Band Of Winds Research Articles

The role of the timing of sudden stratospheric warmings for precipitation and temperature anomalies in Europe

AbstractThe Northern Hemisphere stratospheric polar vortex (SPV), a band of fast westerly winds over the Pole extending from approximately 10 to 50 km altitude, is a key driver of European winter weather. Extremely weak polar vortex states, so called sudden stratospheric warmings (SSWs), are on average followed by dry and cold weather in Northern Europe, as well as wetter weather in Southern Europe. However, the surface response of SSWs varies greatly between events, and it is not well understood which factors modulate this difference. Here, we address the role of the timing of SSWs within the cold season (December–March) for the temperature and precipitation response in Europe. Given the limited sample size of SSWs in the observations, hindcasts of the seasonal forecasting model SEAS5 from the European Centre for Medium‐Range Weather Forecasts are analysed. First, we evaluate key characteristics of stratosphere–troposphere coupling in SEAS5 against reanalysis data and find them to be reasonably well captured by the model, justifying our approach. We then show that in SEAS5, early winter (December and January) SSWs are followed by more pronounced surface impacts compared to late winter (February and March) SSWs. For example, in Scotland, the low precipitation anomalies are roughly twice as severe after early winter SSWs than after late winter SSWs. The difference in the response cannot be explained by more downward propagating SSWs in early winter, or by different monthly precipitation climatologies. Instead, we demonstrate that the differences result from stronger SPV anomalies associated with early winter SSWs. This is a statistical artefact introduced through the commonly used SSW event definition, which involves an absolute threshold, and, therefore, leads to stronger SPV anomalies during early winter SSWs when the stratospheric mean state is stronger. Our study highlights the sensitivity of surface impacts to SSW event definition.

An ice‐free Arctic: what could it mean for European weather?

An ice‐free Arctic: what could it mean for European weather?

Synoptic scale outbreak of self‐initiated upward lightning (SIUL) from tall structures during the central U.S. blizzard of 1–2 February 2011

AbstractA major central U.S. winter cyclone on 1–2 February 2011 produced a band of high winds, up to 75 cm of snow, and numerous reports of thundersnow from Oklahoma into Ontario over a 26 h period. The National Lightning Detection Network (NLDN) recorded 282 flashes comprised of 1153 events which were >96% negative polarity. Hopes of imaging winter sprites associated with energetic positive cloud‐to‐ground events that sometimes accompany such winter storms did not materialize. However, the lack of lightning over the Great Lakes waters, plus media reports of numerous thundersnow events in downtown Chicago, prompted a detailed analysis of the NLDN data. This revealed that >93% of all lightning in the snow band was likely or possibly associated with self‐initiated upward lightning (SIUL) events from a variety of tall, and some not so tall, structures. In addition to 43 events from two Chicago skyscrapers, many shorter structures were involved, including wind turbines (13.1% of the total) and transmission line towers (6.7%). Wind speeds for all events exceeded the 8 m s−1 minimum threshold associated with SIULs in Japanese winter lightning storms. Radar reflectivities at the event locations had a mean of 28 dBZ and were almost always <35 dBZ. While conventional radar displays suggested stratiform precipitation in the thundersnow region, detailed analysis of 3‐D‐gridded NMQ (National Mosaic and Multi‐Sensor Quantitative Precipitation Estimation) radar reflectivity data confirmed elevated embedded cellular convection spanning the −10°C region associated with isentropic lifting above a frontal surface, evidence of noninductive charge generation sufficient to allow upward leader initiation from tall objects.

High‐resolution mapping of Bora winds in the northern Adriatic Sea using synthetic aperture radar

The Adriatic Sea is regularly subjected to strong Bora wind events from the northeast during winter. The events have a strong effect on the oceanography in the Adriatic, driving basin‐scale gyres that determine the transport of biogeochemical material and extracting large amounts of heat. The Bora is known to have multiple surface wind jets linked to the surrounding orography and have been the focus of many studies, but it has not been possible to describe the detailed spatial structure of these jets by in situ observations. Using high‐resolution spaceborne RADARSAT‐1 synthetic aperture radar (SAR) images collected during an active Bora period (23 January–16 February 2003), we created a series of high‐resolution (300 m) maps of the wind field. The obtained winds show reasonable agreement with several in situ wind observations, with an RMS wind speed error of 3.6 m/s, slightly higher than the 2–3 m/s errors reported in previous studies. These SAR images reveal the spatial structure of the Bora wind in unprecedented detail, showing several new features. In the Senj region of Croatia, several images show rhythmic structure with wavelengths of 2–3 km that may reflect Bora pulsation seen at fixed locations by previous investigators. Along the Italian coast, several images show a wide (20–30 km) band of northwesterly winds that abruptly change to the northeasterly Bora winds further offshore. Meteorological model results suggest that these northwesterly winds are consistent with those of a barrier jet forming along the Italian Apennine mountain chain.

