Jet Stream Maximum Research Articles

Long-term upper-troposphere climatology of potential contrail occurrence over the Paris area derived from radiosonde observations

Abstract. Condensations trails (or contrails) that form behind aircraft have been of climatic interest for many years; yet, their radiative forcing is still uncertain. A number of studies estimate the radiative impact of contrails to be similar to, or even larger than, that of CO2 emitted by aviation. Hence, contrail mitigation may represent a significant opportunity to reduce the overall climate effect of aviation. Here we analyze an 8-year data set of radiosonde observations from Trappes, France, in terms of the potential for contrail and induced cirrus formation. We focus on the contrail vertical and temporal distribution and test mitigation opportunities by changing flight altitudes and fuel type. Potential contrail formation is identified with the Schmidt–Appleman criterion (SAc). The uncertainty of the SAc, due to variations in aircraft type and age, is estimated by a sensitivity study and is found to be larger than the radiosonde measurement uncertainties. Linkages between potential contrail formation layers and the thermal tropopause, as well as with the altitude of the jet stream maximum, are determined. While non-persistent contrails form at the tropopause level and around 1.5 km above the jet stream, persistent contrails are located approximately 1.5 km below the thermal tropopause and at the altitude of the jet stream. The correlation between contrail formation layers and the thermal tropopause and jet stream maximum allows to use these quantities as proxies to identify potential contrail formation in numerical weather prediction models. The contrail mitigation potential is tested by varying today's flight altitude distribution. It is found that flying 0.8 km higher during winter and lowering flight altitude in summer reduces the probability for contrail formation. Furthermore, the effect of prospective jet engine developments and their influence on contrail formation are tested. An increase in propulsion efficiency leads to a general increase in the potential occurrence of non-persistent and persistent contrails. Finally, the impact of alternative fuels (ethanol, methane, and hydrogen) is estimated and found to generally increase the likelihood of non-persistent contrails and, to a more limited extent, persistent contrails.

Diabatic generation of negative potential vorticity and its impact on the North Atlantic jet stream

AbstractLocalised regions of negative potential vorticity (PV) are frequently seen on the equatorward flank of the upper‐tropospheric jet streams in analysis and forecast products. Their positioning, on the anticyclonic side of the jet and often close to the jet core, suggest they are associated with an enhancement of jet stream maximum winds. Given that PV is generally positive in the northern hemisphere and is materially conserved under adiabatic conditions, the presence of negative PV is indicative of recent diabatic activity. However, little is understood on the mechanisms for its generation and subsequent impacts. In this paper, aircraft measurements from a recent field campaign are used to provide direct observational evidence for the presence of negative PV on the anticyclonic side of an upper‐tropospheric jet. Theory is then developed to understand the process by which PV can turn negative. The key ingredient is diabatic heating in the presence of vertical wind shear, and the resulting PV anomalies are shown to always result from a flux of PV directed “down the isentropic slope”. This explains why, for the typical situation of heating in a warm conveyor belt, negative PV values appear on the equatorward side of the upper‐tropospheric jet stream close to the jet core. These ideas are illustrated with a semi‐geostrophic model and the processes responsible for the observed negative PV are explored using an operational forecast model with online PV tracer diagnostics. The diabatic influence on jet stream winds and shear is of interest because it is pertinent to the predictability of extreme jet stream events and associated flight‐level turbulence, and is crucial to the propagation of Rossby waves at tropopause level, development of midlatitude weather systems and their subsequent impacts at the surface.

Strong Mixing Events Observed near the Tropopause with the MU Radar and High-Resolution Balloon Techniques

Abstract In the present paper, strong isotropic radar echo enhancements are shown that are related to the existence of nearly statically neutralized layers (40–120 m thick) observed with high-resolution (10 cm) temperature measurements performed during the Middle and Upper Atmosphere Radar, Temperature Sheets and Interferometry (MUTSI) campaign (8–26 May 2000, Japan). These events have been observed in the immediate vicinity of the tropopause, just above a jet stream maximum. They most likely result from strong turbulent mixing generated by shear instabilities and could be one of the mechanisms responsible for the generation of strong temperature gradients (temperature gradient sheets) at the mixed layer edges. Further investigations will be necessary to evaluate the occurrence of this neutralization mechanism and its contribution to the generation of strong temperature gradient sheets within the free atmosphere.

