Synoptic-scale Wind Research Articles

Statistics of Extreme Synoptic-Scale Wind Speeds in Ensemble Simulations of Current and Future Climate

Abstract Statistical analysis of the wind speeds, generated by a climate model of intermediate complexity, indicates the existence of areas where the extreme value distribution of extratropical winds is double populated, the second population becoming dominant for return periods of order 103 yr. Meteorological analysis of the second population shows that it is caused when extratropical cyclones merge in an extremely strong westerly jet stream such that conditions are generated that are favorable for occurrence of strong diabatic feedbacks. Doubling of the greenhouse gas concentrations changes the areas of second population and increases its frequency. If these model results apply to the real world, then in the exit areas of the jet stream the extreme wind speed with centennial-to-millennial return periods is considerably larger than extreme value analysis of observational records implies.

Recycling of suspended particulates by the interaction of sea-land breeze circulation and complex coastal terrain

The dispersion of recycled particulates in the complex coastal terrain surrounding Kangnung city, Korea was investigated using a three-dimensional non-hydrostatic numerical model and lagrangian particle model (or random walk model). The results show that particulates at the surface of the city that float to the top of thermal internal boundary layer (TIBL) are then transported along the eastern slope of the mountains with the sea breeze passage and nearly reach the top of the mountains. Those particulates then disperse eastward at this upper level over the coastal sea and finally spread out over the open sea. Total suspended particulate (TSP) concentration near the surface of Kangnung city is very low. At night, synoptic scale westerly winds intensify due to the combined effect of the synoptic scale wind and land breeze descending the eastern slope of the mountains toward the coast and further seaward. This increase in speed causes development of internal gravity waves and a hydraulic jump up to a height of about 1 km above the surface over the city. Particulate matter near the top of the mountains also descends the eastern slope of the mountains during the day, reaching the central city area and merges near the surface inside the nocturnal surface inversion layer (NSIL) with a maximum ground level concentration of TSP occurring at 0300 LST. Some particulates are dispersed following the propagation area of internal gravity waves and others in the NSIL are transported eastward to the coastal sea surface, aided by the land breeze. The following morning, particulates dispersed over the coastal sea from the previous night, tend to return to the coastal city of Kangnung with the sea breeze, developing a recycling process and combine with emitted surface particulates during the morning. These processes result in much higher TSP concentration. In the late morning, those particulates float to the top of the TIBL by the intrusion of the sea breeze and the ground level TSP concentration in the city subsequently decreases.

Modeling Tropical Cyclone Wave Population of the Great Barrier Reef

Wave information is generated for the population of tropical cyclones that affects the Great Barrier Reef region. Several models are combined to create a wave database at 150,000 locations at a resolution of 1,500 m throughout this large and remote region. An autoregressive time series model, based on historical data, generated time series of position and central pressure for a very large ensemble of synthetic tropical cyclones. An empirical model was used to create the wind field throughout the wave generating area. The wind field had three components: The primary cyclone vortex, a secondary vortex used to model the low-pressure trough in which a cyclone is often embedded, and the external synoptic scale wind field created between the cyclone and the mid latitude high-pressure systems. WAMGBR, a wave generation model especially adapted for conditions in the Great Barrier Reef, was used to simulate a very large ensemble of storms on a nested grid system in transformed spherical coordinates. The results of the study are significant wave heights, peak periods, and mean directions for return periods between 20 and 1,000 years.

Interactions Between Airflow and Valley Topography with Implications for Aeolian Sediment Transport

The local topography of a landscape can have a profound influence on airflow characteristics and cause modifications to broader synoptic scale winds. This paper reports the results of a preliminary field study examining the effects of a valley on wind velocity and direction. Anemometers and wind vanes were used to measure airflow characteristics upwind, within and downwind of a 20 m deep and 175 m wide dry valley in the central Namib desert. The field data indicate an upwind region of flow acceleration, a minimum in flow velocity in the center of the valley, flow acceleration toward a maximum at the downwind valley edge and subsequent deceleration toward starting velocities downwind of this edge. The development of a flow separation region at the leading edge of the valley and the range of flow distortion are affected by the incident angle of the approaching wind to the axis of the valley. A conceptual model indicating the potential implications of these findings for aeolian sediment transport processes in the vicinity of dryland valleys is presented.

