Search results for "Extratropical cyclone"
showing 10 items of 50 documents
Quantifying transport into the lowermost stratosphere using simultaneous in-situ measurements of SF<sub>6</sub> and CO&am…
2009
Abstract. The seasonality of transport and mixing of air into the lowermost stratosphere (LMS) is studied using distributions of mean age of air and a mass balance approach, based on in-situ observations of SF6 and CO2 during the SPURT (Spurenstofftransport in der Tropopausenregion, trace gas transport in the tropopause region) aircraft campaigns. Combining the information of the mean age of air and the water vapour distributions we demonstrate that the tropospheric air transported into the LMS above the extratropical tropopause layer (ExTL) originates predominantly from the tropical tropopause layer (TTL). The concept of our mass balance is based on simultaneous measurements of the two pas…
Cyclogenesis Downstream of Extratropical Transition Analyzed by Q-Vector Partitioning Based on Flow Geometry
2014
Abstract During extratropical transition (ET), tropical cyclones exert a significant impact on the midlatitude circulation. Archetypical features of this impact are jet streak formation, amplification of the downstream trough, and modification of the associated downstream cyclogenesis. This study investigates the relative importance of the jet streak and the upper-level trough for cyclone development by quantifying the respective contributions to midtropospheric vertical motion using the Q-vector partitioning by J. C. Jusem and R. Atlas. Their framework is here extended from quasigeostrophic theory to alternative balance. The Q vector under alternative balance involves the nondivergent wind…
Current and emerging developments in subseasonal to decadal prediction
2020
Weather and climate variations on subseasonal to decadal time scales can have enormous social, economic, and environmental impacts, making skillful predictions on these time scales a valuable tool for decision-makers. As such, there is a growing interest in the scientific, operational, and applications communities in developing forecasts to improve our foreknowledge of extreme events. On subseasonal to seasonal (S2S) time scales, these include high-impact meteorological events such as tropical cyclones, extratropical storms, floods, droughts, and heat and cold waves. On seasonal to decadal (S2D) time scales, while the focus broadly remains similar (e.g., on precipitation, surface and upper-…
Dynamics of tropical–extratropical interactions and extreme precipitation events in Saudi Arabia in autumn, winter and spring
2016
The Extratropical Transition of Tropical Cyclones. Part II: Interaction with the Midlatitude Flow, Downstream Impacts, and Implications for Predictab…
2019
Abstract The extratropical transition (ET) of tropical cyclones often has an important impact on the nature and predictability of the midlatitude flow. This review synthesizes the current understanding of the dynamical and physical processes that govern this impact and highlights the relationship of downstream development during ET to high-impact weather, with a focus on downstream regions. It updates a previous review from 2003 and identifies new and emerging challenges and future research needs. First, the mechanisms through which the transitioning cyclone impacts the midlatitude flow in its immediate vicinity are discussed. This “direct impact” manifests in the formation of a jet streak …
Dynamics and Predictability of a Heavy Dry-Season Precipitation Event over West Africa—Sensitivity Experiments with a Global Model
2009
Abstract In January 2002 the Cape Verde region in tropical West Africa was hit by an exceptionally heavy precipitation event. Rain rates of up to 116 mm (48 h)−1 caused harmful impacts on the local population. The rainfall was triggered by a series of two upper-level disturbances penetrating from the extratropics to the West African coast. This study investigates the dynamics and predictability of this event on the basis of simulations with the global model Global Model Europe (GME) of the German Weather Service [i.e., Deutscher Wetterdienst (DWD)] initialized by the European Centre for Medium-Range Weather Forecasts (ECMWF) analysis data. Free forecasts satisfactorily reproduce the upper-l…
A quantitative analysis of stratospheric HCl, HNO3, and O3 in the tropopause region near the subtropical jet
2014
The effects of chemical two-way mixing on the Extratropical Transition Layer (ExTL) near the subtropical jet are investigated by stratospheric tracer-tracer correlations. To this end, in situ measurements were performed west of Africa (25-32 ◦ N) during the Transport and Composition of the Upper Troposphere Lower Stratosphere (UTLS)/Earth System Model Validation (TACTS/ESMVal) mission in August/September 2012. The Atmospheric chemical Ionization Mass Spectrometer sampling HCl and HNO3 was for the first time deployed on the new German High Altitude and Long range research aircraft (HALO). Measurements of O3, CO, European Centre for Medium-Range Weather Forecasts (ECMWF) analysis, and the tig…
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…
Dry-Season Precipitation in Tropical West Africa and Its Relation to Forcing from the Extratropics
2008
Abstract Precipitation is a major socioeconomic factor in the Guineo-Soudanian zone of tropical West Africa with its distinct summer rainy season from May to October. Albeit rare, precipitation during the dry season can have substantial impacts on the local hydrology and human activities reaching from the rotting of harvests to improved grazing conditions. This study provides an observationally based synoptic and dynamical analysis of an abundant rainfall event during the dry season of 2003/04 that affected the countries of Nigeria, Benin, Togo, and Ghana. The results point to a forcing of the rainfalls from the extratropics in the following ways: 1) Upper-level clouds and moisture to the e…
Sharpness of the extratropical tropopause in baroclinic life cycle experiments
2007
[1] The sharpness of the extratropical tropopause is investigated using baroclinic life cycle experiments. In these simulations, slow synoptic scale dynamics leads to a net tropopause sharpening. The mechanism involves vertical convergence of the ageostrophic wind. The results support earlier suggestions from idealized theoretical studies.