Wintertime characteristic atmospheric circulation in the Grenoble basin and impact on air pollution
Supervisors
Mme Chantal Staquet
M. Charles Chemel
Abstract
This PhD work investigates the transport and spatial distribution of particulate matter in the alpine valley of Grenoble during wintertime. It is motivated by the adverse health effects that impact the inhabitants of the valley as a result of long term exposition to pollutants.
Since detailed numerical modeling of the dispersion of pollutants at fine spatial scales is unpractical over long time scales, the first part of this PhD work attempts to identify large-scale weather types that can be associated with a characteristic atmospheric dynamics in the Grenoble valley. To obtain this weather type decomposition, classical (PCA+Kmeans) and less classical (convolutional LSTM neural network) methods are compared. The introduction of the temporal component as well as constraints linking synoptic and local scales are also studied. The result is that, for a limited number of weather types, only the one corresponding to the winter anticyclonic blocking seems to produce a characteristic local response.
In the second part of the thesis, the impact of the variability within the winter anticyclonic blocking weather type on this local response is studied, via the simulation of four episodes with different characteristics. To this end, High Resolution Mesoscale Simulations (down to about 100 meters horizontal resolution) are used to accurately capture the dynamics in complex terrain. The emission inventory implemented in the numerical model allows, in combination with the dynamics, to obtain an accurate spatial representation of the air pollution.
From a dynamical point of view, it appears that the atmospheric circulation at the valley bottom is very homogeneous from one episode to the next and driven by thermal effects. Conversely, the height of the thermal inversion and the vertical structure of the valley winds depend on the synoptic conditions. Concerning atmospheric pollution, the main result is that the concentration fields at the valley bottom are similar from one episode to the other because the winds flowing along the valley floor are similar. This concentration field contains the same hot spots of pollution whatever the episode, implying that the population living there might be exposed to poor air quality during anticyclonic winter episodes.