Mean Wind Fields Research Articles

Long-term observations of the wind field in the Antarctic and Arctic mesosphere and lower-thermosphere at conjugate latitudes

[1] Mean winds, semidiurnal and diurnal tides, and trends and long-period oscillations spanning a solar cycle (from early 1999 through June 2010) measured by medium frequency (MF) radars at conjugate Antarctic and Arctic latitudes (Syowa, Antarctica, 69°S, 39.6°E, and Andenes, Norway, 69.3°N, 16°E) are described and compared. Zonal mean winds are stronger and more uniform from year to year over the Antarctic, with a stronger eastward winter jet spanning the range of altitudes presented (70 to 96 km). The summer westward jet is also stronger and maximizes at higher altitudes over the Antarctic than over the Arctic. The eastward winter jet over the Arctic, while generally weaker, exhibits a localized maximum in late winter at ∼2 to 3 year intervals. Meridional mean winds likewise achieve somewhat stronger maxima at higher altitudes over the Antarctic than over the Arctic. Semidiurnal tide amplitudes are typically somewhat larger over the Antarctic and similar in the two components, with maxima at ∼85 km or above and narrow responses that tend to cluster from ∼February to May and ∼September to November over the Antarctic and from ∼December to February and ∼June to September over the Arctic. Zonal diurnal tide amplitudes are quite similar between the sites, with maxima extending from ∼70 to 90 km and slightly stronger over the Antarctic. Meridional diurnal amplitudes display more significant growth with altitude, achieve stronger maxima at the highest altitudes presented, and typically exhibit a single and narrow maximum during December to February over the Antarctic and double maxima from ∼May to September over the Arctic. Also discussed are trends and long-period oscillations over a solar cycle observed in these mean and tidal wind fields.

A Change of Large-scale Circulations in the Indian Ocean and Asia Since 1976/77 and Its Impact on the Rising Surface Temperature in Siberia

This study examines the changes of an interdecadal circulation over the Asian continent to find cause of the surface warming in Siberia from 1958 to 2004. According to our study, there is a coherency between a long-term change of sea surface temperature in the Indian Ocean and the rapid increase of air temperature in Siberia since 1976/1977. In this study, we suggest that mean wind field changes induced by the positive sea surface temperature anomalies of the Indian Ocean since 1976/1977 are caused of inter-decadal variations in a large-scale circulation over the Asian continent. It also indicates that the inter-decadal circulation over the Asian continent is accompanied with warm southerly winds near surface, which have significantly contributed to the increase of surface temperature in Siberia. These southerly winds have been one of the most dominant interdecadal variations over the Asian continent since 1976/1977. In addition, we investigated the long-term trend mode of 850 hPa geopotential height data over the Asian continent from the Empirical Orthogonal Function (EOF) analysis for 1958-2004. In result, we found that there was an anomalously high pressure pattern over the Asian continent, it is called 'the Asian High mode'. It is thus suggested that the Asian High mode is another response of interdecadal changes of large-scale circulations over the Asian continent.

Evaluation of a fast response pressure solver for flow around an isolated cube

This work describes and evaluates a pressure solver that has been incorporated into a fast response three-dimensional building-resolving diagnostic wind modeling system. The solver computes the three-dimensional pressure field around buildings and on exterior walls in terms of a coefficient of pressure by solving a simplified pressure Poisson equation (that neglects turbulence stresses in the Navier-Stokes) for incompressible flow. The input to the solver is the three-dimensional mean wind field obtained from a fast response empirical-diagnostic urban wind model. The present study is an evaluation of the pressure solver using wind-tunnel data for flow normal to and at a 45° angle to an isolated cubical building. Results for the normal incident wind angle case indicate that the model satisfactorily reproduces the general spatial patterns and the magnitude of the pressure difference around much of the cube. Details of the flow field that are not satisfactorily predicted include the spatial distribution of pressure on the roof and the lower half of the front side of the building and the magnitude along the sidewalls where pressures are over predicted. The results for the 45° case show reasonable agreement between the model and experiments on the front and the back walls, but over predict pressures on the leading edge of the rooftop. Regions with poor pressure predictions appear to be a result of unsatisfactory mean wind modeling.

