Search results for "Jet stream"
showing 10 items of 11 documents
On the occurrence of strong vertical wind shear in the tropopause region: a 10-year ERA5 northern hemispheric study
2021
A climatology of the occurrence of strong wind shear in the upper troposphere–lower stratosphere (UTLS) is presented, which gives rise to defining a tropopause shear layer (TSL). Strong wind shear in the tropopause region is of interest because it can generate turbulence, which can lead to cross-tropopause mixing. The analysis is based on 10 years of daily northern hemispheric ECMWF ERA5 reanalysis data. The vertical extent of the region analyzed is limited to the altitudes from 1.5 km above the surface up to 25 km, to exclude the planetary boundary layer as well as strong wind shear in higher atmospheric layers like the mesosphere–lower thermosphere. A threshold value of St2=4×10-4s-2 of t…
Analysis of a jet stream induced gravity wave associated with an observed ice cloud over Greenland
2004
International audience; A polar stratospheric ice cloud (PSC type II) was observed by airborne lidar above Greenland on 14 January 2000. It was the unique observation of an ice cloud over Greenland during the SOLVE/THESEO 2000 campaign. Mesoscale simulations with the hydrostatic HRM model are presented which, in contrast to global analyses, are capable to produce a vertically propagating gravity wave that induces the low temperatures at the level of the PSC afforded for the ice formation. The simulated minimum temperature is ~8 K below the driving analyses and ~4.5 K below the frost point, exactly coinciding with the location of the observed ice cloud. Despite the high elevations of the Gre…
Rossby Wave Packets on the Midlatitude Waveguide-A Review
2018
Abstract Rossby wave packets (RWPs) are Rossby waves for which the amplitude has a local maximum and decays to smaller values at larger distances. This review focuses on upper-tropospheric transient RWPs along the midlatitude jet stream. Their central characteristic is the propagation in the zonal direction as well as the transfer of wave energy from one individual trough or ridge to its downstream neighbor, a process called “downstream development.” These RWPs sometimes act as long-range precursors to extreme weather and presumably have an influence on the predictability of midlatitude weather systems. The paper reviews research progress in this area with an emphasis on developments during…
Evidence of small-scale quasi-isentropic mixing in ridges of extratropical baroclinic waves
2019
Abstract. Stratosphere–troposphere exchange within extratropical cyclones provides the potential for anthropogenic and natural surface emissions to rapidly reach the stratosphere as well as for ozone from the stratosphere to penetrate deep into the troposphere, even down into the boundary layer. The efficiency of this process directly influences the surface climate, the chemistry in the stratosphere, the chemical composition of the extratropical transition layer, and surface pollution levels. Here, we present evidence for a mixing process within extratropical cyclones which has gained only a small amount of attention so far and which fosters the transport of tropospheric air masses into the…
Recent changes in air temperature, heat waves occurrences, and atmospheric circulation in Northern Africa
2013
This study documents the time evolution of air temperature and heat waves occurrences over Northern Africa for the period 1979-2011. A significant warming (1°-3°C), appearing by the mid-1960s over Sahara and Sahel, is associated with higher/lesser frequency of warm/cold temperatures, as with longer duration and higher occurrences of heat waves. Heat waves episodes of at least 4 day duration have been examined after removing the long-term evolution. These episodes are associated with specific anomalies: (i) in spring, positive low-level temperature anomalies over the Sahel and Sahara; low and midlevel cyclonic rotation over Morocco associated with a Rossby wave pattern, lessening the Harmatt…
Global Climatologies of Eulerian and Lagrangian Flow Features based on ERA-Interim
2017
Abstract This paper introduces a newly compiled set of feature-based climatologies identified from ERA-Interim (1979–2014). Two categories of flow features are considered: (i) Eulerian climatologies of jet streams, tropopause folds, surface fronts, cyclones and anticyclones, blocks, and potential vorticity streamers and cutoffs and (ii) Lagrangian climatologies, based on a large ensemble of air parcel trajectories, of stratosphere–troposphere exchange, warm conveyor belts, and tropical moisture exports. Monthly means of these feature climatologies are openly available at the ETH Zürich web page (http://eraiclim.ethz.ch) and are annually updated. Datasets at higher resolution can be obtained…
The effects of the Southwest Indian Ocean tropical cyclones on Ethiopian drought
1998
Tropical cyclones are one of the prominent weather systems that are generated over the tropical oceans. The cyclones that develop in the Southwest Indian Ocean (SWIO) usually travel west then southwest and finally recurve to southeast, generally before reaching the East African coast. However, it is shown in this study how SWIO-tropical cyclones/depressions can indirectly affect Ethiopian weather. Using correlation and composite analyses, interannual and intraseasonal rainfall variations in Ethiopia were compared with the frequency and time of occurrence of the tropical cyclones. Years showing the consecutive occurrence of several tropical depressions over the SWIO coincide with the drought…
Tropopause level Rossby wave breaking in the Northern Hemisphere: a feature-based validation of the ECHAM5-HAM climate model
2012
Breaking synoptic-scale Rossby waves (RWB) at the tropopause level are central to the daily weather evolution in the extratropics and the subtropics. RWB leads to pronounced meridional transport of heat, moisture, momentum, and chemical constituents. RWB events are manifest as elongated and narrow structures in the tropopause-level potential vorticity (PV) field. A feature-based validation approach is used to assess the representation of Northern Hemisphere RWB in present-day climate simulations carried out with the ECHAM5-HAM climate model at three different resolutions (T42L19, T63L31, and T106L31) against the ERA-40 reanalysis data set. An objective identification algorithm extracts RWB …
Tropical–extratropical interactions related to upper-level troughs at low latitudes
2007
Abstract Momentum and kinetic energy fluxes associated with low-latitude transient disturbances at upper-levels play an important role in the general circulation of the atmosphere. They are related to eastward and equatorward propagating, positively tilted wave trains from the extratropics. Theoretical, modelling and observational studies show that this particular kind of tropical–extratropical interaction is most common in regions of mean upper-level westerlies at low latitudes, i.e. over the central and eastern Pacific and Atlantic Oceans during boreal winter and spring. The penetration of an upper-level trough into the Tropics is often associated with enhanced convection and the formatio…
Recurrent daily rainfall patterns over South Africa and associated dynamics during the core of the austral summer
2010
This paper investigates the influence of some modes of climate variability on the spatio-temporal rainfall variability over South Africa during the core of the rainy season, December to February (DJF). All analyses are based directly on the rainfall field instead of atmospheric processes and dynamics. An original agglomerative hierarchical clustering approach is used to classify daily rainfall patterns recorded at 5352 stations from DJF 1971 to DJF 1999. Five clusters are retained for analysis. Amongst them, one cluster looks most like the rainfall and circulation mean picture. Another one, representing 37% of the days, describes strong negative rainfall anomalies over South Africa resultin…