenesis, lasting for 6–12 hours. On the other hand, quasigeostrophic results indicate only a local effect of mountain slopes, suggesting possibly a dominant role of the low-level blocking. A hypothetical scenario of Alpine lee cyclogenesis is proposed, based on results obtained here.With 14 Figures     

Simultaneous Measurements of Black Carbon, PM10, Ozone and NOx Variability at a Locally Polluted Island in the Southern Tropics

This paper shows a comparative study of particle and surface ozone concentration measurements undertaken simultaneously at two distinct semi-urban locations distant by 4 km at Saint-Denis, the main city of La Réunion island (21.5° S, 55.5° E) during austral autumn (May 2000). Black carbon (BC) particles measured at La Réunion University, the first site situated in the suburbs of Saint-Denis, show straight-forward anti-correlation with ozone, especially during pollution peaks ( 650 ng/m³ and 15 ppbv, for BC and ozone respectively) and at night-time (90 ng/m³ and 18.5 ppbv, for BC and ozone respectively). NO_x (NO and NO₂) and PM₁₀ particles were also measured in parallel with ozone at Lislet Geoffroy college, a second site situated closer to the city centre. NO_x and PM₁₀ particles are anti-correlated with ozone, with noticeable ozone destruction during peak hours (mean 6 and 9 ppbv at 7 a.m. and 8 p.m. respectively) when NO_x and PM₁₀ concentrations exhibit maximum values. We observe a net daytime ozone creation (19 ppbv, O₃ +4.5 ppbv), following both photochemical and dynamical processes. At night-time however, ozone recovers (mean 11 ppbv) when anthropogenic activities are lower ([BC] 100 ng/m³). BC and PM₁₀ concentration variation obtained during an experiment at the second site shows that the main origin of particles is anthropogenic emission (vehicles), which in turn influences directly ozone variability. Saint-Denis BC and ozone concentrations are also compared to measurements obtained during early autumn (March 2000) at Sainte-Rose (third site), a quite remote oceanic location. Contrarily to Saint-Denis observations, a net daytime ozone loss (14.5 ppbv at 4 p.m.) is noticed at Sainte-Rose while ozone recovers (17 ppbv) at night-time, with however a lower amplitude than at Saint-Denis. Preliminary results presented here are handful data sets for modelling and which may contribute to a better comprehension of ozone variability in relatively polluted areas.     

Trace gas exchange at the air/water interface: Measurements of mass accommodation coefficients

A liquid jet of 90 m diameter and variable length has been utilized to determine absorption rates and, hence, mass accommodation coefficients , of atmospheric trace gases. The compounds investigated are HCl (0.01), HNO₃ (0.01), N₂O₅ (0.005), peroxyacetyl nitrate (>0.001), and HONO (0.005). It is concluded that the absorption of these trace gases by liquid atmospheric water is not significantly retarded by interfacial mass transport. The strengths and limitations of the liquid jet technique for measuring mass accommodation coefficients are explored.     

Influence of heterogeneous land surfaces on surface energy and mass fluxes

Summary Land-surface heterogeneity affects surface energy fluxes. The magnitudes of selected land-surface influences are quantified by comparing observations with model simulations of the FIFE (First ISLSCP Field Experiment) domain. Several plausible heterogeneous and homogeneous initial and boundary conditions are examined, although soilmoisture variability is emphasized. It turns out that simple spatial averages of surface variation produced biased flux values. Simulated maximum latent-heat fluxes were approximately 30 to 40 W m^–2 higher, and air temperatures 0.4 °C lower (at noon), when computations were initialized with spatially averaged soil-moisture and leaf-area-index fields. The planetary boundary layer (PBL) height and turbulent exchanges were lower as well. It additionally was observed that (largely due to the nonlinear relationship between initial soil-moisture availability and the evapotranspiration rate), real latent-heat flux can be substantially less than simulated latent-heat flux using models initialized with spatially averaged soil-moisture fields. Differences between real and simulated fluxes also vary with the resolution at which real soil-moisture heterogeneity is discretized.With 8 Figures     