Tropospheric tidal effects on the middle and upper atmosphere

Numerical experiments that explore the effects of tides of tropospheric origin on the upper and middle atmosphere reveal strong signatures of an eastward propagating zonal wave number 3 diurnal tide (DE3), which peaks near 110 km and penetrates into the upper thermosphere. We demonstrate that nonmigrating tidal dissipation dominated by DE3 effects in the upper mesosphere and lower thermosphere (MLT) strongly accelerates the zonal mean wind field, affecting both the altitude and magnitude of the low‐latitude jets. We also quantify, for the first time, a stationary planetary wave 4 oscillation (sPW4) in the MLT, which is excited by nonlinear interaction between the DE3 and the migrating diurnal tide (DW1). Our results suggest that the sPW4 modulates the DE3 MLT response and may impact the E region dynamo as well as ionospheric and thermospheric signatures at much higher altitudes.

High-Resolution Large-Eddy Simulations of Scalar Transport in Atmospheric Boundary Layer Flow over Complex Terrain

Abstract This paper presents high-resolution numerical simulations of the atmospheric flow and concentration fields accompanying scalar transport and diffusion from a point source in complex terrain. Scalar dispersion is affected not only by mean flow, but also by turbulent fluxes that affect scalar mixing, meaning that predictions of scalar transport require greater attention to the choice of numerical simulation parameters than is typically needed for mean wind field predictions. Large-eddy simulation is used in a mesoscale setting, providing modeling advantages through the use of robust turbulence models combined with the influence of synoptic flow forcing and heterogeneous land surface forcing. An Eulerian model for scalar transport and diffusion is implemented in the Advanced Regional Prediction System mesoscale code to compare scalar concentrations with data collected during field experiments conducted at Mount Tsukuba, Japan, in 1989. The simulations use horizontal grid resolution as fine as 25 m with up to eight grid nesting levels to incorporate time-dependent meteorological forcing. The results show that simulated ground concentration values contain significant errors relative to measured values because the mesoscale wind typically contains a wind direction bias of a few dozen degrees. Comparisons of simulation results with observations of arc maximum concentrations, however, lie within acceptable error bounds. In addition, this paper investigates the effects on scalar dispersion of computational mixing and lateral boundary conditions, which have received little attention in the literature—in particular, for high-resolution applications. The choice of lateral boundary condition update interval is found not to affect time-averaged quantities but to affect the scalar transport strongly. More frequent updates improve the simulated ground concentration values. In addition, results show that the computational mixing coefficient must be set to as small a value as possible to improve scalar dispersion predictions. The predicted concentration fields are compared as the horizontal grid resolution is increased from 190 m to as fine as 25 m. The difference observed in the results at these levels of grid refinement is found to be small relative to the effects of computational mixing and lateral boundary updates.

Net effect of the QBO in a chemistry climate model

Abstract. The quasi-biennial oscillation (QBO) of zonal wind is a prominent mode of variability in the tropical stratosphere. It affects not only the meridional circulation and temperature over a wide latitude range but also the transport and chemistry of trace gases such as ozone. Compared to a QBO less circulation, the long-term climatological means of these quantities are also different. These climatological net effects of the QBO can be studied in general circulation models that extend into the middle atmosphere and have a chemistry and transport component, so-called Chemistry Climate Models (CCMs). In this work we show that the CCM MAECHAM4-CHEM can reproduce the observed QBO variations in temperature and ozone mole fractions when nudged towards observed winds. In particular, it is shown that the QBO signal in transport of nitrogen oxides NOx plays an important role in reproducing the observed ozone QBO, which features a phase reversal slightly below the level of maximum of the ozone mole fraction in the tropics. We then compare two 20-year experiments with the MAECHAM4-CHEM model that differ by including or not including the QBO. The mean wind fields differ between the two model runs, especially during summer and fall seasons in both hemispheres. The differences in the wind field lead to differences in the meridional circulation, by the same mechanism that causes the QBO's secondary meridional circulation, and thereby affect mean temperatures and the mean transport of tracers. In the tropics, the net effect on ozone is mostly due to net differences in upwelling and, higher up, the associated temperature change. We show that a net surplus of up to 15% in NOx in the tropics above 10 hPa in the experiment that includes the QBO does not lead to significantly different volume mixing ratios of ozone. We also note a slight increase in the southern vortex strength as well as earlier vortex formation in northern winter. Polar temperatures differ accordingly. Differences in the strength of the Brewer-Dobson circulation and in further trace gas concentrations are analysed. Our findings underline the importance of a representation of the QBO in CCMs.