Mesoscale predictability under various synoptic regimes

Abstract. Numerical model experiments using slightly rotated terrain are compared to gauge the sentivity of mesoscale forecasts to small perturbations that arise due to small synoptic-scale wind direction errors relative to topographic features. The surface and above surface wind speed errors, as well as the precipitation forecast errors, are examined for a landfalling cold front that occurred during the California Landfalling Jets (CALJET) experiment. The slight rotation in the terrain results in nearly identical synoptic-scale forecasts, but result in substantial forecast errors on the mesoscale in both wind and precipitation. The largest mesoscale errors occur when the front interacts with the topography, which feeds back on the frontal dynamics to produce differing frontal structures, which, in turn, result in mesoscale errors as large as 40% (60%) of the observed mesoscale variability in rainfall (winds). This sensitivity differs for the two rotations and a simple average can still have a substantial error. The magnitude of these errors is very large given the size of the perturbation, which raises concerns about the predictability of the detailed mesoscale structure for landfalling fronts.

A two-dimensional response to a tropical storm on the Gulf of Mexico shelf

Surface current data from drifting buoys and remotely sensed wind data recorded over the continental shelf in the northeastern Gulf of Mexico during the passage of tropical storm Josephine in October 1996 are examined. Drifter data show the existence of a strong surface jet (velocities reaching 1 m s −1) that moves up the west Florida shelf and westward along the Louisiana–Texas shelf, and lasts for nearly 1 week. The coastal jet occurs during an intense synoptic scale wind event where wind speeds reach 15 m s −1. A simple force balance and statistical analysis are performed to assess the role of strong wind forcing. The primary balance shows an Ekman-type current. The role of local acceleration is greatest when winds are directed along bathymetry. A simple two-dimensional strongly forced shelf response model developed from the linear steady-state momentum equations also indicates larger along-shore currents due to both Ekman-type forcing by cross-shore winds and a cross-shore pressure gradient arising from conservation of mass. Model parameters fit empirically are within 15% of theoretical values. The simple model explains 30% and 46% of the variance in the observed along-shore and cross-shore surface currents, respectively.

Dynamics of a Catalina Eddy Revealed by Numerical Simulation

Abstract Through numerical simulations with the Pennsylvania State University–NCAR Mesoscale Model the dynamics of a Catalina Eddy event that formed during the period 26–30 June 1988 off the coast of southern California is examined. A strengthening and veering of the low-level synoptic-scale winds, from climatological northwesterlies to northerlies, results in a more pronounced effect of the coastal orography around the California bight region. In particular, relative vorticity formed by flow over the coastal terrain remains offshore. Prior to the formation of a quasi-steady eddy within the bight, the northerlies are strong enough to advect anomalously high vorticity out of the region. The formation of a mature Catalina eddy relies on a rapid deceleration of the synoptic-scale northerlies on 30 June, such that vorticity, once formed, remains in the bight. The eddy is also strongly modulated by the diurnal cycle. Northwesterly flow around 500 m above mean sea level impinging on the mountains north of the b...

Low-Level Potential Vorticity and Cyclogenesis to the Lee of the Alps

Abstract High-resolution numerical model simulations over the Alpine region are presented that reveal the presence of low-level elongated bands of potential vorticity (PV) downstream of high topography. These PV streamers (or PV banners) occur when the synoptic-scale wind turns into a direction across the Alps. Individual pairs of banners with anomalously positive and negative values of PV can be attributed to flow splitting, either on the scale of the whole of the Alps (primary banners), or on that of individual massifs and peaks of the model topography (secondary banners). The PV bands have amplitudes of up to −2.5 and +5 pvu, a width of 50–150 km, can attain a length of up to 1500 km, and extend in the vertical from the surface up to the 500-hPa level on occasions. The PV banners are associated with zones of enhanced horizontal wind shear. The analysis of daily output from the operational NWP model run of the Swiss Meteorological Institute also demonstrates that such PV streamers are a frequent feature...

Air‐snow interactions and the surface energy and mass balance over the melting zone of west Greenland during the Greenland Ice Margin Experiment

A meso‐γ‐scale atmospheric model coupled to a snow/ice model has been used to study the atmospheric circulation observed during the Greenland Ice Margin Experiment [Oerlemans and Vugts, 1993] and its impact on the ice sheet mass balance. The simulations shown here are two dimensional and cover the July 5–24, 1991, period, a typical fine weather period in that area. The synoptic scale wind, temperature, and humidity evolution are included. Simulated wind and temperature compare reasonably well with available observations. The simulated snow/ice melting is examined, and the sensitivity of the model to the ice‐sheet surface representation is discussed by replacing the snow/ice model by a classical force‐restore model. It is found that the melting‐refreezing of liquid water in the snow pack impacts significantly the surface energy and mass balances of the ice sheet. In particular, the nighttime surface temperature minimum and katabatic wind speed are less marked when the refreezing process is activated in the model.