A global climate model of Titan's atmosphere and surface

We present the formulation of a global climate model (GCM) of Titan's atmosphere and surface and report initial results. The model is a fully three-dimensional, modified version of NCAR's terrestrial global climate model, CAM3. It includes forcing by Saturn's gravitational tides; a treatment of the planetary boundary layer and surface interactions; scattering and absorption of short-wave radiation; absorption and emission of long-wave radiation; thermal conduction in the soil, and a package for computing atmospheric chemistry. The physical properties and distribution of minor species and aerosols are constrained by Cassini observations. The simulations do not include the effects of a methane hydrological cycle, feedback between radiative forcing and transport of radiatively active minor species and aerosol, or aerosol microphysics. We report a set of baseline results obtained after more than ∼30 Titan-years of model integration. The model calculates a tropospheric circulation that is in-family with the results from other published GCMs, with a regime of weak retrograde winds dominating the summer hemisphere and a band of weak prograde winds appearing in the winter hemisphere. In the stratosphere, the model produces a polar jet in the winter hemisphere, but its peak velocity of 12 m s −1 is more than an order of magnitude weaker than the winter polar jet inferred from observations. We find that the globally integrated atmospheric angular momentum (AAM) undergoes a semiannual oscillation similar to that calculated by Tokano and Neubauer [Tokano, T., Neubauer, F.M., 2005. Wind-induced seasonal angular momentum exchange at Titan's surface and its influence on Titan's length-of-day. Geophys. Res. Lett. 32, L24203–L24206], but delayed in seasonal phase by 30° of L s. The associated transfer of angular momentum to the surface implies that Titan's rate of rotation should be accelerating during the epoch of Cassini Mission observations, consistent with results obtained by the Cassini radar investigation [Stiles, B.W., Kirk, R.L., Lorenz, R.D., Hensley, S., Lee, E., Ostro, S.J., Allison, M.D., Callahan, P.S., Gim, Y., Iess, L., Persi del Marmo, P., Hamilton, G., Johnson, W.T.K., West, R.D., 2008. The Cassini RADAR Team: Determining Titan's spin state from Cassini RADAR images. Astron. J. 135, 1669–1680]. The amount of AAM transfer to the surface predicted by the model, when combined with knowledge of the state of rotation, points to a low value for Titan's effective moment of inertia. As has been suggested [Lorenz, R.D., Stiles, B.W., Kirk, R.L., Allison, M.D., Persi del Marmo, P., Iess, L., Lunine, J.I., Ostro, S.J., Hensley, S., 2008. Titan's rotation reveals an internal ocean and changing zonal winds. Science 319, 1649–1651], this may indicate significant decoupling between Titan's outer crust and its mantle, possibly due to the presence of an intervening subsurface ocean. We note, however, that existing Titan GCMs, as presently formulated, do not yield uniform results concerning AAM transfer to the surface and ignore some physical processes that could potentially alter the predicted magnitude and seasonal phasing of the transfer.

Horizontal structures and dynamics of Titan's thermosphere

The Cassini Ion Neutral Mass Spectrometer (INMS) measures densities of gases including N2 and CH4 in situ in Titan's upper atmosphere. We have used data from 13 targeted flybys of Titan (T5–T32) to construct an empirical model that describes the mean state of the thermosphere in the northern hemisphere, giving the N2 and CH4 densities between 1000 and 1600 km as a function of latitude and height. The principal features in the INMS data are well reproduced by this simple model. We find a pronounced oblateness in the thermosphere, with densities above 1100 km altitude increasing by around 70% from the northern (winter) pole to the equator, resulting in isobaric surfaces being ≃45 km higher over the equator than at the northern pole. Thermospheric temperatures derived from the densities tend to decrease with height from 149 ± 10 K to 140 ± 13 K near 1600 km. Considerable latitude differences are present in the temperatures below 1200 km. Near 1000 km altitude, temperatures reach 164 ± 6 K at 20°N and 131 ± 6 K near 80°N. Using our Thermosphere General Circulation Model with this thermal structure imposed, we derive thermospheric horizontal wind speeds reaching ∼150 m s−1, with primarily poleward flow at equatorial latitudes which, northward of around 60°N, is accompanied by a band of prograde zonal winds of up to 50 m s−1. At high latitudes, converging horizontal winds generate regions of strong subsistence. We find thermospheric dynamics to be sensitive to coupling from below. CH4 abundances are enhanced in the northern polar region, which may result from transport by thermospheric winds.