Remote sensing of the nocturnal boundary layer for wind energy applications.

The fine temporal and spatial resolution of Doppler lidar observations has been highly effective in the study of wind and turbulence dynamic in the nocturnal boundary layer during Lamar Low-Level Project in 2003. The High-Resolution Doppler Lidar (HRDL), designed and developed at the National Oceanic and Atmospheric Administration (NOAA) Earth System Research Laboratory (ESRL), measures range-resolved profiles of line-of sight (LOS) Doppler velocity and aerosol backscatter with a pulse repetition frequency of 200 Hz, velocity precision about 10 cm s-1, and a very narrow beam width. The majority of the lidar-measured wind speed and variance profiles were derived using a vertical-scan mode and the application of a vertical binning technique. The profile data were used to calculate quantities important for wind energy applications, including turbulence intensity, wind and directional shear through the layer of the turbine rotor. Profiles of all quantities show a strong variation with height. The mean wind fields, the turbulence, and turbulence intensities show a good agreement with sonic anemometer sodar high confidence (high SNR) measurements. The ability of HRDL to provide continuous information about wind and turbulence conditions at the turbine height and above the range of the tower measurements made HRDL as a powerful instrument for studies of the nighttime boundary layer features. Such information is needed as turbine rotors continue to rise higher into the boundary layer.

Distinct Modes of the East Asian Summer Monsoon*

Abstract Resolution of a complex Hermitian matrix derived from monthly mean 850-hPa wind fields during the summer season (June–August) from 1968 to 2004 revealed four different modes of East Asian summer monsoon (EASM) variability. The leading EASM mode, accounting for 19.6% of the variance, is characterized by two different modes (M11 and M12) or their combination. Both portray a closed cyclonic or anticyclonic circulation anomaly over the western North Pacific (WNP), South China Sea (SCS), and southeastern China; corresponding anomalous geopotential height fields show a wave train structure from the WNP across Japan, the Okhotsk Sea, and Alaska to North America. Thus, the leading EASM mode characterizes the teleconnection pattern of the WNP-EASM. The correlation between M11 (M12) and the dynamic index for the WNP-EASM is 0.85 (0.51). M11 has leading spectral peaks at 15 and 3 yr, whereas M12 displays a predominant peak at 2 yr. It is found that M11 has interdecadal variations, with the transition years being circa 1973 and 1989, respectively. M11 is closely related to air–sea interactions in the SCS and the northwestern Pacific, and its association with the convective heat source over the northwestern Pacific is secondary. In contrast, M12 is closely related to the tropical convective heat source rather than tropical western Pacific sea surface temperature (SST). The second EASM mode, accounting for 12.8% of the variance, is identified and characterized by two distinct and alternating modes or their linear combination (M21 and M22). One mode (M21) closely relates to the dual blocking high pattern detected in anomalous sea level pressure (SLP) and 500-hPa geopotential heights over the Ural Mountains and the Okhotsk Sea. The other (M22) corresponds to a dipole blocking anomaly in anomalous SLP and geopotential heights, with opposing anomalous centers in the south of Japan and the Korean peninsula, and the area between Lake Baikal and the Okhotsk Sea. M22 shows significant correlations with summer mean rainfall in southern and southeastern China. Thus, a single index of EASM is inappropriate for investigating and predicting the EASM.

