A comparison of cloud motion and ship wind observations over the Indian Ocean for the year of FGGE

Cloud motions over the Indian Ocean were compared to ship observations for the FGGE year. The statistics of this comparison show seasonal changes in the cloud-ship relationship as well as geographical and wind-pattern-dependent fetch history changes. Most of these changes follow simple boundary-layer relationships governed by friction and temperature advection. The most significant result is the improvement of the cloud-ship directional shear with wind speed. The mean veering angle between cloud and ship measurements decreased at higher wind speeds along with scatter of the shearing angle. This implies that the ability of cloud motion measurements to indicate the wind stress on the ocean improves for the important situations when the winds are strong.     

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 °.     

Markov chain simulations of vertical dispersion in the neutral surface layer for surface and elevated releases

Vertical dispersion in the neutral surface layer is investigated using a Markov Chain simulation procedure. The conceptual basis of the procedure is discussed and computation procedures outlined. Wind and turbulence parameterizations appropriate to the neutral surface layer are considered with emphasis on the Lagrangian time scale. Computations for a surface release are compared with field data. Good agreement is found for the variation of surface concentration and cloud height to distances 500 m downwind of the source. The functional form of the vertical concentration profile is examined and an exponential with exponent 1.6 is found to give the best fit with simulations.For elevated releases, it is demonstrated that an initial dip of the mass mean height from the simulation can be normalized for various release heights using a non-dimensionalized downwind coordinate incorporating advective wind speed and wind shear. The vertical distribution standard deviation ( _z ), as employed in Gaussian models, shows a fair degree of independence with source height but close examination reveals an optimum source height for maximum _z at a given downwind distance,x. This source height increases with downwind distance. Also the simulations indicate that vertical wind shear is more important than vertical variation of Lagrangian time scale close to the source, with a reverse effect farther downwind.     

Boundary-layer parameterizations for applied dispersion modeling over urban areas

Many applied dispersion models require the knowledge of boundary-layer parameters such as sensible heat flux,Q _H , friction velocity,u _*, and turbulent energy components, _w and _v . Formulas are suggested for calculating these parameters over a wide variety of types of ground surfaces, based on simple observations of wind speed near the ground and fractional cloud cover, and specification of constants such as roughness length, albedo, and soil moisture availability. Observations ofu _*,Q _H , _w , and _v during field experiments in St. Louis and Indianapolis are used to test the formulas for urban sites. Relative errors of about ±20% in the predictions are seen to occur whenu _*,Q _H , _w , and _v are large. However, when these quantities are small (e.g.,u _* < 0.2 m/s), the errors in the predictions are as large as the mean value of the quantity itself.In addition, it is concluded from studies of available field data and theories that the magnitude of _w is not well-known at elevations above about 100m during the late afternoon and night. Some simple parameterizations for _w . are suggested that are consistent with the observed steady decrease in ground-level concentration in the afternoon and the sudden increase in concentration that can occur a few hours after sunset due to wind shears associated with a low-level jet, for continuous plumes emitted from moderate to tall stacks.     

Diurnal variation of horizontal wind direction fluctuations in complex terrain at Geysers,Cal.

Horizontal diffusion in the surface layer is dependent on the standard deviation of wind direction fluctuations . Diurnal variation of this parameter in complex terrain was studied for the July 1979 Geysers, Cal., experiment using data from a network of 11 short meteorological towers in the 25 km² Anderson Creek watershed Valley side slopes are roughly 20 ° and maximum terrain difference is about 1 km.Values of for wind directions sampled for one hour at a height of 10 m are about 35 ° during the daytime. They slowly decrease to about 20 ° by 8 to 10 p.m. as stability increases but wind speeds are still relatively high. After 10 p.m. the drainage flow sets in at most stations, with speeds of 1 to 2 m s^-1, and average increases to about 30° during the period 11 p.m. to 6 a.m. In general, highest values of at night are associated with lowest values of wind speed and greatest static stability. This enhancement of by the terrain suggests that horizontal diffusion at night always conforms to that expected during nearly neutral stabilities. That is, Pasquill class D diffusion applies to the horizontal component all night in complex terrain.     