The von Kármán constant determined by large eddy simulation

The present study explores the extent to which the logarithmic region of the adiabatic atmospheric boundary layer can be modeled using a three-dimensional large eddy simulation. A value of the von Kármán constant (_LES) is obtained by determining the slope of a logarithmic portion of the velocity profile. Its numerical value is found to be dependent on the value of the Smagorinsky-Model Reynolds number, Re_SM: the value of _LES increases with Re_SM. Results indicate that _LES approaches a value of 0.35 as Re_SM reaches about 7.75 × 10⁵ for the largest domain. The sensitivity of _LES to the profile region over which it is evaluated has been tested. Results show that _LES is not sensitive to the depth of this evaluation region when we employ five grids above the sub-grid buffer layer where sub-grid-scale effects dominate. The maximum _LES is obtained when the lower boundary of the evaluation region is just above the top of the sub-grid-scale buffer layer. This result is consistent with modelled mean speed and resolved-scale shear stress profiles.     

Equatorial disturbances,monsoons, and 1982–1983 ENSO

Summary Interannual modes are described in terms of three-month running mean anomaly winds (u,v), outgoing longwave radiation (OLR), and sea surface temperature (T _* ). Normal atmospheric monsoon circulations are defined by long-term average winds (u _n,v _n) computed every month from January to December. Daily winds are grouped into three frequency bands, i.e., 30–60 day filtered winds (u _L,v _L); 7–20 day filtered winds (u _M,v _M); and 2–6 day filtered winds (u _S,v _S). Three-month running mean anomaly kinetic energy (signified asK _L , K _M , andK _S , respectively) is then introduced as a measure of interannual variation of equatorial disturbance activity. Interestingly, all of theseK _L , K _M , andK _S perturbations propagate slowly eastward with same phase speed (0.3 ms^–1) as ENSO modes. Associated with this eastward propagation is a positive (negative) correlation between interannual disturbance activity (K _L , K _M , K _S ) and interannualu (OLR) modes. Namely, (K _L , K _M , K _S ) becomes more pronounced than usual nearly simultaneously with the arrival of westerlyu and negativeOLR (above normal convection) perturbutions. In these disturbed areas with (K _L , K _M , K _S >0), upper ocean mixing tends to increase, resulting in decreased sea surface temperature, i.e.T _* 0. Thus, groups (not individual) of equatorial disturbances appear to play an important role in determiningT _* variations on interannual time scales. HighestT _* occurs about 3 months prior to the lowestOLR (convection) due primarily to radiational effects. This favors the eastward propagation of ENSO modes. The interannualT _* variations are also controlled by the prevailing monsoonal zonal windsu _n, as well as the zonal advection of sea surface temperature on interannual time scales. Over the central Pacific, all of the above mentioned physical processes contribute to the intensification of eastward propagating ENSO modes. Over the Indian Ocean, on the other hand, some of the physical processes become insignificant, or even compensated for by other processes. This results in less pronounced ENSO modes over the Indian Ocean.With 10 FiguresContribution No. 89-6, Department of Meteorology, University of Hawaii, Honolulu, Hawaii.     

Occurrence of roll circulations in a shallow boundary layer

Meteorological measurements taken at the Näsudden wind turbine site during slightly unstable conditions have been analyzed. The height of the convective boundary layer (CBL) was rather low, varying between 60 and 300 m. Turbulence statistics near the ground followed Monin-Obukhov similarity, whereas the remaining part of the boundary layer can be regarded as a near neutral upper layer. In 55% of the runs, horizontal roll vortices were found. Those were the most unstable runs, with -z _i/L > 5. Spectra and co-spectra are used to identify the structures. Three roll indicators were identified: (i) a low frequency peak in the spectrum of the lateral component at low level; (ii) a corresponding increase in the vertical component at mid-CBL; (iii) a positive covariance {ovvw} together with positive wind shear in the lateral direction (V/z) in the CBL. By applying these indicators, it is possible to show that horizontal roll circulations are likely to be a common phenomenon over the Baltic during late summer and early winter.