Seasonal variations of the vertical fluxes of heat and horizontal momentum in the mesopause region at Starfire Optical Range, New Mexico

Lidar observations of wind and temperature profiles between 85 and 100 km, conducted at the Starfire Optical Range (SOR), New Mexico, are used to characterize the seasonal variations of the vertical fluxes of heat and horizontal momentum and their relationships to gravity wave activity in this region. The wind and temperature variances exhibit strong 6‐month oscillations with maxima during the summer and winter that are about 3 times larger than the spring and fall minima. The vertical heat flux also exhibits strong 6‐month oscillations with maximum downward flux during winter and summer. The downward heat flux peaks near 88 km where it exceeds −3 K m s−1 in mid‐winter and is nearly zero during the spring and fall equinoxes. The heat flux is significantly different from zero only when the local instability probability exceeds 8%, i.e., the annual mean for the mesopause region. The momentum fluxes also exhibit strong seasonal variations, which are related to the horizontal winds. Two‐thirds of the time the horizontal momentum flux is directed against the mean wind field.

A regional coupled air–ocean wave model and the simulation of the South China sea summer monsoon in 1998

AbstractWind‐generated ocean waves play a key role in the transfer of momentum, heat, and water vapor between air and sea. In order to reveal the influences of ocean waves on the South China Sea summer monsoon (SCSSM) and to discuss the related mechanisms, a regional atmospheric climate model (with 9 layers in P‐σ coordinate) is coupled to the third generation ocean wave model using the sea‐state‐dependent roughness, and the coupled model (referred to as P‐σRCM‐WAM) is applied to simulate the SCSSM‐developing processes as applicable for 1998. Four simulations are performed and their results compared with one another. The difference between the fully coupled and uncoupled models in terms of the pentad mean wind fields at the 850 hPa level shows a cyclonic circulation over the northern South China Sea (SCS) and southern China before the SCSSM onset. During the onset of the SCSSM the cyclonic circulation changes to a prevailing southwesterly flow over the northern SCS, thus accelerating the SCSSM onset. After the onset the differential wind field shows a divergent pattern. The evolution of precipitation is also modified by the coupling. In response to the air–ocean wave (AW) coupling, the SCSSM rainfall increases in the northern SCS during the SCSSM onset; after the onset the rainfall decreases over the SCS and increases over South China. Considering the atmospheric fields at seasonal timescales, the fully coupled model better simulates the intensity of the 500 hPa geopotential height field and obtains the precipitation with the spatial pattern and amount that are closer to the observations, especially in June, compared with the uncoupled model. The AW feedback in the momentum coupling process results in an anticyclonic differential wind field at the 850 hPa level near the coast. During the SCSSM onset, the thermal positive feedback is dominant in causing changes in the heat fluxes and intensifies the 850 hPa wind field over the Bay of Bengal (BOB) and northern SCS. The BOB and the SCS are both sensitive regions for the onset of SCSSM in the momentum and thermal coupling experiments. Copyright © 2006 Royal Meteorological Society.

Coastal oceanographic regimes of the Northern Argentine Continental Shelf (34–43°S)