Atmospheric dispersion in the arctic: Winter-time boundary-layer measurements

The winter-time arctic atmospheric boundary layer was investigated with micrometeorological and SF₆ tracer measurements collected in Prudhoe Bay, Alaska. The flat, snow-covered tundra surface at this site generates a very small (0.03 cm) surface roughness. The relatively warm maritime air mass originating over the nearby, partially frozen Beaufort Sea is cooled at the tundra surface resulting in strong (4 to 30 °C · (100 m)^-1) temperature inversions with light winds and a persistent weak (1 to 2 °C · (100 m)^-1) surface inversion with wind speeds up to 17 m s^-1. The absence of any diurnal atmospheric stability pattern during the study was due to the very limited solar insolation. Vertical profiles were measured with a multi-level mast from 1 to 17 m and with a Doppler acoustic sounder from 60 to 450 m. With high wind speeds, stable layers below 17 m and above 300 m were typically separated by a layer of neutral stability. Turbulence statistics and spectra calculated at a height of 33 m are similar to measurements reported for non-arctic, open terrain sites and indicate that the production of turbulence is primarily due to wind shear. The distribution of wind direction recorded at 1 Hz was frequently non-Gaussian for 1-hr periods but was always Gaussian for 5-min periods. We also observed non-Gaussian hourly averaged crosswind concentration profiles and assume that they can be modeled by calculating sequential short-term concentrations, using the 5-min standard deviation of horizontal wind direction fluctuations () to estimate a horizontal dispersion coefficient ( _y ), and constructing hourly concentrations by averaging the short-term results. Non-Gaussian hourly crosswind distributions are not unique to the arctic and can be observed at most field sites. A weak correlation between horizontal ( _v ) and vertical ( _w ) turbulence observed for both 1-hr and 5-min periods indicates that a single stability classification method is not sufficient to determine both vertical and horizontal dispersion at this site. An estimate of the vertical dispersion coefficient, _z , could be based on or a stability classification parameter which includes vertical thermal and wind shear effects (e.g., Monin-Obukhov length, L).     

Two-point velocity statistics over Lake Ontario

Cross-spectra between wind speeds on several masts in Lake Ontario have been analyzed. As previously predicted, coherence over water (small intensity of turbulence) between wind speeds measured on masts lined up with the wind appears to be larger than over land, and increases with decreasing Richardson numbers. As a result, in cold air over warm water, wind speed fluctuations are well predictable from upstream measurements. For large angles between the anemometer line and wind, the difference between coherence over land and water disappears. Furthermore, there is no significant difference in vertical coherence between water and land. When the wind is parallel to the anemometer line, small eddies travel, in agreement with Taylor's hypothesis, with the local mean wind speed. Large eddies travel significantly faster. Vertical phase delay increases with increasing Richardson number.     

Observational aspects on a dominant spectral frequency in an airflow over a weeping-lovegrass stand

Mean and fluctuating wind velocities were measured above a flexible stand (weeping-lovegrass). A waving phenomenon Honami appeared over the stand during the observation period. Some spectral parameters were derived from the vertical wind fluctuations. A dependency of frequency on mean horizontal wind velocity was found. The result, n _m = 0.66u, was obtained under the range of wind speeds from 0.9 m s^-1 to 3.1 m s^-1 just above the canopy.     

Variability of Mixed-Layer Heights over the Indian Ocean and Central Arabian Sea during INDOEX, IFP-99

The upper air data collected from the balloon-borne GLASS Sondes launched from the Oceanic Research Vessel (ORV) Sagar Kanya during the Intensive Field Phase of the Indian Ocean experiment (INDOEX, IFP-99;SK-141 Cruise) are utilized forstudying the variability in the mixed-layer heights observed over the western tropical Indian Ocean and central Arabian Sea. During the entire cruise, typical daytime convective mixed-layer heights (roughly corresponding to 1400 LT) obtained from _V and q profiles, were observed to be in the range 200–900 m. Shallowmixed -layer heights are observed, in general, over the Inter-Tropical Convergence Zone (ITCZ). Over the central Arabian Sea, vertical profiles of _V and q demonstrate a double mixed-layer structure of the marine atmospheric boundary layer (MABL), which gradually disappears close to the Indian coastline.     

