Strong Vertical Wind Shear Research Articles

Heat, Moisture, and Momentum Budgets of Isolated Deep Midlatitude and Tropical Convective Clouds as Diagnosed from Three-Dimensional Model Output. Part II: Sensitivity to Ice Phase and Small Changes in Ambient Shear Strength and Low-Level Moisture Supply

Abstract This project uses a three-dimensional anelastic cloud model with a simple ice phase parameterization to evaluate the feedback of isolated deep convective clouds over a horizontal scale comparable to one grid cell in typical mesoscale numerical weather prediction models. A more specific focus in this paper is the sensitivity of the feedback to modest changes in the initial vertical wind shear intensity and low-level moisture supply, as well as to the ice phase. Two parallel sets of comparative simulations are run for a quasi-steady severe Oklahoma supercell thunderstorm in strong vertical wind shear versus a weaker, less persistent, and narrower tropical Atlantic cumulonimbus with a slowly decaying and pulsating updraft in much weaker shear. The horizontal Reynolds averaging approach of Anthes is adopted to diagnose the budgets for heat, moisture, and horizontal momentum. Several similarities and differences between the midlatitude and tropical control experiments were delineated in Part I. The ma...

98/03891 Effect of the heating rate on the pyrolysis of straw pellets

The Nor'westers (severe thunderstorms) that form over northeast India and adjoining Bangladesh region during the pre-monsoon season of 2008 are studied employing observations from ground based radar, Tropical Rainfall Measuring Mission (TRMM) and synoptic stations. Subsequently, an attempt is made to simulate the storms using Weather Research and Forecasting (WRF) model at 9 km horizontal resolution, and 28 vertical levels. Analyses of Radar data for 15 cases out of 108 during the study period showed that the Nor'westers typically propagate in the form of squall lines (parallel bow shaped bands) having horizontal length of about 200 km, reaching more than 400 km on some occasions. They propagate at typical speeds of about 50 km h− 1 from northwest to southeast directions. The model underestimated the strength of the squall lines in terms of wind speed. The simulated results showed the presence of strong vertical wind shear and an advection of warm moist southerly wind from the Bay of Bengal during the formation of Nor'westers. Low level positive vorticity in combination with moist southerly wind from the Bay of Bengal and strong surface heating resulted in the formations of the Nor'westers in all the cases. Cloud tops reached as high as 18–20 km in some of the cases of the severe storms. The altitude of core of maximum precipitation was located between 3–5 km. Average cloud hydrometeor content of the Nor'westers was estimated to be about 3.5 g m− 3.

Comparison of topside and bottomside irregularities in equatorial F region ionosphere

Numerical simulations have been carried out to study and compare the mechanisms of the formations of the topside and bottomside ionospheric irregularities. As is well‐known, the topside ionospheric irregularities have always originated from the bottomside ionosphere due to Gravitational Rayleigh‐Taylor (GRT) instability. From the penetration process we found that the primary bubble with a horizontal scale of tens of kilometers always has a smooth shape in the bottomside ionosphere and breaks into much smaller‐scale structures after penetrating into deep topside ionosphere. Since the plasma bubble is unlikely to break into irregularities in the bottomside ionosphere, the bottomside irregularities probably result from the turbulence in the neutral atmosphere. Examples of simulations are presented to demonstrate that bottomside irregularities may exist in a region with a strong neutral wind jet and seeding gravity wave motion. A strong neutral jet stream always has a strong vertical wind shear, which provides a broad height range of dynamic instability. When the atmospheric gravity wave approaches its critical level, its amplitude will continually grow to break into smaller‐scale waves and turbulence. Then all the waves of different scales as well as the turbulence will become the seeds of the bottomside irregularities.

An overview of a heavy rain event in southeastern Iberia: the role of large-scale meteorological conditions

Abstract. The heavy rain event of 30 January to 4 February 1993 in the Segura Basin in SE Iberia is investigated. The study emphasizes the atmospheric processes that led to thunderstorm development. The meteorological scenario has been separated into preconvective and convective periods and into large-scale and subsynoptic settings. The warm, moist air flow from the Mediterranean Sea, channelled by a high-over-low blocking pattern with a cold front over southeastern Iberia appears to be the main reason that helped trigger a severe weather occurrence. The study of the mesoscale environment parameters showed an enhanced conditional unstability through a deep troposphere layer with a moderate to strong vertical wind shear that promoted conditions to allow organization of long-lived convective structures.