Large-Scale Structure of the June–July 1996 Marine Boundary Layer along California and Oregon

Abstract Data from surface stations, profilers, long-range aircraft surveys, and satellites were used to characterize the large-scale structure of the marine boundary layer off of California and Oregon during June and July 1996. To supplement these observations, June–July 1996 averages of meteorological fields from the U.S. Navy’s operational Coupled Ocean–Atmospheric Mesoscale Prediction System (COAMPS) model were generated for the region. Model calculations show a broad band of fast northerly surface winds exceeding 7 m s−1 extending along the California–Oregon coast. Buoy-measured peaks of 7.1 m s−1 off Bodega Bay, 7.2 m s−1 off Point Piedras Blancas, and 8.8 m s−1 near Point Conception were reported. Mean winds at the buoys located 15–25 km offshore are generally faster than those at coastal stations, and all station winds are faster in the afternoon. The aircraft and station observations confirm that an air temperature inversion typically marks the top of the marine boundary layer, which deepens offs...

Numerical simulations of the Piedmont flood of 4–6 November 1994

A case study of the 1994 Piedmont flood is carried out by performing several numerical experimentsusing the MM5 model. We report on sensitivity tests and analyses that suggest thefollowing description of the events leading to the flood: rising motion ahead of an upper-leveltrough, progressing from Spain towards Italy, initiated deep convection within the conditionallyunstable air existing over the Mediterranean. Thus, a band of intense precipitation and lowlevelsoutherly winds formed and moved eastward with the upper trough. As the rain band andstrong southerlies encountered the western Alps, moist air was lifted, resulting in large amountsof rainfall. The associated latent-heat-related upward motion produced a strong positive vorticityanomaly over the western Alps with associated low-level easterlies in the Po Valley whichappear to have played a major role in retarding the generally eastward progress of the rainband over Piedmont.

The Influence of Mesoscale Features of the Sea Surface Temperature Distribution on Marine Boundary Layer Winds off the Scotian Shelf during the Superstorm of March 1993

Abstract This paper studies the mesoscale wind field during the blizzard of March 1993 off the east coast of North America and examines the influence of the sea surface temperature distribution on surface winds. Can the Gulf Stream and its meanders, by its strong influence on the marine boundary layer, generate mesoscale features in the wind field? Numerical simulations of the storm are carried out using the MC2, a fully elastic nonhydrostatic model. Simulations are conducted at different resolutions (50, 25, 10, 5, and 2 km) with both detailed and smoothed SST fields, so as to examine the influence of these parameters on the marine boundary layer winds. Results from these numerical simulations are compared with surface observations from buoys. The study reveals some mesoscale features in the wind field caused by the Gulf Stream’s meanders and the warm eddies of the SST field. In a stable boundary layer, the meanders shaped a 50–55-kt (26–28 m s−1) band of winds in a general 40–45-kt (21–23 m s−1) wind fi...

Meteorology of the polar vortex: Spring 1997

The 1996–1997 northern hemisphere spring polar vortex was very strong, cold, and symmetric, somewhat similar to those found in the Antarctic spring vortex. The vortex did not form until late in December and was very symmetric from February into late April. The spring vortex was characterized by record low temperatures, record low ozone amounts as measured from the Total Ozone Mapping Spectrometer (TOMS) instruments, and a wide band of strong winds in the lower stratosphere. Spring wave activity was greatly reduced, with 100 hPa February–March eddy heat fluxes that were lower by a factor of 2 from any previously observed values over the last 18 years.