Simulation of Extratropical Synoptic-Scale Stratosphere–Troposphere Exchange Using a Coupled Chemistry GCM: Sensitivity to Horizontal Resolution

Abstract The transport of ozone between the stratosphere and troposphere has been studied with a coupled chemistry GCM (ECHAM4) for two horizontal resolutions (T30: 3.75° × 3.75° and T63: 1.875° × 1.875°). The meteorological conditions pertain to the first half of March 1996, when a large amplification of waves in the upper troposphere resulted in the development of two upper-level subtropical cyclonic vortices in the Northern Hemisphere over the Atlantic Ocean. The focus is on the sensitivity of the model simulations to the horizontal model resolution. A finer, more detailed structure was obtained for the calculated meteorological fields (especially the “streamers” of high potential vorticity (PV) and low specific humidity associated with synoptic disturbances) when the higher resolution was applied. In quantitative terms, main differences were found in the upper-tropospheric PV values and the jet stream maxima (higher at T63). Overall, the higher-resolution results were in excellent agreement with Europ...

Characteristics of the large-scale circulation changes during the sudden onset of the fall transition season

The characteristics of large-scale circulation changes manifest in the abrupt onset of the fall transition season are analysed for the Northern Hemisphere extratropical region. Ten years of daily observational data from the European Centre for Medium Range Weather Forecasts (ECMWF) global model initialization analyses are examined using the vertically integrated total kinetic energy (TKE) as the primary descriptor for circulation intensity. The time series data are bandpass filtered to examine subsets of frequencies representing specific atmospheric time scales, i.e. the low-frequency tropical forced oscillations (29.9–59.9 days), the intermediate-frequency oscillations related to non-linear processes such as weather regimes (10.0–29.7 days) and the high-frequency extratropical synoptic scale (2.5–6.5 days). Times when significant increases in kinetic energy (KE) variability occurred in the area-weighted hemispheric-mean time series for each bandpass were determined, and the onset date for each year and for each bandpass period was designated as the first significant increase in KE variability during the 1 August–30 November time period. Statistically significant abrupt onset events are found, with the majority occurring in August, September and October. The mean onset times vary from 3 October for the 29.9–59.9-day bandpass period to 30 August for the 2.5–6.5-day bandpass period. Composites with respect to the onset time show robust features of the abrupt onset event in both the hemispheric-mean time series and the spatial patterns of KE variability. Centres of maximum change in the magnitude of KE variability are in the western and central Pacific and the Atlantic Ocean sectors coincident with the areas of jet stream maxima. Separate analyses for onset characteristics for area-mean KE time series for the Pacific and Atlantic Ocean areas show that there is little correlation between Pacific and Atlantic onset times for the 29.9–59.9-day bandpass period. However, for the 10.0–29.7-day bandpass period the Atlantic onset times tend to lead the Pacific onset times, while for the 2.5–6.5-day bandpass period the Pacific region onset times tend to occur prior to the Atlantic onset times. Copyright © 2000 Royal Meteorological Society

Southern Hemisphere Storm Tracks, Blocking, and Low-Frequency Anomalies in a Primitive Equation Model

Abstract The results of a study are presented that indicate three-dimensional instability theory is able to generate analogs of a wide variety of Southern Hemisphere observed fluctuations including those associated with cyclogenesis, blocking, and low-frequency teleconnection patterns. This study has been conducted with a two-level primitive equation eigenvalue model and the growing modes for both January and July averaged basic states are examined. In both seasons, the fastest-growing cyclogenesis modes have largest amplitudes in the eastern part of the hemisphere just downstream of the polar jet stream maxima. In July, there is a tendency to form elongated eddies in the region downstream of Australia where growth on both polar and subtropical jet streams occurs. These results are in general agreement with the observational studies of Southern Hemisphere storm tracks. For both January and July, larger-scale slower-propagating dipole or multipole modes are found that are consistent with blocking in some o...