A numerical study of the wind field effect on the growth and observability of equatorial spread F

The neutral wind field effects on the development of the equatorial plasma bubbles have been simulated by a two‐dimensional time‐dependent model similar to that developed by Zalessak and Ossakow. The results indicate that when there exists no neutral wind, any perturbation on the bottom‐side of the ionosphere density profile will be amplified by the gravitational Rayleigh‐Taylor (GRT) instability and an upwelling bubble will form as expected. When a zonal neutral wind field exists, the uplift velocity of the bubble will be enhanced by a uniform neutral wind, but suppressed by vertical shear of the wind field. Secondary structures called plumes will grow out from the side walls of the primary bubble if some secondary perturbation signal with vertical structure is seeded. We notice that patches and multiple plume structures have been observed in the mid‐latitude and low‐latitude ionosphere. So we have developed a two‐dimensional local theory of the generalized GRT instability to calculate the growth rate of a small‐scale perturbation signal in the vicinity of the primary bubble and found that the growth rate increases with time along with the growth of the bubble. It is proved that the irregularities generated by some of our simulations should be able to cascade into observable (e.g., by radar) turbulence in a reasonable period of time after their generation. Similar to the effect on the growth of the primary bubble, a strong uniform wind field may enhance the growth rate of a perturbation signal in the vicinity of the bubble, while a strong vertical wind shear will suppress the growth rate. The main point of this theory is that the growth rate of the GRT instability is controlled by the density gradients and the magnitudes of the velocity of the plasma motion relative to the neutral wind in both directions. In addition, our simulations have proved that a seeding wave with phase velocity matching the background wind speed will generate the fastest growing bubble, disapproving the theory of spatial resonance.

Modulation of equatorial subseasonal convective episodes by tropical‐extratropical interaction in the Indian and Pacific Ocean regions

Composite relationships among outgoing longwave radiation, sea level pressure, surface winds, and upper tropospheric circulation are examined for northern winter during subseasonal episodes of eastward progression of convection from the Indian Ocean to the western Pacific. This evolution often culminates with westerly wind burst events and strong air‐sea interaction associated with regional‐scale convective blowups in the western equatorial Pacific. We first document some of these interactions in the composites for two timescales, the submonthly (6–30 days) and that of the Madden‐Julian Oscillation (MJO) timescale (30–70 days). We then analyze the December 1992 period during the Tropical Ocean Global Atmosphere Coupled Ocean‐Atmosphere Response Experiment (TOGA COARE) to illustrate how these composite relationships are manifested in a case study. Convection in the Indian Ocean for the composites is shown to be associated with a northern hemisphere wave train at 200 mbar, arcing through the midlatitudes, that can contribute to convective blowups farther east on the submonthly (6–30 days) timescale in the Intertropical Convergence Zone (ITCZ) in the eastern Pacific. The eastern Asian trough that is part of this wave train is associated with pressure surges from the northern hemisphere and subsequent convection over Southeast Asia. As the MJO convective envelope moves east to Australasia, midlatitude wave trains in either hemisphere include upper level troughs east of Asia and Australia and pressure surges from either hemisphere that contribute to pressure rises over the Indonesian region and a subsequent shift of the convective envelope to the western Pacific. The vertical wind structure for the December 1992 case study is consistent with the composite surface and upper level winds and also shows strong vertical wind shear in the boundary layer, a sharply defined westerly maximum near 700 mbar and an intensification of the upper level easterlies near 100 mbar. Very deep westerlies (to 200 mbar) are confined to shorter timescales. The case study illustrates the various time and space scale interactions noted in the composites. Reciprocal interactions between the tropics and the midlatitudes on the submonthly and MJO timescales in both the composites and the case study involve pressure surges and wave interaction that influence subsequent convection as the convective envelope migrates eastward from the tropical Indian to Pacific Ocean region.