Equatorial variability and resonance in a wind‐driven Indian Ocean model

A numerical isopycnal ocean model has been designed and applied to model the Indian Ocean north of 25°S. Vertical normal modes are used in the open boundary conditions and for selections of initial layer depths. A 21‐year integration with a reduced Hellerman‐Rosenstein monthly averaged wind stress has been made with 3.5‐layer and 1.5‐layer versions of the model. Both solutions reproduce the main features of the observed wind‐driven seasonal circulation in the Indian Ocean above the main thermocline. The transient semiannual equatorial surface jets are more intense, more coherent, and in better phase agreement with observations when three layers are active. The associated undercurrents below the main thermocline are also included in the 3.5‐layer model solution. Second baroclinic‐mode, reflecting, equatorial Kelvin and Rossby waves combine to give a semiannual, resonant basin mode. Experiments with an equatorial band of semiannual zonal winds suggest a very strong response of the Indian Ocean to wind forcing with this period. Further, the amplitudes of the 28–30 day oscillations in the western equatorial model region are found to be strongly damped with depth; they have upward phase propagation and downward energy propagation.

A numerical study of wind stress curl effects on eddies and filaments off the northwest coast of the Iberian Peninsula

A high-resolution, multi-level, primitive equation ocean model is used to examine the response to the particular conditions of anticyclonic (negative) wind stress curl and equatorward (upwelling favorable) winds along the eastern ocean boundary off the northwest coast of the Iberian Peninsula in the vicinity of the highly productive rias. A band of steady equatorward winds, which are uniform alongshore but contain zonal variability (i.e., anticyclonic wind stress curl), results in an equatorward coastal surface current nearshore and a poleward surface current offshore. In time, the currents become unstable and lead to the development of anticyclonic warm core eddies. In addition, cold core filament formation occurs in areas where the anticyclones advect the upwelled coastal water offshore. The results from the experiments support the hypothesis that wind forcing, particularly the combination of anticyclonic (negative) wind stress curl and equatorward (upwelling favorable) winds, is an important mechanism for generating opposing alongshore surface currents, anticyclonic eddies, and filaments off the northwest coast of the Iberian Peninsula. In a sensitivity study, the results from three numerical experiments are examined to understand, in a limited way, the dependence of the generation of the surface currents on the boundary conditions (free slip and no slip), the use of wind band forcing and the imposed wind stress curl distribution. Results show that, regardless of whether free-slip or no-slip boundary conditions or wind band forcing are used, opposing alongshore surface currents are generated. Because such currents do not develop when there is no wind stress curl, the opposing currents are generated by the anticyclonic wind stress curl.

A wave-like vortex in a cold air stream

A situation is presented in which the satellite images seem to display all the ingredients of a wave on the polar front. It is shown that the cloud features, although situated entirely in the cold air, contained weak frontal characteristics. The system's size, the associated narrow band of strong winds ahead of it and the precipitation pattern, however, match those of some polar lows. It is demonstrated that the shape of the system was due to semi-horizontal deformation, whereas the frontal characteristics were the result of slight baroclinicity in the cold air stream.

A wave-like vortex in a cold air stream

A situation is presented in which the satellite images seem to display all the ingredients of a wave on the polar front. It is shown that the cloud features, although situated entirely in the cold air, contained weak frontal characteristics. The system's size, the associated narrow band of strong winds ahead of it and the precipitation pattern, however, match those of some polar lows. It is demonstrated that the shape of the system was due to semi-horizontal deformation, whereas the frontal characteristics were the result of slight baroclinicity in the cold air stream.

On the Existence of Atmospheric Vortices Downwind of Hawaii during the HAMEC Project

Abstract Aircraft observations west of the island of Hawaii in June 1980 during the Hawaii Mesoscale Energy and Climate (HAMEC) Project have provided the first in situ measurements of airflow within atmospheric vortices downwind of a tall island. A band of low-level westerly winds was observed to extend more than 150 km west of the island along the axis line separating cyclonic vortices to the north of that line from anticyclonic vortices to the south. The theoretical downstream propagation speed of those vortices is obtained from the solution to a quadratic equation, and while previous satellite studies of atmospheric vortices used the larger root (i.e., ∼80% of the ambient flow), the present data are consistent with the smaller root ∼20%). The turbulent Reynolds number for the flow is 140, and the corresponding vortex shedding time is 32 h.

