Doctor of Philosophy

dissertationThe atmospheric boundary layer (ABL) has been widely investigated due to the complexity of its physical processes and its impact on human life. One of the most challenging yet critical topics in this layer is scalar transport. Many efforts have been dedicated to investigating heat and moisture transport in the ABL using experimental and numerical approaches over the last several decades. However, there are still many knowledge gaps that limit the performance of numerical weather prediction models, in particular over complex terrain. For example, insufficient understanding of near-surface processes has resulted difficulties in parameterizing meteorological variables in numerical models. Hence, the main objective of this work is to gain a better fundamental understanding of flow processes and scalar transport in the surface boundary layer over different types of terrain with the ultimate goal of improving numerical weather forecasting models by developing more accurate surface parameterizations. Three different topics are discussed in this dissertation. The first topic is a study of land-atmosphere interactions over a desert playa to better understand the impacts of spatial and temporal heterogeneity in water availability as part of the short-term hydrologic cycle. High evaporation rates and the exponential decay of these rates are observed following occasional rainfall events. Three main factors explained the fast evaporation observed following rain- fall. The first factor is the existence of a powerful positive feedback mechanisms initialized by rainfall events that leads to increasing volumetric water content, decreasing surface albedo and Bowen ratio, followed by increases in net radiation, and eventually the enhancement of evaporation rates. The second factor is the clay soil texture, which has low permeability and high capacity. The soil property makes more water available near the surface for evaporation. The third factor is the non-negligible nocturnal evaporation rates that are correlated with increasing soil moisture content. Moreover, a higher spatial variability of surface soil moisture and evaporation is observed when the surface is dry. The second topic is articulated around a case study of the mechanisms that modulates the evolution of valley fog. A typical shallow, early-morning, short- lived valley fog is observed in a sheltered alpine valley. This work shows that mountain circulations play a critical role in the formation and development of shallow valley fog by modulating temperature and moisture fields through katabatic flow interactions and gravity waves. In particular, internal gravity waves are shown to modulate fog processes by varying the near-surface temperature within a time period of ≈ 20 min. The purpose of the last topic is to better understand the potential temperature variance budget over three different surfaces, a desert playa (dry lakebed), characterized by a flat surface devoid of vegetation; a vegetated site, characterized by a flat valley floor covered with greasewood vegetation, and a mountain terrain site with a slope angle of 2 -4° and covered by high-elevation vegetation. The analysis reveals the presence of a 5-m layer where the production and dissipation terms of potential temperature variance drop rapidly below this level. Within the 5-m layer, turbulent transport of potential temperature variance acts as a sink term at all sites of interest. The ratio of turbulent transport to production of potential temperature variance remains constant as stability decreases. The imbalance ratio between production and dissipation shows no correlation with the stability conditions

Présentation

Heintz Jean-Georges. Présentation. In: Revue d'histoire et de philosophie religieuses, 59e année n°3-4,1979. Mélanges Edmond Jacob. pp. 269-272

A Local Similarity Function for Katabatic Flows Derived From Field Observations Over Steep- and Shallow-Angled Slopes

Katabatic flows are notoriously difficult to model for a variety of reasons. Notably, the assumptions underpinning Monin-Obukhov similarity theory (MOST) are inherently violated by the sloping terrain, causing the traditional flux-gradient relations used in numerical weather prediction models to break down. Focusing on turbulent momentum transport, we show significant flux divergence, further violating MOST assumptions, and that the traditional parameterizations fail even with local scaling for katabatic flow. In response, we propose a modified local-MOST stability-correction function, informed by near-surface turbulence observations collected over two mountainous slopes with inclination angles (α) of α ≈ 7.8° and α ≈ 35.5°. The proposed relation includes a directly, making data from both slopes collapse with unprecedented agreement. RMSE between measured fluxes and estimates from the proposed and Businger et al. (1971, https://doi.org/10.1175/1520-0469(1971)028\u3c0181:FPRITA\u3e2.0.CO;2) relations show significant improvement. Results can be used to inform future development of wall-model and turbulence closures in the katabatic flow layer

Investigating mountain breezes characteristics and their effects on CO2 concentration at three different sites

International audienceThe characteristics of daytime and nighttime mountain breezes have been analysed and compared at three different sites: a) in the foothills of the Guadarrama Mountain range (El Escorial, Spain); b) on a plateau close to The Pyrenees (Lannemezan, France); and c) in the Salt Lake Valley (SLV, Utah, US). A systematic algorithm, based on synoptic and local meteorological conditions, has been used to detect automatically numerous events at each site. On the one hand, the wind characteristics of these mountain breezes depend on the scale of the breeze detected at each site. Their arrivals are observed approximately when the sensible heat flux changes sign, but they are delayed in the sites that are farther away from the mountains. On the other hand, the effects of these breezes on CO2 mixing ratios have been investigated. The typical increases and decreases of CO2 mixing ratios observed around the afternoon and morning transition do not always occur at the same time of the breeze arrival to the tower site, which unlinks these drastic changes in CO2 from the direct horizontal advection produced by the breezes. However, the CO2 mixing ratio is sensitive to changes in wind direction in highly heterogeneous sites, like the SLV site. Besides, the changes in surface turbulence produced by the breezes have an important effect on CO2. Indeed, a clear relationship is found for CO2 mixing ratio and the turbulent kinetic energy in the lowest atmospheric layers during the nighttime events

Investigating mountain breezes characteristics and their effects on CO2 concentration at three different sites

International audienceThe characteristics of daytime and nighttime mountain breezes have been analysed and compared at three different sites: a) in the foothills of the Guadarrama Mountain range (El Escorial, Spain); b) on a plateau close to The Pyrenees (Lannemezan, France); and c) in the Salt Lake Valley (SLV, Utah, US). A systematic algorithm, based on synoptic and local meteorological conditions, has been used to detect automatically numerous events at each site. On the one hand, the wind characteristics of these mountain breezes depend on the scale of the breeze detected at each site. Their arrivals are observed approximately when the sensible heat flux changes sign, but they are delayed in the sites that are farther away from the mountains. On the other hand, the effects of these breezes on CO2 mixing ratios have been investigated. The typical increases and decreases of CO2 mixing ratios observed around the afternoon and morning transition do not always occur at the same time of the breeze arrival to the tower site, which unlinks these drastic changes in CO2 from the direct horizontal advection produced by the breezes. However, the CO2 mixing ratio is sensitive to changes in wind direction in highly heterogeneous sites, like the SLV site. Besides, the changes in surface turbulence produced by the breezes have an important effect on CO2. Indeed, a clear relationship is found for CO2 mixing ratio and the turbulent kinetic energy in the lowest atmospheric layers during the nighttime events