A method for computing single trajectories representing boundary layer transport

For many purposes it is necessary to represent the transport of air pollutants by a single trajectory. Examples are models which simulate the physical and chemical processes within a box moving along a trajectory or the receptor-oriented statistical modelling of air pollutants. Especially within the planetary boundary layer strong vertical wind shear poses serious problems for describing transport processes with a single trajectory. This paper describes a new method for calculating boundary layer trajectories which uses a pollutant mass-weighted transport vector. For evaluating this vector it is assumed that. the pollutant is evenly distributed within the mixed layer. During periods of reduced mixed layer depth (e.g. nighttime) a residual layer may form above the mixed layer and most of the pollutant is transported aloft. However, this residual layer transport is only of interest if the mixed layer height rises again and pollutants are fumigated back to ground-level. Thus, estimation of the most appropriate pollutant transport depth requires the knowledge of mixing heights along the whole trajectory from the starting point to the receptor point. As this information is not a priori available, an iterative scheme was developed which uses the mixing heights along a preliminary trajectory as an input for an improved guess of the final trajectory. Experiments have shown that this method converges very fast. The computation of a first guess trajectory with fixed transport depth and one iteration is sufficient.

Heat, Moisture, and Momentum Budgets of Isolated Deep Midlatitude and Tropical Convective Clouds as Diagnosed from Three-Dimensional Model Output. Part I: Control Experiments

Abstract This project uses a three-dimensional anelastic cloud model with a simple ice phase parameterization to evaluate the feedback between isolated deep convective clouds and their near surroundings. The horizontal Reynolds averaging approach of Anthes is adopted to diagnose the vertical profiles of the individual budget terms for heat, moisture, and horizontal momentum, as well as the resultant effects of each budget as defined by apparent sources or sinks. The averaging area, 33.75 km on a side, is comparable to one grid cell for typical mesoscale numerical weather prediction models. Two comparative simulations are run, one for a severe Oklahoma thunderstorm in strong vertical wind shear and the other for a tropical Atlantic cumulonimbus in much weaker shear. The midlatitude cloud evolves to a vigorous quasi-steady mature stage with several supercell characteristics including an erect large-diameter updraft, a strong and vertically extensive mesolow, and a well-developed highly asymmetric cold pool ...

Probing a Nonprecipitating Cold Front with Doppler Weather Radar

Abstract The utility of Doppler radar to study boundary-layer kinematics of a weak nonprecipitating cold front in Oklahoma on 16 October 1987 was examined with measurements from two radars. Diagnosis was impeded by operation at low antenna elevation angles, short radar ranges, and low signal-to-noise ratios. Further, kinematic parameters computed by single-radar velocity-azimuth-display (VAD) technique for meteorological wavelengths <125 km were significantly smoothed (more than 50% attenuated). Meteorological scales ≥5 km were well resolved (less than 50% attenuated) in wind fields synthesized from dual-Doppler radar observations, but derived parameters were particularly sensitive to the vertical extrapolation of radial velocity measurements in the presence of strong vertical wind shear. Nonetheless, radar-derived wind flows depicted a sequence of events consistent with other instrumentation. In the vicinity of the front, mean-flow divergence, vertical velocity, and deformation, computed from single-rada...

An Observational Study of Strong Vertical Wind Shear over North America during the 1983/84 Cold Season