Forced Planetary Waves in the Northern Hemisphere Winter: Wave-Coupled Orographic and Thermal Forcings

Abstract A more complete and new formulation of the orographic forcing and new thermal forcings are included in a steady state model of the Northern Hemisphere planetary waves. When both forcings are included, the simulation produces excellent results which are compared in detail with observations. In particular, the Siberian high, the tropospheric East Asian trough and subtropical tropospheric East Asian jet stream maxima are well reproduced even though the forcing is primarily extratropical in origin. The modes uses a lower boundary condition in which the orographic forcing is determined by the effects of the total flow, not just the zonal mean basic state. Consequently, the net orographic forcing changes when thermal forcing is added and the tow solution is not equal to the linear sum of the solutions with orographic and thermal forcings separately. The thermally induced orographic forcing is found to be very significant and, in the troposphere, there is strong interaction between the two forcings with...

The Geographical Locations of Southern Hemisphere Storm Tracks: Linear Theory

The instability properties of a three-dimensional climatological January Southern Hemisphere flow field are examined using a five-level spherical quasi-geostrophic spectral model. The growth rates, phase frequencies, disturbance streamfunctions and eddy momentum and heat fluxes are studied for the eight fastest growing modes, all of which we monopole cyclogenesis modes. There is reasonable agreement between instability theory and observations as far as the geographical locations of the storm tracks and eddy fluxes are concerned. The principal storm track is located in the eastern part of the hemisphere slightly downstream and poleward of the jet stream maxima. The usual vertical structure problem of instability theory occurs with the theoretical disturbance streamfunctions and eddy fluxes being relatively too large at the surface compared with values at the tropopause.

A Unified Three-Dimensional Instability Theory of the Onset of Blocking and Cyclogenesis

Abstract The instability characteristics of three-dimensional flow typical of the Northern Hemisphere average winter troposphere are examined in a two-layer spherical quasigeostrophic model. The properties of the four fastest-growing small-amplitude disturbances that develop on the basic state have been analyzed for three cases (case 1, 2a and 3) having increasingly larger static stability parameters. For case 1, the fastest-growing disturbance mode, which is propagating eastward rather rapidly, has a monopole cyclogenesis structure with maximum amplitudes slightly downstream of the jetstream maxima in the Pacific ocean and off the cast coast of North America. Comparisons of streamfunctions squared and momentum and heat fluxes with bandpass filtered observations for transient eddies, which pick out the developing storms, are quite reasonable considering that a two-layer model is used and the contributions from individual linear modes are considered. For case 2a, the fastest growing mode has large-scale hi...

Isolation of the Gravity-Inertial Motion Component in a Nonlinear Atmospheric Model

Abstract Diagnostic methods me considered for isolating gravity-inertial motions in the output of a nonlinear atmospheric numerical model. The gravity-inertial component is defined by the nongeostrophic motions not directly incorporated with the synoptic-scale evolution according to quasi-scostrophic or balance model relationships. The analysis methods am applied to the solutions for a propagating jet stream maximum generated by a simple two-layer hydrostatic numerical model. Results identify a coherent pattern in the gravity-inertial motion component but details of the horizontal structure and propagation characteristics are only partially resolved. Results also elucidate relative merits of a number of physical variables and difference fields for defining the gravity-inertial component.

Northeasterly Cold Surges and Near-Equatorial Disturbances over the Winter MONEX Area During December 1974. Part II: Planetary-Scale Aspects

Abstract In Part I of this pre-winter MONEX study a synoptic analysis on the interaction between cold air surges off the East Asian continent and the synoptic-scale disturbances, especially their associated convection, in the equatorial South China Sea during December 1974 was carried out. In this paper we investigate some kinematic characteristics of the planetary-scale circulation changes during the same period in the midlatitudes and tropics in relation to the cold surge and the synoptic-scale disturbances. The main findings suggest a picture of coherent variations of several planetary-scale circulation features which may he described by the following sequence of events: 1) The East Asia local Hadley circulation strengthens as the cooling due to cold air advection over northern China increases prior to a cold surge on the South China coast. This is accompanied by an intensified jet stream maximum centered over Japan, which is apparently a result of the upstream acceleration by the upper level poleward ...

Sudden stratospheric warmings forced by mountains

A global, quasigeostrophic model of planetary wave interaction with the mean zonal flow has been used to simulate sudden stratospheric warmings forced by mean tropospheric zonal winds interacting with orography. Planetary waves m = 1 and 2 are simultaneously computed and interact with each other only through the mean zonal flow. Orographic forcing is determined by the tropospheric wind field. An increase in the jet stream maximum from 36 ms−1 to 48 ms−1 is sufficient to initiate repeated major warmings with an ∼80 day cycle. These results are in good agreement with the observed anti‐correlation of the annual variation of the winter monthly averaged jet stream strength and polar night jet strength, which is interpreted to be a direct consequence of the increased planetary wave stresses on the mean flow in the stratosphere.

