Strong Horizontal Wind Research Articles

Tornado Generation in a Narrow Cold Frontal Rainband -Fujisawa Tornado on April 20, 2006-

At noon on April 20, 2006, a wind storm occurred in Fujisawa City, Kanagawa Prefecture, Japan with a F0-P1-P1 rank on the Fujita-Pearson-scale (FPP-scale). Damage to over 40 residential buildings was observed. The damage covered an area 2 km by 50 m. The strong winds were accompanied by the passage of a cold front. Eyewitness accounts of a dust column and the recording of a pressure drop (1.5 hPa) indicate the existence of a tornado vortex. A hook-shaped radar echo (5 km in diameter) was observed in the narrow cold frontal rainband, with strong horizontal wind shear above the damage area. The Doppler velocity pattern indicated a misocyclone, which had a diameter of 2 km and a vorticity on the order of 10-2 s-1. The wind storm may have caused the tornado, which formed in the cold frontal wind shear zone.

Quasi‐periodic backscatters from the E region at Gadanki: Evidence for Kelvin‐Helmholtz billows in the lower thermosphere?

Analyzing the field‐aligned coherent radar backscatter observed over Gadanki, India (13.5°N, 79.2°E), with a narrow beam pointing almost vertically, we present convincing experimental evidence for the presence of low‐latitudes tilted sporadic ionization layers close to 10 km in vertical extent that move horizontally through the field of view of the radar. Using the data from high temporal (∼3 s) resolution experiments, we also show that the line‐of‐sight Doppler velocities associated with at least some of the quasi‐periodic striations have very clear vortex‐like structures cutting across a vertical plane inside regions of strong horizontal wind shears. The power as well as the Doppler width peak together, and they often reach their peak values near the center of a vortex, where the magnitude of the Doppler velocity is minimum. The Doppler properties and spatial distribution of the 3 m echoes are explained in terms of a local electrodynamical process that makes ions and electrons move with the vertical neutral wind. Both the wind field and the tilt of the layers are in turn consistent with the presence of Kelvin‐Helmholtz billows. We postulate that the billows themselves are triggered by a shear instability in the large ambient zonal wind; strong zonal wind shears clearly have to be present when sporadic E layers are observed. In our case, the breaking of an originally uniform and horizontal sporadic E layer into tilted pieces aligned more or less parallel to one another, and their motion through the radar field of view in the presence of a mean zonal wind, give the echoes their quasi‐periodic appearance.

Effects of along- and cross-radar-beam winds on Doppler radar spectrum

Abstract. Beam broadening effects on the Doppler radar spectrum induced by along- and cross-radar-beam winds are theoretically investigated in this article. Analytical expression of beam broadening spectrum for a vertical beam subject to a constant vertical wind in the absence of horizontal wind is derived first. We find that the resultant beam broadening spectral shape is in an exponential form, rather than a Gaussian shape. The peak of the exponential spectrum corresponds to the true vertical wind velocity, and the spectral width σ is a function of vertical wind velocity w and the half-power beam width θ1/2 of the radar beam, in accordance with the expression σ=0.09wθ1/22. The beam broadening spectrum for a vertical radar beam subject to both vertical and horizontal winds is numerically studied, and the result shows that the spectral shape is distorted from the Gaussian form. The estimated vertical wind velocity of the distorted spectrum is very close to the true vertical wind velocity with a difference of less than 0.6% for a typical case. However, its spectral width is substantially greater than that of the conventional Gaussian spectrum broadened by horizontal wind only. The difference of the spectral widths Δσ (in unit of percentage) in the two, as a function of the half-power beam width and the ratio of vertical to horizontal wind velocities, can be well approximated by a parabolic equation. With this equation and the conventional Gaussian beam broadening spectral width caused by horizontal wind, the true width of the distorted beam broadening spectrum induced by vertical and horizontal winds can be inferred without complicated numerical computation. Through a transformation of wind velocity from vertical and horizontal winds into along- and cross-radar-beam winds defined on the cross section of an oblique radar beam, the results obtained from the case of vertical radar beam can be directly applied to the case of the oblique beam. We show that in some special cases the beam broadening spectral widths of an oblique beam subject to both vertical and horizontal winds will be exceedingly narrower than those estimated by the existing formula, irrespective of the presence of strong horizontal wind velocity.