The phenomenon of strong vertical wind sheer (SWS) over the North American continent is documented with conventional rawinsonde data for the period 1 October 1983 through 31 March 1984. One of the objectives of this work is to describe the three-dimensional distribution of frontal zones. Since it is not possible to use the coarse horizontal resolution of the sounding network to document fully the existence of frontal zones, we instead use the excellent vertical resolution of the winds to infer the existence of these strong baroclinic zones through use of the thermal wind relation. SWS, defined as a shear exceeding that associated with a horizontal temperature gradient of 1 5.5°C/500 km, is most frequently observed in the lower and upper troposphere, though secondary frequency peaks are observed in the middle troposphere. The SWS phenomenon is preferentially located in the middle latitudes where the strongest baroclinity is typically found. Monthly fields of SWS show the frequency maxima to he closely associated with the mean jet stream and strong baroclinity. An examination of an exceptionally active five-day period of SWS activity shows a very strong baroclinic zone throughout the United States. The most active region of SWS in the eastern area is located in a zone of cyclonic wind shear downstream of a time-averaged though. This particular region of SWS is associated with explosive surface cylogenesis. A study of a corresponding period of exceptionally weak SWS activity reveals a markedly different synoptic pattern in which relatively weak wind maxima and baroclinity are shifted poleward into Canada; significantly weaker surface anticylones and no explosive cyclogenesis occurs. These results suggest that the phenomenon of SWS Phenomenon of SWS can be related to synoptic-scale weather patterns. We demonstrate that SWS phenomenon is preferentially associated with Richardson numbers of less than 1, suggesting a strong association with turbulence. SWS events are associated preferentially with more stable lapse rates than are found otherwise. We also find that SWS events at 700 mb are associated approximately 65% of the time with relative humidities of 75% or less. This result suggests that much of the turbulence observed during these SWS events occurs in cloud-free conditions.

Hailstorm over Telangana

During 11-13 March 1981 Telangana an Neighbourhood was affected by severe hailstorm caused extensive damage to life and property: In this paper an attempt has been made to bring out the synoptic situations. It. has been observed that the hailstorm was associated with marked upper air divergence, which was provided, by a jetstream, movement or a trough in the jet stream with axis located in the break of jet-core and strong Vertical wind shear. The low level convergence and interaction or airmasses or contrasting characteristics were provided by low level anticyclones over Bay or Bengal, Arabian Sea and surface low pressure area with north-south oriented trough and/or north-south low level upper air trough/wind discontinuity over the area.

Interactions between Upper and Lower Tropospheric Gravity Waves on Squall Line Structure and Maintenance

Using a simplified thermodynamic sounding, and variable vertical wind shear, we investigate the role of gravity waves on the structure and propagation of a simulated two-dimensional squall line. Based on an observed squall line environment, the modeled troposphere has been divided into three distinct thermodynamic layers. These consist of an absolutely stable atmospheric boundary layer (ABL), an elevated well-mixed layer, and an upper tropospheric layer of intermediate stability. We find the mixed layer to have a dual role; it has a reduced stability and thus provides abundant buoyancy for the convective scale updrafts, and it provides an ideal layer to trap mesoβ-scale (20–200 km) wave energy generated in the stable layers. The generated waves thus have a significant and lasting impact on the simulation. We also find this thermodynamic structure to be conducive to both strong surface wind perturbations and long-lived squall lines. Experiments that vary the vertical wind shear profile demonstrate that the most vigorous and long-lived squall lines arise with a deep layer of strong vertical wind shear. This result is dependent on the changes in the phase speed and magnitude of the stable layer waves that occur in the sheared versus nonsheared environments. Without flow, waves generated by an initial heat pulse split into symmetric leftward and rightward moving disturbances. Waves generated in the upper tropospheric stable layer are found to move relative to the lower tropospheric waves resulting in a decoupling of deep tropospheric vertical motion and a decrease in strength of the simulated system. With vertical wind shear, the magnitude of the simulated waves is enhanced and an opportunity for sustained coupling between the upper and lower waves exists. It is shown that the upper and lower tropospheric waves in a sheared environment account for many of the circulation features typically associated with two-dimensional squall lines. A simple mechanism for the rear-to-front middle-level jet and surface wake low is also presented.

The Influence of Sea-Surface Temperature on Surface Wind in the Eastern Equatorial Pacific: Seasonal and Interannual Variability

