Synoptic scale meteorology | EPFL Graph Search

In meteorology, the synoptic scale (also called the large scale or cyclonic scale) is a horizontal length scale of the order of or more. This corresponds to a horizontal scale typical of mid-latitude depressions (e.g. extratropical cyclones). Most high- and low-pressure areas seen on weather maps (such as surface weather analyses) are synoptic-scale systems, driven by the location of Rossby waves in their respective hemisphere. Low-pressure areas and their related frontal zones occur on the leading edge of a trough within the Rossby wave pattern, while high-pressure areas form on the back edge of the trough. Most precipitation areas occur near frontal zones. The word synoptic is derived from the Ancient Greek word (), meaning "seen together". The Navier–Stokes equations applied to atmospheric motion can be simplified by scale analysis in the synoptic scale. It can be shown that the main terms in horizontal equations are Coriolis force and pressure gradient terms; therefore, one ca

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

Modeling Snow Saltation: The Effect of Grain Size and Interparticle Cohesion

Daniela Brito Melo, Francesco Comola, Michael Lehning, Varun Sharma, Armin Sigmund

The surface of the Earth is snow-covered at least seasonally over large areas. This snow surface is highly dynamic, particularly under the influence of strong winds. The motion of snow particles driven by the wind not only changes the snow cover but has important consequences for the atmosphere in that it adds mass and moisture and extracts heat. Large scale meteorological and climatological models neglect these surface dynamics or produce conflicting results from too simplified process representation. With recent progress in the detailed understanding of the saltation process, in particular with respect to sand saltation, and the advancement of numerical models, we can systematically investigate the influence of snow properties on saltation. This contribution uses a Large Eddy Simulation model with full surface particle dynamics to investigate how snow cohesion and size distribution influence saltation dynamics and in particular the total mass flux. The model reproduces some known characteristics of the saltation system such as a focus point or a constant near surface particle speed. An interesting result is that cohesion and grain size heterogeneity can increase the overall saltation mass flux at high friction velocities. Moreover, some simplified models agree reasonably well with the simulations for given bed characteristics, while others clearly do not. These results are valid for continuous saltation while intermittent saltation, which often occurs in nature, needs further investigation. In order to successfully parameterize saltation in large scale models, progress must be made in correctly representing snow surface properties in these models, in particular cohesion.

AMER GEOPHYSICAL UNION2022

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

2021