Correlations between microphysical properties of large-scale semi-transparent cirrus and the state of the atmosphere

By making use of TOVS Path-B satellite retrievals and ECMWF reanalyses, correlations between bulk microphysical properties of large-scale semi-transparent cirrus (visible optical thickness between 0.7 and 3.8) and thermodynamic and dynamic properties of the surrounding atmosphere have been studied on a global scale. These clouds constitute about half of all high clouds. The global averages (from 60°N to 60°S) of mean ice crystal diameter, D e, and ice water path (IWP) of these clouds are 55 μm and 30 g m −2, respectively. IWP of these cirrus is slightly increasing with cloud-top temperature, whereas D e of cold cirrus does not depend on this parameter. Correlations between D e and IWp of large-scale cirrus seem to be different in the midlatitudes and in the tropics. However, we observe in general stronger correlations between D e and IWP and atmospheric humidity and winds deduced from the ECMWF reanalyses: D e and IWP increase both with increasing atmospheric water vapour. There is also a good distinction between different dynamical situations: In humid situations, IWP is on average about 10 gm −2 larger in regions with strong large-scale vertical updraft only that in regions with strong large-scale horizontal winds only, whereas the mean D e of cold large-scale cirrus decreases by about 10 μm if both strong large-scale updraft and horizontal winds are present.

Mesoscale structures viewed by SAR and AVHRR near the Canary islands

Comparison of Synthetic Aperture Radar and Advanced Very High Resolution Radiometry images of sea surface backscatter and temperature during the peak of the summer Trade winds reveals many aspects of the regional oceanography of the Canary Islands. A strong correspondence occurs between the SAR and AVHRR signals. The generally uniform wind field is perturbed as it flows past the islands producing regions of calm immediately downstream. These are bounded by lines of strong horizontal wind shear that coincide with temperature fronts between warmer lee waters and the cooler surrounding ocean. Weaker winds prevail up to 50 km downstream while enhanced wind speed on the boundaries of the lee can extend more than 150 km. Lee waves are excited in the atmospheric inversion layer where the wind passes over abrupt island topography. Quantitative estimates of wind speed made with the CMOD4 algorithm are unreliable in the downstream region because wind direction is variable and unknown and because spatial gradients in air-sea temperature difference affect atmospheric boundary layer stability. A large anticyclone south of Tenerife strongly influenced the estimated wind speed probably because higher ocean temperatures in its centre caused atmospheric instability and increased radar backscatter. The temperature fronts marking boundaries of the upwelling filament and strong eddies observed in AVHRR appear as lines of current shear in the SAR images.

Mesocyclone and Microburst Signature Distortion with Dual PRT Radars

Abstract A simple model is used to illustrate the limitations of dual pulse repetition time (PRT) radars for measurements of wind fields with strong horizontal wind gradients. In such conditions, errors may occur even if the radial velocity measurements in the individual rays are not aliased. However, basic circulation and divergent signatures are seen and some improvement is possible with further postprocessing of the data. This is significant, as radars using this procedure are being used operationally for severe weather applications in Australia and Canada.

A Numerical Simulation of the Two-Day Wave near the Stratopause

Abstract The middle atmosphere version of the National Center for Atmospheric Research Community Climate Model 2 (MACCM2) mechanistic model is used to simulate several aspects of the observed two-day wave near the stratopause and its possible connection with inertial instability. Model experiments show that, for a prescribed initial wind condition with strong horizontal wind variation in the Tropics, inertial instability can trigger the two-day wave. An increase in horizontal wind curvature fostered by inertial instability circulation barotropically destabilizes the easterly jet in low summer latitudes and allows first the growth of the wavenumber-4 and then the wavenumber-3 component of the two-day wave. Near the stratopause, the two-day wave Eliassen–Palm flux is directed equatorward away from the wave’s critical line source and westerly momentum is transported into the easterly jet core by the wave. While much of the wavenumber-4 energy is confined near the stratopause, the wavenumber-3 energy can prop...

