Corée du Sud

Résumé

La Corée du Sud (en 한국 ; ), nom usuel de la forme officielle république de Corée (en 대한민국 ; ), est un État souverain d'Asie de l'Est qui couvre la moitié sud de la péninsule coréenne. Sa capitale est Séoul. La langue officielle est le coréen, dont l'écriture est le hangeul, et la monnaie le won. L'animal représentant le pays est le tigre. Au nord, son unique frontière terrestre d'une longueur de avec la Corée du Nord, est constituée par la zone démilitarisée (DMZ). La Corée du Sud est bordée par la mer Jaune à l’ouest, par la mer du Japon à l'est (dont l'appellation est dénoncée par les Sud-Coréens qui la désignent par « mer de l'Est ») et par le détroit de Corée au sud. Actuellement, la Corée du Sud est la dixième puissance économique mondiale selon le calcul du produit intérieur brut en parité de pouvoir d'achat, et la quinzième selon le critère monétaire traditionnel. En 2016, elle est à la fois un des pays d’Asie de l’Est avec l’IDH le plus élevé et le taux de fécondité le

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https://fr.wikipedia.org/wiki/Cor%C3%A9e_du_Sud

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Chargement

Identification of snowfall microphysical processes from Eulerian vertical gradients of polarimetric radar variables

Alexis Berne, Josué Etienne Gehring, Noémie Planat, Etienne Gabriel Henri Vignon

Polarimetric radar systems are commonly used to study the microphysics of precipitation. While they offer continuous measurements with a large spatial coverage, retrieving information about the microphysical processes that govern the evolution of snowfall from the polarimetric signal is challenging. The present study develops a new method, called process identification based on vertical gradient signs (PIVSs), to spatially identify the occurrence of the main microphysical processes (aggregation and riming, crystal growth by vapor deposition and sublimation) in snowfall from dual-polarization Doppler radar scans. We first derive an analytical framework to assess in which meteorological conditions the local vertical gradients of radar variables reliably inform about microphysical processes. In such conditions, we then identify regions dominated by (i) vapor deposition, (ii) aggregation and riming and (iii) snowflake sublimation and possibly snowflake breakup, based on the sign of the local vertical gradients of the reflectivity Z(H) and the differential reflectivity Z(DR). The method is then applied to data from two frontal snowfall events, namely one in coastal Adelie Land, Antarctica, and one in the Taebaek Mountains in South Korea. The validity of the method is assessed by comparing its outcome with snowflake observations, using a multi-angle snowflake camera, and with the output of a hydrometeor classification, based on polarimetric radar signal. The application of the method further makes it possible to better characterize and understand how snowfall forms, grows and decays in two different geographical and meteorological contexts. In particular, we are able to automatically derive and discuss the altitude and thickness of the layers where each process prevails for both case studies. We infer some microphysical characteristics in terms of radar variables from statistical analysis of the method output (e.g., Z(H) and Z(DR) distribution for each process). We, finally, highlight the potential for extensive application to cold precipitation events in different meteorological contexts.

