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Use of long-term measurements for a better evaluation of surface-atmosphere interactions in climate and weather prediction models – MOSAI Project (H/F)

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Date Limite Candidature : lundi 28 juin 2021

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General information

Reference : UMR5560-FABLOH-002
Date of publication : Monday, June 7, 2021
Scientific Responsible name : Supervisor: Fabienne Lohou, co-supervisors: Marie Lothon, Guylaine Canut, Jean-Charles Dupont
Type of Contract : PhD Student contract / Thesis offer
Contract Period : 36 months
Start date of the thesis : 1 October 2021
Proportion of work : Full time
Remuneration : 2 135,00 € gross monthly

Description of the thesis topic

The meteorological phenomena draw their energy from the Earth's surface and dissipate most of their energy close two the surface. The land surface, through its topography, soil moisture, temperature or vegetation activity, impacts the atmosphere from daily to seasonal time scale (Dirmeyer and Halder, 2016, Betts et al., 2017): afternoon convection, cloud formation and evolution (Milovac et al., 2016), mesoscale circulations (Taylor et al., 2012) or planetary waves (Koster et al., 2014). Land surface processes have been shown influential for extremes events, such as atmospheric drought and heat waves (Wang et al., 2015). Through these phenomena, the land surface can consequently impact climate trajectories (Seneviratne et al., 2010). An accurate assessment of the Land-Atmosphere (L-A) exchanges, and their correct representation, are therefore essential for weather and climate forecasts.
The Global Energy and Water cycle Exchanges (GEWEX) and World Climate Research Program (WCRP) have pointed out for the last ten years the importance of the L-A coupling for weather and climate models. The Working Group on Numerical Experimentation (WGNE) survey on systematic errors (Feb. 2019) established that the outstanding errors in the modelling of surface fluxes of momentum and sensible and latent heat is the second most important issue. Earth System Models (ESM) and numerical Weather Prediction (NWP) often have large biases in their representation of surface-atmosphere flux when compared to observations. The detailed quantification and reduction of these biases are still on-going efforts in many modelling centres. The MOSAI project aims at contributing to this effort.
The first step to achieve this objective is to conduct a fair and correct evaluation of the L-A interactions simulated by ESM and NWP. This is based on reliable references against which the simulated L-A exchanges can be evaluated.

The main objective of this thesis corresponds to the first scientific objective of the MOSAI project and is to investigate and determine the uncertainty and representativeness of L-A exchanges measured over heterogeneous landscapes. The PhD will take advantage of long-term measurements acquired at ACTRIS and ICOS instrumented sites and will systematically estimate the measurement errors, the
surface energy balance (SEB) non-closure issue, and, the representativeness of the local measurements in the heterogeneous landscape, at the grid mesh scale. This is a necessary step towards a fair evaluation of the models in order to avoid blaming the ESM for wrong reasons or doubting the measurements.

To achieve this objective dedicated field experiments are needed to document the variability of the L-A exchanges within a grid mesh of 5 x 5 km2 around the ACTRIS sites. One year-field campaign per site are planned at Meteopole (Toulouse, 07/2020 to 07/2021), at SIRTA (Palaiseau, en 2022) and at P2OA (Lannemezan, en 2023). Up to five surface patches with different vegetation covers, chosen according to the high-resolution land-use map created by CESBIO, will be instrumented. The length of one year enables to take into account the seasonal variability of the L-A exchanges, due to vegetation and meteorological changes from one season to the other. A full suite of
sensors monitoring the soil-vegetation-atmosphere system will be deployed, covering flux, radiation, soil characteristics and conditions, vegetation type and state, and boundary-layer main characteristics. In order to extend the observation to the vertical dimension of the atmospheric boundary layer and increase the understanding of the
horizontal variability, the EOP at P2OA (the most rural site) will be reinforced with some specific and non-permanent observations along the year, including (i) frequent profiles of the CBL with re-usable radiosondes, (ii) in-situ turbulence measurements with Remotely Piloted Airplane Systems (RPAS) around the main P2OA tower, (iii) spatialized information of the L-A exchange by IR and microwave scintillometers, and (iv) spatialized information of soil moisture using the Global Navigation Satellite System (GNSS) . The thesis student will participate to the two experiments in SIRTA and P2OA.