Climatological “models”; of cold air mesocyclones derived from SSM/I data

A major limitation of satellite infrared data used to detect mesocyclones occurring in cold airstreams over datavoid oceanic regions is the inability to document the basic structure of these systems. In this research we use the integrated water vapor (IWV), integrated cloud liquid water (ICLW), precipitation, and surface wind speed information retrieved from the SSM/I (Special Sensor Microwave/Imager) of the DMSP (Defense Meteorological Satellite Program) platform to develop climatological (statistical mean) “models”; of mesocyclone structure and evolution for the southern oceans. Composite analysis of sea level pressures and 500 hPa heights shows that mesocyclones frequently develop in the region between strong synoptic‐scale pressure and height anomalies, and over positive differences in sea‐air temperature. Climatological “models”; (statistical means, standard deviations) of marine atmospheric variables retrieved from the SSM/I indicate that mesocyclones have associated strong horizontal wind shear: highest wind speeds occur in the northwest quadrant and lowest wind speeds in the southeast quadrant of these storms. The composite IWV ranges from low to moderate values (5–15 kg m‐2) that increase from south to north within mesocyclones. Solid precipitation seems to dominate over rain in most of these systems. An estimate of the surface sensible heat flux for one mesocyclone outbreak suggests that surface energy plays an important role in mesoscale cyclogenesis over the oceans.

The nucleation and growth of ice particles in the upper mesosphere

The highest clouds and the coldest temperatures in the Earth's atmosphere exist in the region of the summer mesopause at high latitudes. The presence of ice particles in the region leads to several unique phenomena, including sharply bounded layers in which electrons, and occasionally positive ions, are severely depleted (“biteouts”), and intense radar echoes. This paper reviews some of the recent advances in our understanding of the ways in which these ice particles form and grow. Nucleation can occur on heavy positive ions, but the smoke and dust particles resulting from meteor ablation are more likely condensation nuclei. Ice particles probably form mainly near the mesopause and grow as they sediment downward, but the presence of strong horizontal and vertical winds in the region complicate this simple picture. While biteouts are now generally recognized as being due to scavenging of electrons by particles, the reasons for their existence in narrow sharply bounded layers remain unclear. While the subvisible ice particles are likely to be negatively charged under normal conditions, it is pointed out that under conditions of low ionization in the region, negatively and positively charged particles probably exist in roughly equal numbers, leading to the possibility of enhanced growth by coagulation of oppositely charged particles.

An observational study of cyclogenesis in the lee of the Japan central mountains

An observational study is conducted of the lee cyclogenesis that occurred on 11 December 1990 in the southeast region of the Japan central mountains (JCM). The area of the region, roughly defined by the 1000m above sea level is 250×250 km. The principal results are summarized as follows: i) The parent low, which triggered the lee cyclogenesis, was initiated by an upper-level vortex and accompanied by a low-level jet and surface cold front on the upstream side of the JCM. The system moved eastward along the northern edge of the JCM and dissipated. ii) The surface cold front approached the JCM, became blocked in the middle of the JCM, and stagnant for about three hours. South of the JCM, however, the cold front advanced eastward with no blocking. This resulted in the formation of a strong horizontal wind shear zone along the southeastern edge of the JCM, between the warmer air on the downstream side of the JCM and the colder air to the south. A shallow cyclonic vortex, confined to within 3 km above ground level was subsequently generated in the horizontal wind shear zone. iii) The vortex moved eastward along a meso-scale coastal warm front, which previously existed for more than ten hours over the Kanto Plain, since the morning of 11 December. The vortex coupled with the upper-level vortex when it moved over the warm sea area east of the Japanese Islands, and then rapidly developed (12hPa/12h).