The climate of the eastern Pacific exhibits a pronounced equatorial asymmetry. Boundary layer air originating in the Southern Hemisphere trades crosses the equator and flows into the intertropical convergence zone (ITCZ), whose southern limit is nearly always located at least 4° to the north of the equator. The sea-surface temperature (SST) distribution is characterized by a prominent “cold tongue” centered ∼ 1°S, a strong frontal zone centered ∼ 2°N, and a warm eastward current centered near 5°N. The surface wind field exhibits a pronounced horizontal divergence as the air flows northward across the oceanic frontal zone. These features vary in strength in response to the annual cycle and the El Niño/Southern Oscillation phenomenon. The northward cross-equatorial surface winds, the cold tongue and the frontal zone all tend to be strongest during the cold season (July through November). During the cold season of the coldest years, when the cold tongue is most prominent, the cross-equatorial flow tends to be weaker than normal and the northward flow across 5°N stronger than normal. It is shown that within a few degrees of the equator the meridional equation of motion for the surface winds reduces to a balance between the pressure gradient force and the frictional term that involves the vertical derivative of the vertical flux of momentum by subgrid scale processes. Some of the seasonal and interannual variability of the surface winds appears to be a response to the hydrostatic sea-level pressure changes induced by variations in the strength of the cold tongue. However, that the maximum divergence of the surface winds is observed directly above the oceanic frontal zone rather than over the cold tongue appears to be due to the reduction in vertical wind shear within the lowest 100 m that occurs as air parcels pass northward from the cold tongue to the much warmer waters or the North Equatorial Countercurrent. As evidence of the existence of strong vertical wind shear in the stable boundary layer regime over the cold tongue, we note that northward velocities just 100 m above sea level at the Galapagos Islands have been reported to be on the order of 15 m s−1; more than twice as strong as the surface winds.

Airborne lidar observations of clouds in the Antarctic troposphere

In January 1986, SRI International made exploratory airborne observations of Antarctic tropospheric clouds with a downward‐viewing lidar onboard an LC‐130 supply aircraft. Frequency of observations depended upon the schedule of supply missions. Two types of clouds were observed: relatively opaque, midlevel cloud layers 3.0 to 4.0 km below flight level (about 4.0 to 4.5 km MSL); and higher altitude optically transparent cirrus clouds exhibiting long trails or curtains of ice crystals that extended from flight level downward to the top of the midlevel clouds and, frequently, to ground level. The midlevel clouds were often multilayered and, at times, showed wave or cellular structure associated with cloud streets. The ice crystal trails from the cirrus clouds showed evidence of the presence of strong vertical wind shear, and were observed to “seed” the midlevel cloud layers, producing large breaks in the overcast. These exploratory observations attest to the utility of lidar for atmospheric research studies in the Antarctic region.

梅雨前線帯のmeso-&amp;alpha;-scale convective systemの発達と微細構造

Part I of the present study has shown that a long lived meso-α-scale convective system (MαCS) during 14-15 July 1979, which was accompanied by a weak Baiu frontal depression consisted of meso-β-scale convective systems (MβCSs) and the development/decay and movement of MβCSs had strong influence on the behavior of the MαCS.In Part II, the fine structure of precipitation was studied using dense surface observation data over western Japan (Kyushu). MβCSs consisted of a few meso-γ-scale convective systems (MγCSs), which had the life-time of∼3 hours and brought precipitation of 1-5×108 tonne. Although MγCSs developed in the eastern, central and trailing portions of the depression, the large part of the precipitation over Kyushu was brought about by MγCSs generated around the trailing portion. These MγCSs were generated over the western coast of Kyushu.Under the condition of strong vertical wind shear in the trailing portion, MγCSs showed remarkable band structures generated over the western coast of Kyushu. The intense precipitation (maximum 100mm/hour) in this area was not associated with strong gusts nor with temperature fall.The orographic influence of Kyushu on the generation of MγCSs and the possible role of symmetric instability in the strong vertical shear flow on the formation of precipitation bands were discussed.

Propagation of turbulence structures detected by VHF radar

In the high latitude wintertime mesosphere VHF radar measurements usually reveal several turbulence layers at heights between 65 and 85 km which are closely related to strong vertical wind shear. The turbulence layers are superposed by turbulence bursts, which often form sequences with periods similar to those of simultaneously observed velocity oscillations. The horizontal propagation velocity of the resulting turbulence structures can be obtained by cross-correlating the signal power time series measured at three antenna beam positions. A statistical study using a total of 71 events shows that there is a significant correlation between the propagation velocity of turbulence structures and the mean wind, being consistent with the assumption that turbulence is advected by large scale motions. It is suggested that the observed turbulence bursts are due to secondary static instabilities, which for their part are generated by primary Kelvin-Helmholtz instabilities in regions of strong wind shear.