Cases-97: Late-Morning Warming And Moistening Of The Convective Boundary Layer Over The Walnut River Watershed

Aircraft, radiosonde, surface-flux, and boundary-layer windprofiler data from the Cooperative Atmosphere Surface Exchange Study's 1997 field project, CASES-97, are combined with synoptic data to study the evolution of the vertically-averaged mixed-layerpotential temperature []and mixing-ratio [Q] onthree nearly-cloudless days from 1000 CST to 1200CST (local noon is approximately 1230 CST). This was achieved through examination of the terms in the time-tendency (`budget')equations for []and [Q]. We estimate three of the terms –local time rate of change, vertical flux divergence, andhorizontal advection. For the [Q]-budget, vertical flux divergence usually dominates, buthorizontal advection is significant on one of the three days. The [Q]-budget balances for two of the three days to within the large experimental error. For the -budget,vertical flux divergence accounts for most of the morningwarming, with horizontal advection of secondary importance.The residual in the -budget has the same sign for all three days, indicating that not all the heating is accounted for. We can balance the []-budgets to within experimental error on two of the three days by correcting the vertical-flux divergence for apparent low biases in the flux measurements of one of the aircraft and in the surface fluxes, and accounting for direct heating of the mixed layer by radiative flux divergence allowing for the effects of carbonaceous aerosols. The [];-budget with these corrections also balances on the third day if horizontal gradients from synoptic maps are used to estimate the horizontal advection. However, the corrected budget for this day does not balance if the horizontal gradient in the advection term is estimated using CASES-97aircraft and radiosondes; we suggest that persistent mesoscale circulations led to an overestimate of the horizontal gradient andhence horizontal advection.     

Small-Scale Variability in the Coastal Atmospheric Boundary Layer

The influence of the main large-scale wind directions on thermally driven mesoscale circulations at the Baltic southwest coast, southeast of Sweden, is examined. The aim of the study is to highlight small-scale alterations in the coastal atmospheric boundary layer. A numerical three-dimensional mesoscale model is used in this study, which is focused on an overall behaviour of the coastal jets, drainage flows, sea breezes, and a low-level eddy-type flow in particular. It is shown that synoptic conditions, together with the moderate terrain of the southeast of Sweden (max. height h₀ 206 m), governs the coastal mesoscale dynamics triggered by the land-sea temperature difference T. The subtle nature of coastal low-level jets and sea breezes is revealed; their patterns are dictated by the interplay between synoptic airflow, coastline orientation, and T.The simulations show that coastal jets typically occur during nighttime and vary in height, intensity and position with respect to the coast; they interact with downslope flows and the background wind. For the assigned land surface temperature (varying ±8 K from the sea temperature) and the opposing constant geostrophic wind 8 m s^-1, the drainage flow is more robust to the opposing ambient flow than the sea breeze later on. Depending on the part of the coast under consideration, and the prevailing ambient wind, the sea breeze can be suppressed or enhanced, stationary at the coast or rapidly penetrating inland, locked up in phase with another dynamic system or almost independently self-evolving. A low-level eddy structure is analyzed. It is governed by tilting, divergence and horizontal advection terms. The horizontal extent of the coastal effects agrees roughly with the Rossby radius of deformation.     

The KABEG’97 field experiment: An aircraft-based study of katabatic wind dynamics over the Greenland ice sheet

The aircraft-based experiment KABEG97 (Katabatic wind and boundary-layer front experiment around Greenland) was performed in April/May 1997. During the experiment, surface stations were installed at five positions on the ice sheet and in the tundra near Kangerlussuaq, West Greenland. A total of nine katabatic wind flights were performed during quite different synoptic situations and surface conditions, and low-level jets with wind speeds up to 25m s^-1 were measured under strong synoptic forcing of the katabatic wind system. The KABEG data represent a unique data set for the investigation of katabatic winds. For the first time, high-resolution and accurate aircraft measurements can be used to investigate the three-dimensional structure of the katabatic wind system for a variety of synoptic situations.Surface station data show that a pronounced daily cycle of the near-surface wind is present for almost all days due to the nighttime development of the katabatic wind. In a detailed case study the stably-stratified boundary layer over the ice and the complex boundary-layer structure in the transition zone ice/tundra are investigated. The katabatic wind system is found to extend about 10 km over the tundra area and is associated with strong wind convergence and gravity waves. The investigation of the boundary-layer dynamics using the concept of a two-layer katabatic wind model yields the results that the katabatic flow is always a shooting flow and that the pure katabatic force is the main driving mechanism for the flow regime, although a considerable influence of the large-scale synoptic forcing is found as well.     