Updraft Evolution: A Perspective from Cloud Base

A Queen Air, instrumented to make 1-Hz measurements of the kinematic, dynamic, and thermodynamic fields, and radar, mesonet, and soundings from the Cooperative Convective Precipitation Experiment 1981 is used to monitor the evolution of the updraft at cloud base of a large cumulus congestus over the High Plains. The environment is characterized by modest instability and strong horizontal wind shear. Twelve passes completed by the Queen Air just below cloud base from the late growth to the dissipation stage reveal that the main updraft splits into two with the south updraft rotating cyclonically and moving to the right of the mean winds. This cell is associated with a pressure perturbation in excess of 1 mb that is most likely caused by the interaction of the updraft with the shear of the horizontal wind. Saturation-point analyses of the updraft and the subcloud layer demonstrate that in the early stages air from near the surface ascended into the cloud, but as the cloud ages, air from the upper subcloud and transition layers contributes to the updraft. This air has little or no buoyancy, which loads to cloud collapse. Mass flux and saturation-point analyses predict the cloud's demise adequately, in contrast to the trends of vertical velocity, virtual potential temperature, or moisture at cloud base. A pressure perturbation caused by updraft–shear interaction is an important mechanism for cloud intensification, but it must act in concert with another forcing mechanism, typically a gust front, to tap the most unstable air found in the lower subcloud layer in the High Plains. The observations support the numerical simulations of cumulonimbi in the presence of strong shear, albeit for a much smaller cloud. Congestus clouds merit attention as they are suitable targets for a variety of platforms and will lead to a more complete understanding of the convective cloud spectrum.

Explosive Marine Cyclogenesis in a Three-Layer Model with a Representation of Slantwise Convection: A Sensitivity Study

Abstract A three-layer primitive equation model with a representation of slantwise convection is used to study explosive marine cyclogenesis. A simple representation of boundary layer and shallow and deep cloud types is considered. Apart from the control simulation. experiments are conducted to test the sensitivity of the bent-back warm front and the explosive deepening to various model parameters. The major results are as follows: 1) The control simulation produces a realistic explosive cyclone. 2) The cloud mass fraction at the top of the boundary layer, which is equal to the fractional area of slantwise ascent in a grid box, essentially determines the cloud mass flux transported into the large-scale atmosphere. The thermal gradient in the bent-back warm frontal region and the final deepening strongly depend on this parameter. 3) Shallow clouds are relatively insignificant in affecting the dynamics of explosive cyclones. 4) The surface drag force weakens the development of strong horizontal wind shear a...

Smoke-Column Observations from Two Forest Fires Using Doppler Lidar and Doppler Radar

To demonstrate the usefulness of active remote-sensing systems in observing forest fire plume behavior, we studied two fires, one using a 3.2-cm-wavelength Doppler radar, and one more extensively, using Doppler lidar. Both instruments observed the kinematics of the convection column, including the presence of two different types of rotation in the columns, and monitored the behavior of the smoke plume. The first fire, a forest fire that burned out of control, was observed by the Doppler radar during late-morning and afternoon hours. Strong horizontal ambient winds produced a bent-over convection column, which the radar observed to have strong horizontal flow at its edges and weaker flow along the centerline of the plume. This velocity pattern implies that the column consisted of a pair of counterrotating horizontal vortices (rolls), with rising motion along the centerline and sinking along the edges. The radar tracked the smoke plume for over 30 km. It also provided circular depolarization ratio measurements, which gave information that the scattering particles were mostly flat or needle shaped as viewed by the radar, perhaps pine needles or possibly flat ash platelets being viewed edge on. The second fire, observed over a 5-h period by Doppler lidar, was a prescribed forest fire ignited in the afternoon. During the first hour of the fire the lidar observed many kinematic quantities of the convection column, including flow convergence and anticyclonic whole-column, rotation of the nearly vertical column, with a vorticity of approximately 10−2 s−1 and an estimated peak vertical velocity w of 1 5 m s−1. After the first hour ambient meteorological conditions changed, the whole-column rotation ceased, and the convection column and smoke plume bent over toward the lidar in stronger horizontal flow. At two times during this later stage, w was estimated to be 24 and 10 m s−1. Lidar observations show that the smoke plume of this second fire initially went straight up in the convection column to heights of over 2 km, so most of the smoke was injected into the atmosphere above the unstable, afternoon, convective boundary layer, or mixed layer. Later, as the horizontal winds increased, a larger friction of the smoke remained in the mixed layer. Finally, very late in the afternoon, after ignitions had ceased and the fire was smoldering, almost all of the smoke remained within the mixed layer. These analyses show that lidar and radar can provide valuable three-dimensional datasets on kinematic quantities and smoke distribution in the vicinity of fires. This kind of information should be of great value in understanding and modeling convection-column dynamics and smoke-plume behavior.