The Effect of the Arbitrary Level Assignment of Satellite Cloud Motion Wind Vectors on Wind Analyses in the Pre-thunderstorm Environment

The impact of satellite-derived cloud motion vectors on SESAME rawinsonde wind fields was studied in two separate cases. The effect of wind and moisture gradients on the arbitrary assignment of the satellite data is assessed to coordinate surfaces in a severe storm environment marked by strong vertical wind shear. Objective analyses of SESAME rawinsonde winds and combined winds are produced and differences between these two analyzed fields are used to make an assessment of coordinate level choice. It is shown that the standard method of arbitrarily assigning wind vectors to a low level coordinate surface yields systematic differences between the rawinsonde and combined wind analyses. Arbitrary assignment of cloud motions to the 0.9 sigma surface produces smaller differences than assignment to the 825 mb pressure surface. Systematic differences occur near moisture discontinuities and in regions of horizontal and vertical wind shears. The differences between the combined and SESAME wind fields are made smallest by vertically interpolating cloud motions to either a pressure or sigma surface.

Possible Triggering Mechanisms for Severe Storms in SESAME-AVE IV (9–10 May 1979)

The SESAME-AVE IV (9–10 May 1979) rawinsonde data were analyzed to uncover possible triggering mechanisms for severe storms that developed over western Oklahoma and Texas. The high frequency of observations (at 3 h intervals) and high vertical resolution of reported data (at 25 mb intervals) at all stations permitted investigation of the diurnal variation of the planetary boundary layer on the synoptic scale. Thunderstorms developed first just ahead of a stationary front over the Texas panhandle on the afternoon of 9 May. This area was characterized by the absence of a strong inversion (or “lid”) that represented an interface between very warm and dry air aloft, and warm moist tropical air below. Apparently, mesoscale low-level ascending motion associated with frontal lifting and/or the inland sea breeze effect led to the removal of the lid. Another noteworthy feature in this storm event was the strong vertical wind shear at low and middle levels over the storm area. When combined with the development of a deep boundary layer with weak stratification during the daytime, the Richardson number became less than one in the boundary layer in the prestorm situation. The results of our numerical linear stability analysis indicate that the observed basic states were indeed symmetrically unstable. This may suggest that the triggering processes were argumented by symmetric instability. Although a well defined dry line was present, it does not seem to have contributed directly to initiation of storms in this case. It also was observed that, as the thermal low began to weaken in the early evening, the cold air behind a stationary front started advancing eastward and helped to extend the line of thunderstorms deep into central Texas. This may be another process whereby some storms prefer to develop in the late afternoon or early evening.

A Multiple Structured Frontal Zone-Jet Stream System as Revealed by Meteorologically Instrumented Aircraft

Abstract The distribution of wind, temperature, ozone, and turbulence within a multiple structured frontal zone-jet stream system is described using a combination of direct horizontal measurements by meteorologically-instrumented research aircraft and conventional aerological soundings. Results document the spatial continuity of these zones and associated intrusions of ozone-rich stratospheric air. Aircraft turbulence measurements show the zones with strong vertical wind shear and near-critical values of Richardson number are preferred regions of CAT encounter. Diagnostic calculations of the terms of the gradient thermal wind equation illustrate the importance of the sign and magnitude of the air trajectory curvature and its vertical derivative in maintaining regions of strong vertical wind shear and small Richardson number in the presence of weak horizontal thermal gradient.

A Statistical Analysis of Wind and Temperature Variables Associated with High Altitude Clear Air Turbulence (HICAT)

Abstract Chi-square and Kolmogorov-Smirnov statistical tests are applied to distributions of wind and temperature variables measured in conjunction with Project HICAT U-2 flights from worldwide locations. A distinction is made between turbulent and nonturbulent situations. Turbulence is found to occur in layers that exhibit either large decreases in temperature with height or strong vertical wind shears. Turbulent outnumbered nonturbulent cases nearly 4 to 1 for Richardson numbers <15. A high correlation exists between the magnitudes of in-flight measured temperature variations and the sum of the gust velocity components. Examples of time histories show that the high correlation does not reflect complexities of the temperature change for individual turbulence encounters.