COPERNICUS GESELLSCHAFT MBH2021

Chargement

Precipitation is the result of a chain of meteorological processes ranging from the large- to the micro-scale. While the transport of moisture and lifting mechanisms leading to cloud formation are mostly governed by dynamical processes, the formation and growth of hydrometeors are ultimately determined by microphysical processes. A proper understanding of the complex interactions between atmospheric dynamics and microphysics is of paramount importance to accurately forecast precipitation. In particular, snowfall microphysics is greatly influenced by dynamical processes, such as turbulence and updraughts. Yet, the impact of atmospheric dynamics on snowfall microphysics remains poorly understood.In this thesis, meteorological radars and atmospheric models are combined to investigate how dynamical processes can influence snowfall microphysics. We exploit the synergies between measurements collected with an X-band polarimetric radar (named MXPol), a W-band Doppler radar, and a multi-angle snowflake camera (MASC). Hydrometeor classifications are used to identify the key microphysical processes at play. The objectives of this thesis are twofold: (i) collect data on clouds and precipitation during two field campaigns in South Korea and Antarctica, and (ii) leverage this data to investigate how dynamical processes influenced the microphysics of two snowfall events. First, a 4-month dataset of ground-based radar and in situ measurements collected in South Korea is presented. The dataset includes 9 precipitation events with a total accumulation of 195 mm of equivalent liquid precipitation. Second, measurements of clouds and precipitation during a 3-month campaign at Davis station, Antarctica are introduced. Altogether, both datasets collected during this thesis represent an opportunity to study snowfall microphysics thanks to the complementarity of Doppler radar data and snowflake photographs in two regions where such measurements were not available before.The second part of this thesis is devoted to case studies of two snowfall events. First, a case study of an intense precipitation event in South Korea reveals that a warm conveyor belt associated with a low pressure system provided ideal conditions for riming and aggregation through the generation of turbulence and SLW in the large-scale ascent. These processes led to enhanced precipitation rates of up to 12 mm/h and a total accumulation of 57 mm in 21 h. The second case study shows that orographic gravity waves (OGWs) determined the evolution and distribution of snowfall during an atmospheric river event over Davis in the Vestfold Hills, Antarctica. Despite the intense moisture advection by the atmospheric river, little precipitation was observed at Davis due to intense snowfall sublimation by foehn winds associated with the OGWs. We propose that this mechanism could contribute to the extremely dry climate of the Vestfold Hills, one of the rare ice-free regions of Antarctica.Altogether, this thesis offers two unique datasets which are available to the scientific community and can be used for future studies on snowfall microphysics. It also contributes to a better understanding of how atmospheric dynamics can influence snowfall microphysics. In particular, it illustrates how processes occurring at different spatial scales can determine the dominating snowfall microphysical processes.

EPFL2021

Chargement

Microphysics and dynamics of snowfall associated with a warm conveyor belt over Korea

Alexis Berne, Nikola Besic, Josué Etienne Gehring, Nicolas Jean François Jullien, Etienne Gabriel Henri Vignon

On 28 February 2018, 57mm of precipitation associated with a warm conveyor belt (WCB) fell within 21 h over South Korea. To investigate how the large-scale circulation influenced the microphysics of this intense precipitation event, we used radar measurements, snowflake photographs and radiosounding data from the International Collaborative Experiments for Pyeongchang 2018 Olympic and Paralympic-Winter Games (ICE-POP 2018). The WCB was identified with trajectories computed with analysis wind fields from the Integrated Forecast System global atmospheric model. The WCB was collocated with a zone of enhanced wind speed of up to 45ms(-1) at 6500ma.s.l., as measured by a radiosonde and a Doppler radar. Supercooled liquid water (SLW) with concentrations exceeding 0.2 g kg(-1) was produced during the rapid ascent within the WCB. During the most intense precipitation period, vertical profiles of polarimetric radar variables show a peak and subsequent decrease in differential reflectivity as aggregation starts. Below the peak in differential reflectivity, the specific differential phase shift continues to increase, indicating early riming of oblate crystals and secondary ice generation. We hypothesise that the SLW produced in the WCB led to intense riming. Moreover, embedded updraughts in the WCB and turbulence at its lower boundary enhanced aggregation by increasing the probability of collisions between particles. This suggests that both aggregation and riming occurred prominently in this WCB. This case study shows how the large-scale atmospheric flow of a WCB provides ideal conditions for rapid precipitation growth involving SLW production, riming and aggregation. Future microphysical studies should also investigate the synoptic conditions to understand how observed processes in clouds are related to large-scale circulation.

2020

Chargement

EPFL2021

Microphysics and dynamics of snowfall associated with a warm conveyor belt over Korea

Alexis Berne, Nikola Besic, Josué Etienne Gehring, Nicolas Jean François Jullien, Etienne Gabriel Henri Vignon

2020

Identification of snowfall microphysical processes from Eulerian vertical gradients of polarimetric radar variables

Alexis Berne, Josué Etienne Gehring, Noémie Planat, Etienne Gabriel Henri Vignon

COPERNICUS GESELLSCHAFT MBH2021