Cyclone extratropical | EPFL Graph Search

vignette|Carte météorologique fictive d'un cyclone extratropical affectant la Grande-Bretagne et l'Irlande. Les flèches bleues et rouges entre les isobares indiquent la direction des vents, alors que le symbole D marque le centre de la dépression, soit l'endroit de la plus basse pression atmosphérique. Un cyclone extratropical ou dépression extratropicale est un système météorologique de basse pression, d'échelle synoptique, qui se forme entre la ligne des tropiques et le cercle polaire. Il est associé à des fronts, soit des zones de gradients horizontaux de la température et du point de rosée, que l'on nomme aussi zones baroclines. Les dépressions extratropicales ont des caractéristiques différentes des cyclones tropicaux, alimentés par la convection atmosphérique, et des cyclones polaires plus au nord. Ils sont en fait les dépressions météorologiques qui passent quotidiennement sur la majorité du globe. Avec les anticyclones, ils régissent le temps sur la Terre, produisant nuages, p

Impact of transatlantic transport episodes on summertime ozone in Europe

Isabelle Bey

This paper reports on the transport of ozone

{O}_{3}

and related species over the North Atlantic ocean and its impact on Europe. Measurements of nitrogen dioxide (

{NO}_{2}

)and carbon monoxide (CO) columns from the GOME and MOPITT satellite instruments, respectively, are used in conjunction with the GEOS-CHEM global model of transport and tropospheric chemistry to identify the major events of long range transport that reach Europe over the course of summer 2000. Sensitivity model simulations are used to analyse observed

{O}_{3}

distributions with respect to the impact of long range transport events. For that purpose, we used in-situ

{O}_{3}

observations taken at the mountain site of Jungfraujoch as well as O3 vertical profiles taken in the vicinity of central European cities. Over the course of summer 2000, we identified 9 major episodes of transatlantic pollution transport; 7 events are associated with transient cyclones while 2 events occur through zonal transport (e.g. by advection in the strong low-level westerly winds established in summer between the Azores anticyclone and transient cyclones). We find that on average three episodes occur per month with the strongest ones being in June. The number and frequency of long range transport events that reach Europe are driven by the position and strength of the Azores anticyclone. Model sensitivity simulations indicate that the summer mean North American

{O}_{3}

contribution ranges from 3 to 5 ppb (7–11%) in the planetary boundary layer and 10 to 13 ppb (18–23%) in the middle and upper troposphere. During particular episodes, North American sources can result in

{O}_{3}

enhancements up to 25–28 ppb in the layer between 800–600 hPa and 10–12 ppb in the boundary layer. The impact of the zonal transport events on

{O}_{3}

distribution over Europe is more clearly seen below 700 hPa as they tend to transport pollution at lower levels while the events associated with transient cyclones are more likely to have an impact on the middle and upper troposphere (i.e. above 600 hPa). The air mass origins found in the GEOS-CHEM model are clearly confirmed by back trajectory analyses. During most of the 9 events, a strong contribution in North American

{O}_{3}

is in general associated with only little European

{O}_{3}

and viceversa (in particular at the Jungfraujoch). A substantial North American contribution (e.g., 30% or higher) to

{O}_{3}

over Europe does not always result in pronounced

{O}_{3}

enhancements in the observations during our period of study.

2006

Precipitation over the Southern Ocean: synoptic analysis and model evaluation using ground-based remote sensing and in-situ measurements

Irina Gorodetskaya, Michael Lehning, Katherine Colby Leonard, Etienne Gabriel Henri Vignon

Precipitation is still a poorly known variable in the Southern Ocean/Antarctica due to the lack of measurements. Unique precipitation measurements were carried out during the Swiss Polar Institute"s Antarctic Circumnavigation Expedition (ACE) (December 2016 - March 2017). High temporal resolution measurements of precipitation were performed by a Snow Particle Counter (SPC) and by a micro rain radar (MRR) aboard the RV Akademik Tryoshnikov. Radiosondes were launched periodically to observe the vertical structure of the atmosphere. Additionally, MRR and radiosonde measurements from Dumont D"Urville station (DDU) were available when the expedition was in the Mertz Glacier region. These data offer a rare opportunity to evaluate model and reanalysis products performance in a region without regular precipitation measurements. In this study, ECMWF"s ERA5 reanalysis product and Antarctic Mesoscale Prediction System (AMPS) model data are evaluated using ACE and DDU in-situ observations. Two snowfall events that occurred around Mertz Glacier during the ACE campaign were chosen to compare ERA5 and AMPS data with in-situ measurements. The first event on 2 February 2017 was associated with an extratropical cyclone east of Adelie Land and a moderate along-shore moisture transport. The second event on 8-10 February 2017 was associated with a cyclone west of Mertz blocked by a high-pressure ridge, directing an intense moisture transport (identified as an atmospheric river) and precipitation to DDU. To assess if ERA5 reanalysis and AMPS (Antarctic Mesoscale Prediction System using Polar-WRF model) are able to represent these different types of precipitation events, we analyse the differences in precipitation amount between in-situ, model and reanalysis data and compare modelled vertical profiles with radiosonde measurements.

2021