The combined effects of synoptic-scale and local-scale meteorological events on bed stress and sediment transport on the inner shelf of the Middle Atlantic Bight

Vertical profiles of near-bottom currents and suspended sediment concentrations were measured on the inner shelf at 20 m depth off the US Army Corps of Engineers Field Research Facility at Duck, NC, U.S.A. over the month of October 1994. The observations embraced an initial period of calm conditions and a prolonged and evolving high-energy event. The data were analyzed to examine the effects of both synoptic and local-scale meteorological events on bed stress and resulting suspended-sediment transport. Four storm phases were identified and related to distinct sets of bottom boundary layer responses: (I) during the initial calm period of low bottom stress, non-diffusive suspended-sediment transport was confined within the thin wave boundary layer; (II) during the onset of the storm, increasingly diffusive suspended-sediment transport and active bedform changes prevailed in association with rapidly increasing wave boundary layer thickness and bed stress; (III) at the peak of the storm, wave boundary layer thickness and bed stress reached their maxima and caused high suspended-sediment transport and sheetflow conditions; and (IV) during the swell-dominated post-storm stage, bedforms developed and suspended-sediment transport was confined within the thick wave boundary layer because of the small current shear. Synoptic-scale winds with local wind perturbation controlled the near-bottom flow during Phase II, whereas local winds dominated during Phase III. Wave-current interactions were most intense during Phase III and were responsible for the high bottom stress.

Simulation of Atmospheric Circulation during the GIMEX 91 Experiment Using a Meso-γ Primitive Equations Model

A meso-gamma-scale atmospheric model has been used to simulate atmospheric circulations observed during the Greenland Ice Margin Experiment (GIMEX). The simulations shown here are two-dimensional and cover the 12-13 July 1991 period, a typical summer situation in this area. The synoptic-scale wind forcing is included. The tundra topography is assumed to be either flat, or averaged over a 50-km-wide cross section centered on the GIMEX transect Simulated wind, temperature, humidity, and turbulent fluxes compare reasonably well with available observations. The simulated heat used to melt snow or ice is also shown. The sensitivity of the model results to the synoptic-scale wind forcing is significant. The impact of a tundra much warmer than the ocean on the ice sheet melting is discussed. It is found that weak easterly synoptic-scale winds are able to overwhelm this impact, especially when the tundra is assumed to be flat.

Development and testing of meteorology and air dispersion models for Mexico City

Los Alamos National Laboratory and Instituto Mexicano del Petróleo are completing a joint study of options for improving air quality in Mexico City. We have modified a three-dimensional, prognostic, higher-order turbulence model for atmospheric circulation (HOTMAC) and a Monte Carlo dispersion and transport model (RAPTAD) to treat domains that include an urbanized area. We used the meteorological model to drive models which describe the photochemistry and air transport and dispersion. The photochemistry modeling is described in a separate paper. We tested the model against routine measurements and those of a major field program. During the field program, measurements included: (1) lidar measurements of aerosol transport and dispersion, (2) aircraft measurements of winds, turbulence, and chemical species aloft, (3) aircraft measurements of skin temperatures, and (4) Tethersonde measurements of winds and ozone. We modified the meteorological model to include provisions for time-varying synoptic-scale winds, adjustments for local wind effects, and detailed surface-coverage descriptions. We developed a new method to define mixing-layer heights based on model outputs. The meteorology and dispersion models were able to provide reasonable representations of the measurements and to define the sources of some of the major uncertainties in the model-measurement comparisons.

Further Study on the Predictability of Landscape-Induced Atmospheric Flow

The effect of the synoptic-scale wind on the predictability of landscape-induced atmospheric flow as a function of surface characteristics is studied by means of over 200 two-dimensional numerical simulations. The predictability measure is defined based on domain-averaged signal to noise ratio. The decomposition of the signal into two different parts is also used to further understand the effect of the synoptic-scale wind on the mesoscale atmospheric predictability. It is emphasized that different predictability measures should be used for different purposes, and different predictability results can be obtained based on different measures. For the cases of surface heat patches with patch sizes from L = 0 to 150 km and constant large-scale mean winds of U = 0, 3, 5, 7, and 9 m s−1, it is found that the potential temperature θ, the deviation of θ from its horizontally domain-averaged value θ′, the horizontal velocity u, and u′ are all predictable at any time during the day when L = 10 to 150 km regardless of the mean wind. In contrast to θ and u, the stability of the vertical velocity W (which is the same as w′) at any time during the day is a very complicated function of both the men wind U and the patch size L for all cases. However, based on the temporal- and domain-averaged signal to noise ratio during the day, w is predictable when L = 10 to 100 km. For the cases of sinusoidal hills with horizontal wavelengths of L = 2 to 100 km, bill heights of h = 0.2 to 2 km. and large-scale mean winds of U = 0, 3, and 7 m s−1, it is found that θ, θ′, u, and u′ are all perdictable at any time during the day for cases of L = 60 and 100 km, regardless of the mean wind. It is also found that w and w′ are predictable at any time during the day for cases of L = 60 and 100 km and h = 0.6 to 2 km.