The oceanographic regimes of the Northern Argentine Continental Shelf (NACS, 34°–43°S) are derived from advected waters of subantarctic origin, local sources of continental run-off, and a locally generated salinity maximum. Based on 3690 CTD profiles, monthly mean wind fields at coastal stations, and river discharge data, we define the oceanographic regimes over the shelf by analyzing salinity characteristics and spatial distribution: (1) a maximum in salinity (33.7–34.2) originating from the Gulf of San Matías; (2) a relative salinity minimum (30.0–33.3) of the El Rincón estuarine system; (3) a salinity minimum (0–33.0) originating in the Río de la Plata; and (4) waters of the continental shelf (33.5 and 33.7). Temperature over the shelf is controlled by sea–air heat exchange coupled with bathymetry. An analysis of the Simpson parameter of stability ( ϕ) provided an objective definition of a vertically homogenous coastal zone separated from seasonally stratified shelf waters south of 37°S. Bottom temperature gradients and synoptic sections in the winter and spring indicate the presence of a shallow sea front at the 40–50-m isobaths south of 37°S, persistent throughout the year. We define two seasonal periods, autumn–winter and spring–summer, based on seasonality in monthly mean winds fields, continental run-off, fresh water balance and the spatial distribution of salinity signals. Maximum seasonal variation in the extent and location of the oceanographic regimes occurs within the coastal zone. In the autumn–winter period, we observe a northward extension of the Río de la Plata and Gulf of San Matías waters, as well as a reduction of the El Rincón and Continental Shelf waters near the coast. The spring–summer period is characterized by Río de la Plata waters flowing to the south and east, a reduction of Gulf of San Matías waters and an invasion of El Rincón and Continental Shelf waters into the coastal areas. In a general sense, waters across the NACS undergo a seasonal oscillation in distribution and extension that implies a spring–summer reversal of the characteristic shelf-wide north–northeastward direction of flow within the coastal zone.

A west wind climate simulation of the mountainous Yukon

The wind climate of the mountainous terrain in the southern Yukon is simulated using the Wind Energy Simulation Toolkit (WEST) developed by the Recherche en Prévision Numérique (RPN) group of Environment Canada and is compared to measurements in the field. WEST combines two models that operate at different spatial scales. The Mesoscale Compressible Community (MC2) model is a mesoscale numerical weather prediction model that produces simulations over large domains of the order of a thousand kilometres. The MC2 model uses long‐term synoptic scale wind climate data from the analysis of radiosonde and other observations to simulate mean wind fields at tens of metres above the ground using a horizontal resolution of a few kilometres. The mesoscale results are used as input to MS‐Micro/3 (Mason and Sykes (1979) version of the Jackson and Hunt (1975) model version for microcomputers/3‐dimensional; MS‐Micro hereafter), a more computer‐efficient, microscale model with simpler linearized momentum equations and a domain restricted to a few tens of kilometres with horizontal grid sizes of tens or hundreds of metres. MS‐Micro provides wind field results at specific wind generator hub heights (typically 30 to 50 m above ground level (AGL)) which are of interest to researchers and developers of wind farms. WEST shows relatively strong correlations between its simulated long‐term mean wind speed and the measurements from ten wind energy monitoring stations. However, in the mountainous terrain of the Yukon, WEST tends to predict wind speeds which are about 40% too high. The model also produces erroneous wind directions and some were perpendicular to valley orientations. The most likely cause of the wind speed and direction errors is the substantially modified 5‐km grid‐spaced mesoscale terrain used in MC2. The WEST simulation was also found to double the wind speeds observed at airport stations and there was poor correlation between the simulated and observed wind speeds. The bias in the model could be attributed to a number of factors, including the use of smoothed topography by the model, the discrepancy between the neutral atmosphere assumed in MS‐Micro and the normally observed stable atmosphere, the application of MS‐Micro to every third grid point of the MC2 output, abnormally high sea level wind speeds in the input climate data for MC2, and a certain degree of disagreement between the land surface characteristics used in the model and those found in the field. At comparatively low computer cost, WEST predicts a wind climate map that compares favourably to the wind measurements made in several locations in the Yukon. However, the problem of the modified terrain in the mountainous regions is the most pressing problem and needs to be addressed before WEST is used in the mountainous regions of Canada.