Wind and temperature profile prediction for diabatic surface layers including strong inversion cases

The micrometeorological research program in Antarctica has provided extensive data on wind and temperature profile structure under strong to extreme inversion conditions (Dalrymple et al., 1966; Lettau et al., 1977). The basic similarity hypotheses and limiting conditions for prediction of diabatic surface layer profiles are summarized. The model by Businger et al. (1971) for dimensionless shear and temperature gradients is revised to conform with the new results for strong stability. A novel similarity hypothesis is introduced to complete the step from shear and gradient prediction to prediction of absolute wind speed, wind energy, and temperature on the basis of prescribed external factors of surface layer structure. The physics of interactions between predicted profile tilting and curving are discussed and used to explain several micrometeorological paradoxes, including that of the elevated minimum of air temperature observed occasionally near the active surface when the energy budget is of the nocturnal type.     

Some observations of turbulence and turbulent transport within and above plant canopies

Observations have been made of the structure of turbulence and turbulent exchange within plant canopy layers. A new three-dimensional anemometer was used to measure the eddy fluxes of heat and momentum, and the related cospectra, within and above a corn crop and above a red pine forest. Measured values of momentum and heat fluxes, at each height within the corn canopy, were relatively constant proportions of the flux above the canopy, for the period of a day's observation. Extensive regions obeying a –5/3 power relation were found. Isotropy was found above the forest at high frequencies while above and within the corn crop, the ratios of the lateral and vertical spectral densities to the longitudinal component were less than the expected value in the – 5/3 region. In all situations, the vertical velocity spectra were more peaked than a universal curve, particularly a vertical velocity spectrum from above the forest. It is suggested that the additional variance results from the mixing caused by the individual roughness elements. As expected, the spectra could not be normalized using the height above the soil surface to calculate a non-dimensional frequency, but scaling heights were estimated by matching the frequencies of the peak of each curve with that of the universal curve. Cospectra of uw and wT within the corn canopy were of similar shape and frequency regime, and were basically similar in shape to cospectra above the crop. All of the cospectra were more sharply peaked than universal cospectral curves.     

Spectra,variances and length scales in a marine stable boundary layer dominated by a low level jet

A flow situation over coastal waters of the Baltic Sea is studied. The boundary layer was characterized by stable stratification and the presence of a pronounced low level jet at very low height, 30–150 m, above the surface of the sea. The atmospheric surface layer was apparently extremely shallow; thus the non-dimensional wind gradients and temperature gradients derived from measurements at 8 m do not show adherence to Monin-Obukhov similarity, in sharp contrast to findings from the same site at similar stability conditions but with no low level jet. Instead these quantities are shown to be governed by scales characteristic of stable shear flow away from the surface. The height to the jet centre appears to be an important quantity. Thus, for the cases with the lowest jet height values (30–50 m), some turbulent characteristics of the flow (non-dimensional velocity standard deviations and the correlation between the longitudinal and vertical velocity) have values similar to those found for the zero pressure-gradient laboratory boundary layer over a flat plate (the so called canonical boundary layer) rather than the typical values found in atmospheric boundary-layer flow. It was inferred that the large scale fluctuations known as inactive turbulence, as well as gravity waves, were suppressed in this case.     