The thesis student will build two new indicators to complete the measured surface flux at the ACTRIS sites.

• The first indicator concerns the horizontal representativeness of the local measured surface flux in the heterogeneous landscape at the scale of the ESM or NWP grid. The quantification of the horizontal representativeness of L-A exchanges measurements in heterogeneous landscape need two steps. The first step deals with the footprint study. The flux footprint depends on the height of the measurement, surface roughness length, wind speed and direction, and atmospheric stability. The surface homogeneity within the footprint is important to consider when using the observations to evaluate the numerical models. The study of the footprint variability according to the season and the meteorological conditions together with the use of the land-use map (from satellite) will inform in what extent the measured flux represents a homogeneous surface or a panel of several surfaces. The EOP observations will provide L-A exchange heterogeneities at the model grid scale considering the measurements made on the representative covers in the vicinity of the site. The second step will combine the footprint with the measured L-A exchange heterogeneities at the model grid scale and will provide the indicator of the measured flux representativeness. In case of homogeneous surface in the footprint, the indicator must quantify the representativeness of the L-A exchange over this peculiar surface in the range of L-A exchanges in the model grid. In case of heterogeneous surface in the footprint, the
indicator must quantify in what extent the aggregated flux in the footprint represents the aggregated flux at the model grid scale. This indicator will guide the comparison between the measured and the simulated fluxes (WP2).

• The second indicator will quantify the surface fluxes uncertainties (random and systematic errors, and, SEB non-closure -Finkelstein and Sims, 2001; Kroon et al., 2010; Hollinger and Richardson, 2005; Richardson et al., 2006; Reed et al. 2018). A special focus will be considered for the evapotranspiration term and the soil heat flux, due to larger difficulty to operate accurate measurements for these fluxes (Foken et al., 2006, Mauder et al., 2018). In particular, at one of the ICOS sites, soil heat flux measurements will be supplemented by thermal conductivity and mass heat capacity sensors.

At last, the heterogeneity indicator and the measurement uncertainty due to SEB non-closure will be cross-analysed to test the possible effect of significant horizontal advective flux, and/or an insufficient sampling of large-scale turbulent eddies due to landscape heterogeneity.

This Thesis is funded by the MOSAI project (ANR 2020). Seven French Laboratories are partners in this project: LAERO, CESBIO, CNRM, GET, LMD, LATMOS, IGE and ISPRA. The expected results of this thesis work constitute deliverables that are necessary for other scientific objectives of the project MOSAI. In addition, the indicators that will be developed during this thesis will be implemented in an operational manner in the database AERIS to support the daily flow measurements downloaded by the three ACTRIS sites.

Work Context

The LAERO (Laboratoire d'Aérologie) is part of the observatory OMP (Observatoire Midi-Pyrénées). The OMP federates all laboratories of science of the universe, planet and environment from the University Paul Sabatier (UPS-Toulouse III) with assignments in research, observation, teaching, diffusion of scientific culture and international cooperation. Scientific activities at LAERO are mostly dedicated to the observation and numerical modelling of atmospheric processes, especially small and mesoscale dynamics, physics and chemistry. LAERO coordinates and animates the P2OA instrumented site, especially its 60 m tower equipped with 5 levels of meteorological and SEB instrumentation fully involved in MOSAI. It participates to ACTRIS-FR infra-structure through the surface flux, sky imager, wind profiling among others, and coordinates their associated working groups in the infra-structure. LAERO expertise involved in MOSAI covers observation and understanding of surface/atmosphere interaction and boundary layer dynamics. The role of surface heterogeneity and interaction between surface layer and boundary layer structures has been a topic for a long time in the laboratory.

This doctoral work will have to be done in close connection with the persons in charge of the measurements on the experimental sites as well as with the specialists of the parameterizations representing the surface-atmosphere interactions (see collaborations).
1. Collaborations with the tow other long-term measurement sites: Jean-Charles Dupont/IPSL - SIRTA, Guylaine Canut, William Maurel / CNRM météo-France - Météopole.
2. Collaborations with the modeling community.

Additional Information

Profile and skills required:
Student with a master's degree in atmospheric physics who has taken courses on the boundary layer and surface layer.
Knowledge of a programming language is essential.
Student with an interest for both measurement and numerical modeling.

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