Application of a Multigrid Method to a Mass-Consistent Diagnostic Wind Model

Abstract A multigrid numerical method has been applied to a three-dimensional, high-resolution diagnostic model for flow over complex terrain using a mass-consistent approach. The theoretical background for the model is based on a variational analysis using mass conservation as a constraint. The model was designed for diagnostic wind simulation at the microscale in complex terrain and in urban areas. The numerical implementation takes advantage of a multigrid method that greatly improves the computation speed. Three preliminary test cases for the model’s numerical efficiency and its accuracy are given. The model results are compared with an analytical solution for flow over a hemisphere. Flow over a bell-shaped hill is computed to demonstrate that the numerical method is applicable in the case of parameterized lee vortices. A simulation of the mean wind field in an urban domain has also been carried out and compared with observational data. The comparison indicated that the multigrid method takes only 3%–5% of the time that is required by the traditional Gauss–Seidel method.

Modelling local winds over the Salento peninsula

AbstractA three‐day mesoscale numerical simulation has been performed over the narrow Salento peninsula (south‐eastern Italy) during summer conditions characterised by weak synoptic forcing. These atmospheric conditions favour the development of complex sea‐breeze systems and convergence zones on the peninsula. The aim of this work is to investigate the ability of an atmospheric mesoscale model to reproduce the surface fields of meteorological variables in the presence of local‐scale forcing and breeze circulations, which are fundamental in applications such as air pollution modelling and nowcasting.The modelled fields have been compared with available surface measurements and sodar data. Results indicate that the model can simulate the general mean wind field in a realistic way. The diurnal evolution of the wind is well reproduced and the maximum deviations mostly occur during the night, being associated with calm conditions.Statistical analysis indicates that the typical mean bias is found to be about 1 m s−1 for hourly averaged wind speed, less than 20° for wind direction and about 1°C for temperature. The root mean square error (rmse) varies from 1 to 3 m s−1 for wind speed, from 50° to 70° for wind direction, and is about 2.4°C for temperature. All the values of the numerical indexes are within ranges which are characteristic of those found for other state‐of‐the‐art models applied to similar cases studies.Despite a good overall agreement between predictions and observations, some discrepancies were found in the individual profiles due both to the limited spatial representation of the local details and to the complex wind field which makes the space–time matching between the model and the observations quite critical.The structures of the thermal mixed layer and the breeze convergence zone are similar to numerical studies relative to more idealised conditions. Copyright © 2004 Royal Meteorological Society

Modeling study of mesospheric planetary waves: genesis and characteristics

Abstract. The Numerical Spectral Model (NSM) extends from the ground into the thermosphere and incorporates Hines' Doppler Spread Parameterization for small-scale gravity waves (GWs). In the present version of the model we account for a tropospheric heat source in the zonal mean (m=0), which reproduces qualitatively the observed zonal jets near the tropopause and the accompanying reversal in the latitudinal temperature variations. In the study presented here, we discuss the planetary waves (PWs) that are solely generated internally, i.e. without the explicit excitation sources related to tropospheric convection or topography. Our analysis shows that PWs are not produced when the zonally averaged heat source into the atmosphere is artificially suppressed, and that the PWs are generally weaker when the tropospheric source is not applied. Instabilities associated with the zonal mean temperature, pressure and wind fields, which still need to be explored, are exciting PWs that have amplitudes in the mesosphere comparable to those observed. Three classes of PWs are generated in the NSM. (1) Rossby type PWs, which slowly propagate westward relative to the mean zonal flow, are carried by the winds so that they appear (from the ground) to propagate, respectively, eastward and westward in the winter and summer hemispheres below 80km. Depending on the zonal wave number and magnitudes of the zonal winds, and under the influence of the equatorial oscillations, these PWs typically have periods between 2 and 20 days. Their horizontal wind amplitudes can exceed 40 m/s in the lower mesosphere. (2) Rossby-gravity waves, which propagate westward at low latitudes and have periods around 2 days for zonal wave numbers m=2 to 4. (3) Eastward propagating equatorial Kelvin waves, which are generated in the upper mesosphere with periods between 1 and 3 days depending on m. A survey of the PWs reveals that the largest wind amplitudes tend to occur below 80km in the winter hemisphere; but above that altitude the amplitudes are larger in the summer hemisphere where the winds can approach 50m/s. This pattern in the seasonal variations also appears in the baroclinity of the zonal mean (m=0). The nonmigrating tides in the mesosphere are significantly larger for the model with the tropospheric heat source, in which PWs are apparently generated by the instabilities that arise around the tropopause.

