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

Tropical Channel NEMO-OASIS-WRF Coupled Simulations at Very High Resolution

International audienc

Process-evaluation of tropospheric humidity simulated by general circulation models using water vapor isotopic observations: 2. Using isotopic diagnostics to understand the mid and upper tropospheric moist bias in the tropics and subtropics

Evaluating the representation of processes controlling tropical and subtropical tropospheric relative humidity (RH) in atmospheric general circulation models (GCMs) is crucial to assess the credibility of predicted climate changes. GCMs have long exhibited a moist bias in the tropical and subtropical mid and upper troposphere, which could be due to the mis-representation of cloud processes or of the large-scale circulation, or to excessive diffusion during water vapor transport. The goal of this study is to use observations of the water vapor isotopic ratio to understand the cause of this bias. We compare the three-dimensional distribution of the water vapor isotopic ratio measured from space and ground to that simulated by several versions of the isotopic GCM LMDZ. We show that the combined evaluation of RH and of the water vapor isotopic composition makes it possible to discriminate the most likely cause of RH biases. Models characterized either by an excessive vertical diffusion, an excessive convective detrainment or an underestimated in situ cloud condensation will all produce a moist bias in the free troposphere. However, only an excessive vertical diffusion can lead to a reversed seasonality of the free tropospheric isotopic composition in the subtropics compared to observations. Comparing seven isotopic GCMs suggests that the moist bias found in many GCMs in the mid and upper troposphere most frequently results from an excessive diffusion during vertical water vapor transport. This study demonstrates the added value of water vapor isotopic measurements for interpreting shortcomings in the simulation of RH by climate models

Les modèles climatiques gagnent en précision

International audienc

Back-transport of sulfur species in the high-southern latitudes

Ora

Back-transport of sulfur species in the high-southern latitudes

Ora

Impact of Synoptic Wind Intensification and Relaxation on the Dynamics and Heat Budget of the South Senegalese Upwelling Sector

International audienceAbstract In addition to their well-known seasonal cycle, eastern boundary upwelling systems (EBUS) undergo modulation on shorter synoptic to intraseasonal time scales. Energetic intensifications and relaxations of upwelling-favorable winds with 5–10-day typical time scales can impact the EBUS dynamics and biogeochemical functioning. In this work the dynamical effects of wind-forced synoptic fluctuations on the South Senegalese Upwelling Sector (SSUS) are characterized. The region geomorphology is unique with its wide continental shelf and a major coastline discontinuity at its northern edge. The ocean response to synoptic events is explored using a modeling framework that involves applying idealized synoptic wind intensification or relaxation to a five-member climatological SSUS ensemble run. Model evaluation against sparse midshelf in situ observations indicates qualitative agreement in terms of synoptic variability of temperature, stratification, and ocean currents, despite a moderate but systematic bias in current intensity. Modeled synoptic wind and heat flux fluctuations produce clear modulations of all dynamical variables with robust SSUS-scale and mesoscale spatial patterns. A mixed layer heat budget analysis is performed over the continental shelf to uncover the dominant processes involved in SSUS synoptic variability. Modulations of horizontal advection and atmospheric forcing are the leading-order drivers of heat changes during either wind intensification or relaxation while vertical dynamics is of primary importance only in a very localized area. Also, modest asymmetries in the oceanic responses to upwelling intensification and relaxation are only identified for meridional velocities. This brings partial support to the hypothesis that synoptic variability has a modest net effect on the climatological state and functioning of upwelling systems dynamics

Mesoscale ocean-atmosphere coupling in the Peru-Chile upwelling system

International audienceUnderstanding the dynamics of Eastern boundary upwelling systems such as the Peru-Chile region is of major interest as these regions host an intense biological activity and productive fisheries. Moreover, they are generally poorly represented in global climate models due to biases in low cloud cover and a misrepresentation of mesoscale processes. The mesoscale activity of these systems has been studied quite extensively with observations and with ocean models which generally do not take into account the feedback of the ocean mesoscale (eddies, filaments and fronts) on the atmospheric forcing. However it has been evidenced that oceanic thermal gradients associated with oceanic eddies induce an atmospheric response, in particular in the surface wind, which is the main forcing in upwelling systems. As a consequence, the Humboldt current system appears to be a fully coupled system in which mesoscale air-sea interactions are to be taken into account. Using a regional coupled model (WRF-NEMO) at ~9 km resolution, we study the interactions between wind stress and sea surface temperature (SST) mesoscale structures in the Peru region. The relevant coupling scales are isolated and the spatial and temporal variations of these interactions are characterized. Correlations between mesoscale wind stress and SST compare well with results from satellite data when model fields are smoothed on the observations grid (~50 km), while model small-scale structures (at ~10km resolution) are more strongly coupled. The large-scale wind intensity and steadiness modulate the coupling intensity spatially and at seasonal time scales. The physical mechanisms of the atmosphere response to an SST gradient are also investigated, including the role of the sea surface oceanic current