On the reality of the Venus winds

An examination of the effect of assumptions in the interpretation of the Venera wind data is made as a rebuttal to the suggestion by A.T. Young that the 140 m/sec Venera 8 horizontal wind at 45 km may be either spurious or anomalous. The Venera measurements of wind speed along with the Mariner measurements of a lower region of strong turbulence are evidence for a wide band of variable high-speed retrograde horizontal winds which girdle Venus at the equator. In the prevalent interpretation of the Mariner 10 uv photographs, the region of the top of the visible cloud is characterized by variable high-speed retrograde horizontal winds which orbit Venus with an average period of 4 Earth days, and by many features indicating vertical convection. This interpretation, together with the possibility of atmospheric corotation due to frictional coupling, suggests that the Venera-Mariner band of winds at 45 km extends well beyond the top of the visible cloud, and that the upper region of strong turbulence detected by the Mariners may result in part from vertical convection currents carried along by high-speed horizontal winds. In an alternate interpretation of the Mariner 10 uv photographs Young suggests that the predominant motions may be traveling wavelike disturbances with a 4-day period rather than bulk motion of the atmosphere. For this case the upper region of strong turbulence is interpreted as due mostly to vertical wind shear resulting from a rapid decrease in wind speed within a relatively short distance above the Venera-Mariner band of high-speed winds.

The axis of the tropical easterly jet stream over India and Ceylon

Rawin observations of a number of stations for the months June to September for the five-year (1961-65) have been analysed to study the vertical structure of the tropical easterly jet stream over the Peninsular India and over the regions to the south of it upto the equator. Study of the daily rawin trajectories of Madras and Trivandrum at intervals of 0. 5 kill upto 17 km shows that the axis ( core) of the easterly jet stream (altitude about 14.5 km) is nearer to Trivandrum in June and August and nearer Colombo in September. In July there is a broad band of high winds. It is found that in this region Trivandrum mean maximum wind is highest in all the months June to September, the maximum occurring generally at about 0.5 km higher at Madras than over Trivandrum and Colombo. Observations at Goa (Lat. 15° 29'N) have shown simultaneous existence of another speed maximum in the neighbourhood of Goa at about 16 km.

The Circulation of the Tropical Stratosphere in the Years 1954–1960

Abstract The circulation is described in terms of three components: the long-term mean, the annual cycle and a newly-discovered 26-month cycle, referred to as the equatorial stratosphere wind oscillation. Mean winds are everywhere easterly, except that very weak westerlies may possibly exist near the equatorial tropopause. The easterlies increase with height and are strongest between latitudes 10N and 15N. The amplitude of the annual cycle rises from near zero at the equator to a maximum of about 10 m/sec at approximately 25N and undergoes little change with height. Peak easterlies occur in late July or early August. The amplitude of the 26-month cycle is greatest near the equator, attaining a value in excess of 20 m/sec at the 25-mb level. Below this level the amplitude decreases, and the oscillation fades away in the vicinity of the tropopause. In the poleward direction it is still faintly detectable near 30N. The phase varies with height, each band of easterly or westerly winds appearing first at the h...

The Jet Chart

In a previous paper [5] the authors have indicated the observed relationship between the jet at 200 mb, the −60C isotherm at 200 mb and tornado occurrences. The location of this jet has become important in severe local storm forecasting procedures. Often, the lack of a band of strong winds at 500 mb, especially during the summer months, made this level ineffective as an aid in locating the jet some height above. The 20-mb data are not transmitted until more than six hours after time of observation; thus, the formulation of a chart which is not a constant-pressure chart and called by SELS “The JET CHART” was conceived to remedy this situation. The preparation, analysis, and use of this chart in severe local weather forecasting are set forth here.

THE GENERAL CIRCULATION OF THE TROPICAL AND EQUATORIAL ATMOSPHERE

Abstract Recent experiences with tropical meteorology around the world have brought to light the frequent existence of a zonal band of west winds in the vicinity of the equator, which appears not to be associated with the westerlies of middle latitudes, but which rather seems to be found between the trades of the two hemispheres. The boundaries of the equatorial westerlies are usually characterized by increased cloudiness and poor weather. In this paper an attempt is made to prove that the westerlies of equatorial regions are not simply transitory phenomena but instead should be considered as an integral part of the normal general circulation of the atmosphere. Seasonal variation of the latitude of maximum insolation is the factor introduced to explain the existence of the west winds. A second explanatory theory is discussed in which radiation from the cloud tops of the “intertropical convergence zone” locally reverses the equatorial solenoidal field to produce two new lines of convergence, one on each si...