Flow around a Complex Building: Comparisons between Experiments and a Reynolds-Averaged Navier–Stokes Approach

Abstract An experiment investigating flow around a single complex building was performed in 2000. Sonic anemometers were placed around the building, and two-dimensional wind velocities were recorded. An energy-budget and wind-measuring station was located upstream to provide stability and inflow conditions. In general, the sonic anemometers were located in a horizontal plane around the building at a height of 2.6 m above the ground. However, at the upwind wind station, two levels of the wind were measured. The resulting database can be sampled to produce mean wind fields associated with specific wind directions such as 210°, 225°, and 240°. The data are available generally and should be useful for testing computational fluid dynamical models for flow around a building. An in-house Reynolds-averaged Navier–Stokes approach was used to compare with the mean wind fields for the predominant wind directions. The numerical model assumed neutral flow and included effects from a complex array of trees in the vicinity of the building. Two kinds of comparisons are presented: 1) direct experimental versus modeled vector comparisons and 2) a numerical metric approach that focuses on wind magnitude and direction errors. The numerical evaluation generally corroborates the vector-to-vector inspection, showing reasonable agreement for the mean wind fields around the building. However, regions with special challenges for the model were identified. In particular, recirculation regions were especially difficult for the model to capture correctly. In the 240° case, there is a tendency for the model to exaggerate the turning effect in the wind caused by the effect of the building. Two different kinds of simulations were performed: 1) predictive calculations with a reasonable but not high-fidelity representation of the building's architectural complexity and 2) postexperiment calculations in which a large number of architectural features were well represented. Although qualitative evidence from inspection of the angles of the vectors in key areas such as around the southeast corner of the building indicated an improvement from the higher-fidelity representation of the building, the general numerical evaluation indicated little difference in the quality of the two solutions.

Modeling the Variability of Sydney Harbor Wind Measurements

The time-dependent behavior in the variability of wind measurements is investigated using bivariate generalized autoregressive conditional heteroscedastic models. These models express the current level of short-timescale wind variability in terms of previous observed values of the fluctuations from mean wind fields. As such, these models provide a useful descriptive model that can be applied to short-term forecasting of variability of wind fluctuations around local mean levels.

Evaluating Large-Eddy Simulations Using Volume Imaging Lidar Data

The authors apply data analysis techniques that demonstrate the power of using volume imaging lidar observations to evaluate several aspects of large-eddy simulations (LESs). They present observations and simulations of an intense and spatially evolving convective boundary layer on 13 January 1998 during the LakeInduced Convection Experiment (Lake-ICE). To enable comparison of observed and simulated eddy structure, aerosol scattering was estimated from LES output of relative humidity, a passive tracer, and liquid water. Spatial and temporal correlation functions of aerosol structure from horizontal planes reveal the mean and turbulent convective structure. The correlation functions of the observed and simulated aerosol backscatter are presented as a function of altitude and offshore distance. Best-fit ellipses to the closed contours encircling the origin of the correlation functions are used to obtain the mean ellipticity and orientation of the structures. The authors demonstrate that these two parameters are not sensitive to minor changes in the functional relationship between humidity and optical scattering. The lidar-derived mean wind field is used as a reference for evaluating the LES mean flow. The ellipses from lidar data indicate that structures near the surface tend to be aligned with the mean wind direction, while in the entrainment zone they are aligned perpendicular to the mean wind direction. In the middle of the mixed layer, convective plumes tended to be circular and, therefore, had no preferred orientation at small lags. At longer lags, however, the correlation functions from the middle of the mixed layer show that the observed convective plumes were organized into linear bands oriented perpendicular to the mean wind direction. The perpendicular bands suggest the important role of gravity waves in organizing convective structures. The study shows that the model generates reasonable coherent structures (open cells) where the LES technique is expected to perform poorly (near the surface) and fails to capture the wind-perpendicular organization of closed cells in the middle of the mixed layer where the LES technique is expected to be robust. The authors attribute this failure to the boundary conditions that limited the growth of waves above the mixed layer.

