Vicinity Of Jet Streams Research Articles

Past and Future Trends in Clear‐Air Turbulence Over the Northern Hemisphere

AbstractClear‐Air Turbulence (CAT) is associated with wind shear in the vicinity of jet streams in upper atmospheric levels. This turbulence occurs in cloudless regions and causes most weather‐related aircraft accidents. Recent studies have shown that in response to climate change, CAT could significantly increase over certain regions as a consequence of strengthening of jet streams. In this study we use several atmospheric reanalyses and coupled model experiments database to evaluate CAT recent and future changes in the Northern Hemisphere. Several CAT diagnostics are computed to assess the sensitivity of the results to different turbulence representations. A significant positive trend in CAT frequency is found in the reanalyses in different Northern Hemisphere regions over the period 1980–2021. The signal‐to‐noise analysis shows that over North Africa, East Asia and Middle East the increase of CAT occurrence in the last decades is likely attributed to the response to anthropogenic forcing. In contrast, over the North Atlantic and North Pacific the response to external forcing is not detectable due to a weak signal to noise ratio related to large internal variability. Future climate projections show that over several Northern Hemisphere regions, CAT is projected to increase with a high model agreement and independently of the CAT diagnostic used. The largest increase in CAT is projected to occur over East Asia. In the North Atlantic, large uncertainty remains due to lack of model agreement and different results between the various CAT diagnostics.

Mesoscale temperature fluctuations in the stratosphere

Abstract. An airborne instrument that measures altitude temperature profiles is ideally suited for the task of characterizing statistical properties of the vertical displacement of isentrope surfaces. Prior measurements of temperature fluctuations during level flight could not be used to infer isentrope altitude variations because lapse rate information was missing. The Microwave Temperature Profiler instrument, which includes lapse rate measurements at flight level as a part of temperature profiles, has been used on hundreds of flights to produce altitude versus ground track cross-sections of potential temperature. These cross-sections show isentrope altitude variations with a horizontal resolution of ~3 km for a >6 km altitude region. An airborne isentrope-altitude cross-section (IAC) can be compared with a counterpart IAC generated from synoptic scale data, based on radiosondes and satellite instruments, in order to assess differences between the altitudes of isentrope surfaces sampled at mesoscale versus synoptic scale. It has been found that the synoptic scale isentropes fail to capture a significant component of vertical displacement of isentrope surfaces, especially in the vicinity of jet streams. Under the assumptions that air parcels flow along isentrope surfaces, and change temperature adiabatically while undergoing altitude displacements, it is possible to compute mesoscale temperature fluctuations that are not present in synoptic scale back trajectory parcel temperature histories. It has been found that the magnitude of the mesoscale component of temperature fluctuations varies with altitude, season, latitude and underlying topography. A model for these dependences is presented, which shows, for example, that mesoscale temperature fluctuations increase with altitude in a systematic way, are greatest over mountainous terrain, and are greater at polar latitudes during winter.

Applications of the NERC MST radar facility in mesoscale studies

AbstractThe NERC radar facility located near Aberystwyth (52.4°N,4.1°W) was established for remote sensing of the structure and dynamics of the mesosphere, stratosphere and troposphere; hence the name MST radar. The facility records scatter and reflection from metre‐scale irregularities in refractive index produced by atmospheric waves and turbulence. The measurement of the three‐dimensional wind field at tropospheric and lower stratospheric heights represents the primary capability of the facility and is expected to be particularly useful in short‐range local weather forecasting operations. At the same time, the temporal and height resolution achievable lend themselves to studies of smaller‐scale, time‐varying phenomena, such as waves and turbulence. Particular attention has been paid to the characteristics and vertical propagation of mountain waves and inertia‐gravity waves, and estimates made of the vertical flux of horizontal momentum associated with such waves, known to play a significant role in the large‐scale atmospheric circulation. The turbulence associated with the dissipation of these waves, and also generated in the vicinity of jet streams, has also been examined. Quasi‐specular reflection of radar waves associated with changes in temperature gradients has helped monitor the height and sharpness of the tropopause and also the structure of frontal zones. The technical capabilities of the facility and the continuity of data provided by its all‐weather and extended operation have been shown to be well suited to both mesoscale meteorological and research applications. Copyright © 1999 Royal Meteorological Society