Concentrations of condensation nuclei in the vicinity of jetstream maxima

Ozone and condensation nuclei were measured on 29 March 1977 (and 30 March GMT) from a Sabreliner in a frontal zone south of Denver, Colorado. The ozone served as a tracer for stratospheric air and the condensation nuclei as a tracer for tropospheric air. Both the Langer condensation nucleus counter and the Rich-100 counter can be used to measure condensation nucleus concentrations in the vicinity of the tropopause. However, the former counter is more sensitive to low concentrations. The nuclei measured were found to be almost entirely larger than 0.005 μm in radius, consisting of thousands of molecules. The mesostructure of the ozone and condensation nucleus concentrations agreed well with a double structure of the frontal zone that became apparent from a detailed analysis of radiosonde data.

The Effects of Long Planetary Waves on the Regions of Cyclogenesis: Linear Theory

Abstract A study is made of the effect of a long Planetary wave on the regions of preferential development, eddy momentum and heat fluxes, and the growth rates and phase speeds of growing baroclinic disturbances. The model used is a linear, spherical, two-layer quasi-geostrophic model with a basic state consisting of a 30° jet and an upper layer long planetary wave which together provide an approximate representation of the observed average Northern Hemisphere winter flow. The results of the baroclinic instability theory are compared with the observed regions of most actively developing baroclinic disturbances and eddy momentum and heat fluxes. Considering the crude vertical structure of two-layer models, the agreement between the theoretical and observed results is noteworthy. Details such as the observed latitudinal gaps between the jet-stream maxima and the regions of most actively developing eddies are reproduced in the model. Both these gaps and much of the horizontal variations of the theoretical an...

Vertical Wind Velocities from Superpressure Balloons: A Case Study using Eole Data

A method for determining vertical wind velocities using the pressure- and temperature-change data from superpressure (constant-density) balloons was proposed in an earlier paper (1975). In this paper, we present a case study using actual balloon data to test the method. Data from the Eole balloon experiment were used to estimate the sign of vertical wind velocities, as well as the horizontal velocity field, near 200 mb over Australia. The resulting patterns of vertical velocity compare favorably with satellite pictures and with current models of flow near jet-stream maxima. Additionally, this study provides evidence that areas of vertical motion in the troposphere extend upward into the lower stratosphere without changing sign.

Statistical Characteristics of Subtropical jet-Stream Features in Terms of MRIR Observations from Nimbus II

A sample of 60 individual cases of southwesterly jet-stream maxima located over the continental United States during the Nimbus II period is studied in relation to the MRIR characteristics surrounding the jet. The analysis has been conducted primarily from a statistical point-of-view applied to the equivalent black-body temperature (TBB) patterns of channels 1, 2 and 4, and to hypsometric combinations of these TBB values representing layer-mean temperatures. It was found that certain grow features of the fields of convergence and divergence associated with the moisture distribution in certain quadrants surrounding the jet are consistently indicated in these patterns. A stepwise, multiple regression analysis for specification of the maximum wind speeds gives predictive skill in several channels, particularly in the 5–30 μ channel, indicating that baroclinic effects can be discerned by MRIR observations applied to suitably stratified data samples.

Clear-Air Turbulence Near the Jet-Stream Maxima *

The development of turbo-jet aircraft has made high-level clear air turbulence a major problem for aviation interests. This paper emphasizes the association of the majority of this turbulence with the pronounced vertical wind shear in and near the maximum wind speed centers that move along the jet stream. A physical model is proposed as a possible explanation of clear air turbulence, the associated cirrus bands and wind streaks in the jet maxima. This model is supported by an analogy drawn with similar low-level phenomena studied by Woodcock and others. The model can explain distribution of these features in the horizontal by means of helical vortices which are dependent upon proper vertical wind shear and stability conditions. The observed multiple layers in the vertical are also explained by this model. It is believed that the reason why most of the clear-air turbulence is found near the jet-stream maxima is simply because the necessary shear and stability conditions associated with this turbulence are most frequently fulfilled in that region.