A Thermal Plume Model for the Martian Convective Boundary Layer

The Martian Planetary Boundary Layer [PBL] is a crucial component of the Martian climate system. Global Climate Models [GCMs] and Mesoscale Models [MMs] lack the resolution to predict PBL mixing which is therefore parameterized. Here we propose to adapt the "thermal plume" model, recently developed for Earth climate modeling, to Martian GCMs, MMs, and single-column models. The aim of this physically-based parameterization is to represent the effect of organized turbulent structures (updrafts and downdrafts) on the daytime PBL transport, as it is resolved in Large-Eddy Simulations [LESs]. We find that the terrestrial thermal plume model needs to be modified to satisfyingly account for deep turbulent plumes found in the Martian convective PBL. Our Martian thermal plume model qualitatively and quantitatively reproduces the thermal structure of the daytime PBL on Mars: superadiabatic near-surface layer, mixing layer, and overshoot region at PBL top. This model is coupled to surface layer parameterizations taking into account stability and turbulent gustiness to calculate surface-atmosphere fluxes. Those new parameterizations for the surface and mixed layers are validated against near-surface lander measurements. Using a thermal plume model moreover enables a first order estimation of key turbulent quantities (e.g. PBL height, convective plume velocity) in Martian GCMs and MMs without having to run costly LESs.Comment: 53 pages, 21 figures, paper + appendix. Accepted for publication in Journal of Geophysical Research - Planet

Net exchange reformulation of radiative transfer in the CO2 15um band on Mars

International audienceThe Net Exchange Formulation (NEF) is an alternative to the usual radiative transfer formulation. It was proposed by two authors in 1967, but until now, this formulation has been used only in a very few cases for atmospheric studies. The aim of this paper is to present the NEF and its main advantages, and to illustrate them in the case of planet Mars. In the NEF, the radiative fluxes are no more considered. The basic variables are the net exchange rates between each pair of atmospheric layers i,j. NEF offers a meaningful matrix representation of radiative exchanges, allows to quantify the dominant contributions to the local heating rates and provides a general framework to develop approximations satisfying reciprocity of radiative transfer as well as first and second principle of thermodynamic. This may be very useful to develop fast radiative codes for GCMs. We present a radiative code developed along those lines for a GCM of Mars. We show that computing the most important optical exchange factors at each time step and the others exchange factors only a few times a day strongly reduces the CPU time without any significant precision lost. With this solution, the CPU time increases proportionally to the number N of the vertical layers and no more proportionally to its square N^2. We also investigate some specific points such as numerical instabilities that may appear in the high atmosphere and errors that may be introduced if inappropriate treatments are performed when reflection at the surface occurs

Net-Exchange parameterization of infrared radiative transfers in Venus' atmosphere

International audienceThermal radiation within Venus atmosphere is analyzed in close details. Prominent features are identified, which are then used to design a parameterization (a highly simplified and yet accurate enough model) to be used in General Circulation Models. The analysis is based on a net exchange formulation, using a set of gaseous and cloud optical data chosen among available referenced data. The accuracy of the proposed parameterization methodology is controlled against Monte Carlo simulations, assuming that the optical data are exact. Then, the accuracy level corresponding to our present optical data choice is discussed by comparison with available observations, concentrating on the most unknown aspects of Venus thermal radiation, namely the deep atmosphere opacity and the cloud composition and structure

High resolution simulation of the South Asian monsoon using a variable resolution global climate model

International audienceThis study examines the feasibility of using a variable resolution global general circulation model (GCM), with telescopic zooming and enhanced resolution (~35 km) over South Asia, to better understand regional aspects of the South Asian monsoon rainfall distribution and the interactions between monsoon circulation and precipitation. For this purpose, two sets of ten member realizations are produced with and without zooming using the LMDZ (Laboratoire Meteorologie Dynamique and Z stands for zoom) GCM. The simulations without zoom correspond to a uniform 1° × 1° grid with the same total number of grid points as in the zoom version. So the grid of the zoomed simulations is finer inside the region of interest but coarser outside. The use of these finer and coarser resolution ensemble members allows us to examine the impact of resolution on the overall quality of the simulated regional monsoon fields. It is found that the monsoon simulation with high-resolution zooming greatly improves the representation of the southwesterly monsoon flow and the heavy precipitation along the narrow orography of the Western Ghats, the northeastern mountain slopes and northern Bay of Bengal (BOB). A realistic Monsoon Trough (MT) is also noticed in the zoomed simulation, together with remarkable improvements in representing the associated precipitation and circulation features, as well as the large-scale organization of meso-scale convective systems over the MT region. Additionally, a more reasonable simulation of the monsoon synoptic disturbances (lows and disturbances) along the MT is noted in the high-resolution zoomed simulation. On the other hand, the no-zoom version has limitations in capturing the depressions and their movement, so that the MT zone is relatively dry in this case. Overall, the results from this work demonstrate the usefulness of the high-resolution variable resolution LMDZ model in realistically capturing the interactions among the monsoon large-scale dynamics, the synoptic systems and the meso-scale convective systems, which are essential elements of the South Asian monsoon system

On the impact of transport model errors for the estimation of CO2 surface fluxes from GOSAT observations

A series of observing system simulation experiments is presented in which column averaged dry air mole fractions of CO2 (XCO2) from the Greenhouse gases Observing SATellite (GOSAT) are made consistent or not with the transport model embedded in a flux inversion system. The GOSAT observations improve the random errors of the surface carbon budget despite the inconsistency. However, we find biases in the inferred surface CO2 budget of a few hundred MtC/a at the subcontinental scale, that are caused by differences of only a few tenths of a ppm between the simulations of the individual XCO2 soundings. The accuracy and precision of the inverted fluxes are little sensitive to an 8-fold reduction in the data density. This issue is critical for any future satellite constellation to monitor XCO2 and should be pragmatically addressed by explicitly accounting for transport errors in flux inversion systems

7. Les paramétrisations physiques

Les modèles de circulation générale atmosphérique Les prévisions des évolutions futures du climat reposent sur la mise en œuvre sur des supercalculateurs de modèles numériques complexes qui intègrent dans le temps et dans l’espace les équations qui régissent la météorologie sur la base d’équations bien connues, comme les équations de Navier-Stokes (cf II-1). Les ordinateurs ayant une capacité de mémoire finie, l’échelle de la représentation de ces modèles climatiques est limitée (cf. V-4). Le..

Paramétrisation de la couche limite atmosphérique convective et représentation du cycle diurne des nuages dans un modèle de climat

PALAISEAU-Polytechnique (914772301) / SudocSudocFranceF