Turbulence fields for atmospheric dispersion models in horizontally non-homogeneous conditions

When atmospheric pollutant dispersion is simulated over complex terrain, the turbulence input parameters are often prescribed according to standard parameterisation based on surface layer quantities. These last are available from literature. In the past, several different parameterisations for the first few moments of the turbulence velocity statistic and for the Lagrangian time scale have been developed and tested in different stability conditions. The main shortcomings for using these parameterisations are their inadequateness in predicting the turbulence field in horizontally non-homogeneous boundary layer (like for example in complex terrain or in urban heat island) and the essentially local nature of the prescribed turbulence. The purpose of this work is to suggest proper methods for predicting turbulence field for dispersion model over complex terrain and, more generally, in horizontally non-homogeneous conditions. The modelling system RMS (RAMS–MIRS–SPRAY) is applied to the present study. We introduced new non-local turbulence closures in the RAMS circulation model, which provides both mean wind and turbulence fields. Then, through the interface code (MIRS), the input quantities needed to the Lagrangian stochastic dispersion model (SPRAY) are computed. The model system is validated against a tracer experiment performed in a wind tunnel over a schematic two-dimensional valley.

Influence of Cumulus Convection Schemes on the ENSO-like Phenomena Simulated in a CGCM

The features of ENSO-like phenomena simulated in a coupled ocean-atmosphere model (CGCM) incorporated with two different convection schemes, the Kuo scheme (KUO) and the prognostic Arakawa-Schubert scheme (PAS), are compared. There are the significant differences in the model ENSO properties between KUO and PAS. The sea surface temperature (SST) anomaly in KUO shows a standing oscillation pattern in the eastern equatorial Pacific, while it propagates westward in the western equatorial Pacific. In contrast, the SST anomaly propagates eastward slowly across the entire equatorial Pacific basin in PAS. The amplitude of Nino3 SST anomaly is larger in PAS and smaller in KUO than observations. The strongest equatorial westerly wind anomalies during the mature phase of the ENSO are displaced slightly eastward in PAS but westward in KUO compared to observations, associated with the difference in the mean equatorial SST. The examination of heat balance shows that the SST anomaly is strongly influenced by the thermocline depth anomaly in PAS, which is favorable for the eastward phase propagation of SST anomalies. In KUO, on the other hand, the westward phase propagation of SST anomalies in the western equatorial Pacific is related to the anomalous horizontal advection. Comparing PAS with KUO, the mean thermocline depth in the western equatorial Pacific is shallower in PAS than KUO, while the mean zonal SST contrast in the western equatorial Pacific is stronger in KUO than PAS. They are responsible for the difference in the phase propagation of SST anomalies between KUO and PAS. In KUO, the Kelvin waves reflected at the western boundary seem to contribute to the reversal of the model ENSO. In PAS, on the other hand, the western equatorial wind anomalies appear during the mature phase of the model ENSO, and then induce the forced-Kelvin waves playing an important role in the termination of the model ENSO. These Kelvin waves cross the equatorial Pacific as an air-sea coupled mode with a slow phase speed, compared to the reflected Kelvin waves in KUO. As a result, the time needed for Kelvin waves acting as a negative feedback to propagate in the eastern equatorial Pacific is longer in PAS than KUO, leading to the amplification of Nino3 SST anomalies. These differences in ENSO properties in turn are related to the difference in the mean states that result from the two different convection schemes. A comparison of the mean wind fields suggests that the difference in ENSO characteristics can be attributed to the difference in the mean wind fields over the western Pacific.