A precision kite or balloon-borne mini-sonde for wind and turbulence measurements

A highly mobile system for accurate measurements of wind speed and horizontal turbulence in the lowest few hundred meters of the atmosphere is presented. It consists of a light-weight sonde (only 50 g, including batteries that permit 12 h of continuous operation) which can be easily lifted by a small kite in winds below 5 m/s and up to at least 25 m/s. In winds below 5 m/s, a small kytoon may be used instead. The signals from the sonde are received by a standard FM-radio equipped with a frequency converter, and data are recorded on ordinary cassette tapes. Field tests against towermounted precision instruments were performed at two sites during neutral and unstable conditions with the sonde suspended 25 m below a small kite, the measuring heights being 11 and 18 m respectively during the two test series. Mean wind speeds are found to be accurate to within ±0.2 m/s. Wind speed spectra obtained with the flying sonde can be evaluated up to 0.5 Hz and are found to agree closely with the spectra of the longitudinal component recorded simultaneously by the tower-mounted instrument at the same height. After correction for high frequency loss, which amounted to 5% at this low height (it is expected to decrease rapidly with height), the standard deviation of the wind recorded by the sonde agreed to within 2% with that obtained by the reference instrument. A notable result of the field tests is that there was no sign of degradation of the performance of the sonde in strong turbulence conditions.     

Shear convection

The paper discusses convection in the presence of wind shear, a condition analysed previously by Zilitinkevich (1971). This region (between the forced and fully developed convective layers) was also considered by Betchov and Yaglom (1971). In the present paper the author endeavors to develop a consistent analysis from the basic hypothesis of a very weak interaction between the vertical convective motions and mechanical turbulence, employing a new similarity model of the turbulent regime. Additional experimental data are introduced. Unlike the notation used in the references quoted above, this regime is termed shear convection rather than free convection. The latter is traditionally regarded as synonymous with the terms pure or fully developed convection.     

Structure of the marine boundary layer over north western Indian Ocean during 1983 summer monsoon

The spatial variability of the structure of the lower troposphere over the northwestern Indian Ocean for the period 12th July to 2nd September, 1983 has been studied using upper air data collected during the first scientific cruise of ORV Sagar Kanya.An analysis of thermodynamic structure and kinematics of the marine boundary layer for different zonal and meridional sections revealed the following features: (a) Temperature and humidity inversions were generally absent over the study area except over a few locations in the western region; (b) Large-scale subsidence was found over the central equatorial Indian Ocean; (c) The convective activity over the western Indian Ocean was found to be moderately suppressed as compared to the eastern region; (d) The zonal and meridional components of winds along the equator and 10° N zonal section exhibited a mirror-image-like distribution.     

The stably stratified internal boundary layer for steady and diurnally varying offshore flow

A two-dimensional numerical mesoscale model is used to investigate the internal structure and growth of the stably stratified internal boundary layer (IBL) beneath warm, continental air flowing over a cooler sea. Two situations are studied — steady-state and diurnally varying offshore flow. In the steady-state case, vertical profiles of mean quantities and eddy diffusion coefficients (K) within the IBL show small, but significant, changes with increasing distance from the coast. The top of the IBL is well defined, with large vertical gradients within the layer and a maximum in the coast-normal wind component near the top. Well away from the coast, turbulence, identified by non-zero K, decreases to insignificant levels near the top of the IBL; the IBL itself is characterised by a critical value of the layer-flux Richardson number equal to 0.18. The overall behaviour of the mean profiles is similar to that found in the horizontally homogeneous stable boundary layer over land.A simple physical model is used to relate the depth of the layer h to several relevant physical parameters viz., x, the distance from the coast and U, the large-scale wind (both normal to the coastline) and g/, being the temperature difference between continental mixed-layer air and sea surface, is the mean potential temperature and g is the acceleration due to gravity. Excellent agreement with the numerical results is found, with h = 0.014x ^1/2 U (g/)^–1/2.In the diurnally varying case, the mean profiles within the IBL show only small differences from the steady-state case, although diurnal variations, particularly in the wind maximum, are evident within a few hundred kilometres of the coast. A mesoscale circulation normal to the coast, and superimposed upon the mean offshore flow, develops seawards of the coastline with maximum vertical velocities about sunset, of depth about 2 km and horizontal scale 500 km. The circulation is related to the advection, and subsequent decay, of daytime convective turbulence over the sea.     

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.