Atmospheric trace gas studies in Antarctica

Atmospheric trace gas studies in Antarctica have led to an increased understanding of the global effects from anthropogenic emissions of various compounds. These studies have also contributed to the identification of various atmospheric circulation patterns and mixing mechanisms. Time trend analyses have revealed that although the atmospheric concentrations of F‐11, F‐12, CH3CCl3, and other halocarbons are still increasing, their rate of increase has slowed from peak increasing rates observed in the 1970s. For CO2, CO, and CH4, time trend analyses have also been used to analyze seasonal concentration cycles and to establish relationships between sources and sinks at various times throughout the year. Latitudinal gradient studies have generally revealed that for most compounds with significant anthropogenic sources, atmospheric concentrations are higher in the northern hemisphere than in the southern hemisphere. This is mainly a result of the overwhelming majority of anthropogenic sources existing in the northern hemisphere. Vertical concentration profiles in Antarctica are dependent upon the seasonal variations in circulation patterns associated with final warming and breakup of the polar vortex. Since final warming and breakup of the polar vortex may not always occur at the same time each year, profiles for a given time period from separate years may be different. Stratospheric‐tropospheric air exchange over Antarctica occurs primarily as a result of (1) general circulation patterns with subsidence over the polar regions, (2) stratospheric air injection in the vicinity of jet streams and injection from tropopause folding upstream of troughs, and (3) mountain waves of sufficient magnitude to displace air parcels across the tropopause.

Vertical mass- and trace constituent transports in the vicinity of jet streams

Detailed analyses of radiosonde and aircraft data were used to identify regions of stratospheric air incursions into the troposphere and of tropospheric air inflow into the stratosphere. The latter flow phenomenon was associated with a distinct separation between potential vorticity tropopause and stability tropopause. It occurred in the region of confluence between jet branches of different origin and was connected with strong anticyclonic shears which are associated with the presence of a wind maximum above the stability tropopause.

On the Global Distribution of Aerosols

Aerosol concentrations were measured during a 54-hour circum-navigation of the earth at altitudes between 238 and 162 millibars and concurrently by a separate flight at lower altitudes over the Pacific Ocean. The aerosol concentrations were found to be symmetrically distributed about the earth. Concurrent meteorological measurements indicate that tropospheric aerosols enter the stratosphere in the vicinity of jet streams and that surface aerosols are carried aloft over the intertropical convergence and Antarctic polar front.

Trace Constituents in the Vicinity of Jet Streams

Abstract An extensive series of experiments is being undertaken to determine the concentrations and transport of trace atmospheric constituents in the stratosphere and troposphere in the vicinity of jet streams during tropopause “folding” episodes. The first experiments were undertaken in March and April 1968 and the results are described and discussed. Both propeller-driven aircraft and U. S. Air Force jet aircraft were used, supplemented by ozonesonde and lidar measurements. Trace constituents investigated included ozone, sulfates, various cations, freezing nuclei, aerosol particles in general, and radioactive nuclides. A high, positive correlation was observed between the intensity of radioactivity and the sulfate concentrations. Relative concentrations were consistent with transport of air from the troposphere to the stratosphere on the anticyclonic side of the jet stream. Concentrations of freezing nuclei were on the average much lower but much more uniform in the stratosphere than in the troposphere.

On the origin of ice nuclei

Aerosol particles from the Earth's surface, man-made aerosols, and particles of extraterrestrial origin are discussed as three independent sources of ice nuclei in the atmosphere. Results of airborne measurements of ice nucleus concentration show extreme variability of the concentration over long distances. On a few occasions the highest concentrations were observed in the vicinity of jet streams. In view of these findings, interest in the Findeisen-Bowen hypothesis is being revived and modifications proposed to account for experimental findings.

A Stratospheric Air Tracer Experiment Using Zinc Sulfide

The feasibility of using the fluorescent properties of zinc sulfide powder to study air motions in the vicinity of jet streams was studied by releasing 230 kg of the substance above the subtropical jet in central Australia and later in Western Australia. Zinc sulfide was detected in some of the air samples obtained at the ground in Australia and New Zealand as far as 7000 km from the source and at various altitudes over eastern Australia. It is concluded that this tracer could be useful for studying air motions on such a large scale but that transport mechanisms involving clouds may also be important. Very rapid downward transport was a feature of both experiments. A relationship appears to exist between ice nucleus concentrations and the fallout pattern of the zinc sulfide.

Cloud Distributions in the Vicinity of Jet Streams

Cloud observations made by crew members during flights of Project Jet Stream are used to determine the frequency of occurrence of various types of clouds in the vicinity of jet streams. These cloud...

A Frontal-Jet Stream Cross Section

The validity of the thermal wind relationship for determining high level winds in the vicinity of jet streams is discussed.
 A cross section wind field was constructed utilizing only the thermal field for levels above 700 mb. This wind field is compared to one obtained from rawin reports.
 The results indicate that a reasonable picture of the high level jet winds is obtained when vertical extrapolation is based on the thermal field analysed with respect to three-dimensional fronts.