Ozone variations in the Northern Hemisphere due to dynamic processes in the atmosphere

In this paper we have tried to show that the ozone variation along the polar front zone is very dependent on dynamical processes in the atmosphere. We have used the 500 hPa level to show that there is ascending motion at 100 hPa for south and southwesterly wind flows at 500 hPa, whereas west and northwesterly flows give descending motion at 100 hPa level. The vertical velocities at 100 hPa are remarkably large. We have examples of 7 hPa/h, which corresponds to approximately 500 m/h. These strong vertical velocities are usually set un by mountain ranges and strong horizontal wind flows. Since the vertical motion is transported eastwards by the strong westerlies, the effect on the ozone values will show up to the east of the strongest vertical motions. The dominating wind flow over the Atlantic in the winter is southwesterly. This results in low stratospheric temperatures and low ozone values over the Scandinavian sector. Over western Siberia and Russia a high pressure dominates the general winter circulation. To the east of the high the dominating wind flow will be west and northwesterly, and, combined with the mountain ranges east of the Baikal Sea, this leads to general descending motion over eastern Siberia and Kamchatka. This descending motion will raise the temperature in the lower stratosphere and is well correlated with the increase in ozone. The ozone increase is, however, caused by the lowering of the tropopause and, thereby, the vertical stretching of the ozone layer.

Low-level nocturnal wind maximum over the central Amazon basin

A low-level nocturnal wind maximum is shown to exist over extensive and nearly undisturbed rainforest near the central Amazon city of Manaus. Analysis of meteorological data collected during the 1985 and 1987 Amazon Boundary Layer Experiments (ABLE 2A and 2B) indicates the presence of this nocturnal wind maximum during both the wet and dry seasons of the Central Amazon Basin. Daytime wind speeds which are characteristically 3–7 m s-1 between 300 and 1000 m increase to 10–15m s-1 shortly after sunset. The wind speed maximum is reached in the early evening, with wind speeds remaining high until several hours after sunrise. The nocturnal wind maximum is closely linked to a strong low-level inversion formed by radiational cooling of the rainforest canopy. The night-time inversion extends up to 300 m with strong vertical shear of the horizontal wind below the inversion top and uniformly strong horizontal winds above the inversion top. Frictional decoupling of the air above the inversion from the rough forest below, however, is responsible for only part of the observed increase. Surface and low-level pressure gradients between the undisturbed forest and the large Amazon river system and the city of Manaus are shown to be responsible for much of the nocturnal wind increase. The pressure gradients are interpreted as a function of the thermal differences between undisturbed forest and the river/city. The importance of both the frictional decoupling and the horizontal pressure gradient suggest that the nocturnal wind maximum does not occur uniformly over all Amazonia. We suspect that stronger low-level winds are pervasive under clear skies and strong surface cooling and that, in many places (i.e., near rivers), local pressure gradients enhance the low-level nocturnal winds.