A comparison of satellite‐derived and aircraft‐measured regional surface sensible heat fluxes over the Beaufort Sea

Motivated by the importance of quantifying the regional surface heat balance over Arctic sea ice in studying climate processes, Lindsay and Rothrock (1994) developed a methodology for computing regional surface sensible heat fluxes using readily available advanced very high resolution (AVHRR) IR satellite imagery. Their technique is based upon the determination of the pixel‐by‐pixel sea ice surface temperature from which estimates of sensible heat fluxes are then made. We compare the sensible heat fluxes over the Beaufort Sea computed using their methodology with those measured by a gust probe system on the National Oceanic and Atmospheric Administration P‐3 aircraft on April 18, 1992, during the Leads Experiment. We use an AVHRR image recorded during the P‐3 flight at 2303 UTC. We show that individual lead heat fluxes can be large, 115 W m−2 for 1‐km average fluxes obtained from flight legs that included a 300‐m‐wide lead, but that regional values of sensible flux over 50–200 km of sea ice were small and positive, ∼8 W m−2. The sensible heat flux computed from the P‐3 showed that a value of CS = 1.1×10−3, where CS is the heat transfer coefficient relative to 10 m, is appropriate for the spring Beaufort Sea. We suggest that more realistic winds derived, for example, from the National Meteorological Center sea level pressure analyses be used instead of the constant value of 5 m s−1 now employed by Lindsay and Rothrock. We also found that the maximum value of ΔT, the difference between air and surface temperature, used in the calculation of sensible heat flux using the Lindsay and Rothrock technique was underestimated by a factor of 1.9 when compared with direct measurements. Use of this ΔT correction factor, synoptic scale winds, and the calculated value of CS gave a good comparison between the AVHRR approach and aircraft fluxes measured over the region. The effective regional momentum drag coefficient CD relative to 10 m was 2.1×10−3, typical of Arctic pack ice.

HEIFE領域における境界層の風速場

Simultaneous wind data collected from micrometeorological tower stations and automatic weather stations have been used to investigate the principal features and seasonal variations of the wind fields in the HEIFE experimental area. Besides wind statistics from a less modified data set, a MASCON wind model was adopted in an objective investigation of the three-dimensional atmospheric boundary layer flow during IOP-2 and IOP-3. The results have shown that synoptic scale, mesoscale and sub-mesoscale motions play different roles in different places. Under strong wind conditions, a northwesterly wind is predominant in the area with significant features of mechanical constraint by the mesoscale topography. On fair weather days, the flow patterns exhibit strong diurnal and spatial variations. In the sites adjacent to the north slope of Qilian Mountain and along the valley axis, flows featuring thermal forcing by mesoscale terrain are apparent, but daytime flows show significant seasonal shifts due to the link with the synoptic winds. Around the northern part of the experimental area, diurnal variations of the flows show a more complicated site-dependent behavior, which is caused by the simultaneous influences of synoptic-scale wind and the flows induced by small-scale topography.

Single-station integral measures of atmospheric stagnation, recirculation and ventilation

Mathematical definitions of integral quantities used to characterize the stagnation, recirculation and ventilation potential of various airsheds are proposed. These integral quantities can be calculated from wind data collected at fixed time intervals and at fixed heights in the atmosphere, and could be calculated, for example, from data from ground-based remote wind profilers. These integral quantities, since they are calculated from data at single stations, provide useful characterizations of the flow at individual measurement points, but are true measures of the transport of a plume only under idealized homogenous wind conditions. The utility of these single-station measures for characterizing the air pollution transport potential of an airshed is illustrated using three months of hourly surface and radar profiler measurements of horizontal wind speed and direction collected at three locations in the Colorado Plateaus Basin region of Arizona during the winter of 1990. A surface station at Bullfrog Basin, located on a sheltered basin floor and exposed to diurnal wind systems, experienced stagnations 62% of the time, recirculations 34% of the time, and ventilations 8% of the time. A surface station at Desert View, located on the south rim of the Grand Canyon and exposed to synoptic-scale wind systems, experienced stagnations 8% of the time, recirculations 4% of the time, and ventilations 35% of the time. A radar profiler station at Page, Arizona, experienced stagnations about 20% of the time and recirculations about 25% of the time during the winter at heights below ∼ 400 m a.g.l.; above this height, to levels near 1100 m a.g.l. (the approximate height of surrounding plateaus), the frequency of stagnations and recirculations dropped rapidly, and the frequency of ventilations ranged from 40 to 70%.

Tropical mesoscale rainfall patterns: the opening of the monsoon in the Townsville area, December 1988

Rainfall during six days of monsoonal activity in the 1988/89 wet season is examined in terms of the influence of synoptic scale winds on mesoscale rainfall activity over the Townsville coastal plain (3450 km2). The resulting rainfall patterns. which were based on observations from a dense mesoscale network of 133 gauge points. are presented as isohyetal maps of twenty‐four hour totals for each day in the study period.The study has shown that the distribution of rainfall is related to the influence of synoptic scale wind vectors, which provide upper level steering of convective activity. In addition. the interaction between local winds and synoptic circulation is deemed important for inducing convergence and precipitation. Rainfall generated is then directed by the prevailing upper synoptic flow. Orographic effects also appear to be significant, in that rainfall distribution tends to favour slopes on the windward side of the synoptic scale winds. The rainfall patterns described are probably typical of many of those that occurred throughout the 1988/89 wet season, due to the likely persistence of these mesoscale precipitation mechanisms. Similar mechanisms are also likely to occur over the study area in other wet seasons and elsewhere in the seasonally wet and dry tropics.

Airborne observations of thermal mesoscale circulations in the coastal marine boundary layer

Detailed observations obtained from airborne meteorological measurements in an area along the Baltic coast in the southeast of Sweden are presented. The measurements were taken during late spring/early summer when the water in the Baltic is generally cool compared to typical daytime land surface temperatures which generated significant surface temperature contrasts between land and water. The measurements show the coastal marine boundary layer to be very far from the homogeneity often assumed in numerical models. The cases presented focus mainly on mesoscale circulations driven or modified by this temperature contrast and related turbulence structure such as thermal jets and sea breeze circulations and show a high degree of temporal as well as small‐scale spatial variability. Even though the synoptic scale wind direction in this area is highly variable, due to traveling mid‐latitude cyclones, the surface temperature patterns show great consistency from day to day, with the coldest water always being along the coast with a pronounced minimum localized at the southeastern part of the coastline. It is not clear to what extent the sea surface temperature inhomogeneities are responsible for some of the observed small‐scale variability, nor is it clear if they are results of ocean processes alone or are interacting in any significant way with the marine boundary layer close to the coast.

Modeling of Summertime Flow and Dispersion in the Coastal Terrain of Southeastern Australia

Abstract The Latrobe Valley is situated in a coastal region of complex terrain in southeastern Australia. During typical summertime conditions of light synoptic winds and clear skies, the low-level regional wind field is dominated by sea-breeze and slope-wind circulations. Westerly winds at heights between 1500 and 3000 m are observed to move progressed down to the surface during the night and early morning hours. This unusual feature of the diurnal wind cycle occurs when the synoptic pressure gradient up to 700 hPa is weak, producing a 3–4 m s−1 geostrophic wind from the northeast. A three-dimensional mesoscale model with detailed boundary-layer physics is employed to examine this behavior. Good agreement with observed winds can only be obtained when a negligible synoptic-scale wind is specified. Laboratory experiments and use of a nested model with a larger outer domain show that this occurs because blocking of the northeasterly synoptic flow by upstream orography produces a stagnation region over the m...

Numerical Investigation of Mesoscale Circulations over the Los Angeles Basin. Part II: Synoptic Influences and Pollutant Transport

Abstract This paper describes the influence of large-scale flows on mesoscale circulations and pollutant transport in the Los Angels Basin. National Meteorological Center (NMC) gridded data for twenty-six basin smog episodes were composited, and summer and winter synoptic-scale wind and temperature profiles were created. The 24-h evolution of the atmosphere in and around the basin was simulated, both with and without large-scale winds, using a three-dimensional mesoscale model. Light large-scale winds had little influence on the strong summertime mesoscale circulations, but stronger large-scale winds exerted considerable influence on the weak mesoscale wintertime circulations. During both seasons, daytime upslope flows near the mountains ventilate the basin, inducing basinwide airflow and augmenting the sea breeze. Air from the basin's planetary boundary layer (PBL) travels upslope and rises over the ridgetops, where it mixes convectively to great heights; winds aloft then transport the air from the regio...