Solar wind energy dissipation in the upper atmosphere

The dissipation of solar wind energy makes an important contribution to the energy budget of the earth's upper atmosphere. Heating and momentum transfer by this energy source generate a permanent disturbance zone in the polar region which is characterized by an increase in the temperature and pressure, strong vertical and horizontal winds, and significant changes in the density structure. Heavier gases like Ar, O 2 and N 2 are enhanced, He depleted, and O moderately enhanced or depleted depending on the altitude. The extension and latitudinal structure of this disturbance zone sensitively depend on a number of parameters including the level of magnetic activity, the season, local, universal and storm time, and solar activity. The same parameters also determine the magnitude of the perturbation. Changes outside the disturbance zone are relatively small and are characterized by an increase of all gas constituents. Whereas present models do provide a first order description, more sophisticated algorithms are needed for a satisfactory representation of the disturbance effects.

The Structure and Vorticity Budget of an Early Summer Monsoon Trough (Mei-Yu) over Southeastern China and Japan

One of the most persistent rain-making events over East Asia is the development of an early summer monsoon trough (Mei-Yu) which extends from southeastern China to southern Japan. This work studies the structure and vorticity budget of a Mei-Yu system for the period 10–15 June 1975. Subjectively analyzed grid-point data are time composited with respect to the trough axis along three cross sections over southeastern China (western section), southern East China Sea (central section) and southern Japan (eastern section), respectively, during the mature and decaying stages of the trough. The results indicate that the structure of the eastern and central sections resembles a typical midlatitude baroclinic front with strong vertical tilt toward an upper level cold core and a strong horizontal temperature gradient. On the other band, the western section resembles a semitropical disturbance with an equivalent barotropic, warm core structure, a weak horizontal temperature gradient, and a rather strong horizontal wind shear in the lower troposphere. Cumulus convection activity south of the 850 mb trough is significant in all three sections and contributes substantially to the thermally direct secondary circulation, but the large-scale organizing mechanism differs from one section to another. In the eastern and central sections it is mainly due to differential vorticity advection while in the western section it is due to Ekman pumping (CISK). The generation of cyclonic vorticity is counteracted by cumulus damping in the eastern section and by boundary layer friction in the mountainous western section.

Anisotropic high‐level turbulence

AbstractA study is made of several cases of high‐level turbulence in which the intensity varied with the direction of flight of the aircraft. The direction of maximum turbulence is found to coincide fairly well with the direction of the contour lines. A theory is developed which attributes this type of turbulence to quasi‐horizontal gusts associated with strong horizontal wind shear. Gravity waves are examined as a possible alternative explanation and tentatively rejected.

A Case Study of Jet Stream Clouds

Certain specific cloud forms are frequently associated with strong winds aloft. Investigation has shown that these clouds are usually observed in regions of strong horizontal and vertical wind shear, and in most cases in or above regions where negative absolute vorticity (dynamic instability) and low Richardson's numbers coincide. The small-scale details in jet stream clouds may be the result of turbulent motion in layers brought to or near saturation by large-scale vertical motions in the atmosphere. This paper contains a detailed study of a typical case.

On the frictional influence of horizontal wind shear on vertical motion

AbstractIt is suggested that a reason for the association between anticyclonic vorticity and subsidence lies in the divergence produced by the frictional forces when a strong horizontal wind shear is present. In particular the region just to the south of a strong westerly upper wind (northern latitudes) is favourable for subsidence.A case in which such conditions remained steady for five days has been examined quantitatively, and it is found that a subsidence velocity of a few centimetres per second is accounted for by assuming an isentropic turbulent interchange coefficient (K) of 5 × 109 cms2/sec. This is the value found by Rossby and independently by Grimminger in investigation of other phenomena on a similar scale.The frictional divergence depends on the third space derivative of velocity across the current, and so can only be obtained when the velocity field is mapped in very exact detail. Scope for practical application of this theory in synoptic work is therefore limited.