Turbulent mixing and its impact on lower tropospheric moisture over tropical ocean

The variability of lower-tropospheric humidity is a crucial feature of the tropical climate. Among the processes that impact moisture budget, the vertical transport by turbulent mixing is generally overlooked. Using observations from CINDY/DYNAMO campaign, this is a first attempt to quantify it over the tropical ocean. Turbulent patches of ~100 m depth are observed in relation with large vertical gradients of specific humidity. Intense mixing is diagnosed within these intermittent patches. Three approaches are used to diagnose the overall effect of this intermittent turbulence. Large uncertainties on the corresponding eddy diffusivity coefficient arise from parameters hard to experimentally constrain. However, dry conditions are associated with steep moisture vertical gradients above the boundary layers. Owing to the uncertainties on the eddy diffusivity, these gradients can correspond to negligible or to significant moisture tendencies (~0.5-1 g kg-1 day-1) during the recovery following a dry intrusion or the preconditioning stage of an MJO

Observation of moisture tendencies related to shallow convection

Tropospheric moisture is a key factor controlling the global climate and its variability. For instance, moistening of the lower troposphere is necessary to trigger the convective phase of a Madden-Julian oscillation (MJO). However, the relative importance of the processes controlling this moistening has yet to be quantified. Among these processes, the importance of the moistening by shallow convection is still debated. The authors use high-frequency observations of humidity and convection from the Research Vessel (R/V) Mirai that was located in the Indian Ocean ITCZ during the Cooperative Indian Ocean Experiment on Intraseasonal Variability/Dynamics of the MJO (CINDY/DYNAMO) campaign. This study is an initial attempt to directly link shallow convection to moisture variations within the lowest 4 km of the atmosphere from the convective scale to the mesoscale. Within a few tens of minutes and near shallow convection occurrences, moisture anomalies of 0.25-0.5 g kg-1 that correspond to tendencies on the order of 10-20 g kg-1 day-1 between 1 and 4 km are observed and are attributed to shallow convective clouds. On the scale of a few hours, shallow convection is associated with anomalies of 0.5-1 g kg-1 that correspond to tendencies on the order of 1-4 g kg-1 day-1 according to two independent datasets: lidar and soundings. This can be interpreted as the resultant mesoscale effect of the population of shallow convective clouds. Large-scale advective tendencies can be stronger than the moistening by shallow convection; however, the latter is a steady moisture supply whose importance can increase with the time scale. This evaluation of the moistening tendency related to shallow convection is ultimately important to develop and constrain numerical models

Rôle de l'Interaction Océan-Atmosphère dans la Variabilité Intrasaisonnière de la Convection Tropicale.

Atmospheric tropical convection is strongly modulated at intraseasonal timescales (periods of 20 to 90 days). Ocean may play a critical role in atmospheric convective activity at this timescale. Sea Surface Temperature (SST) variations would indeed perturb surface fluxes and act to destabilise the atmosphere and trigger convection on the large scale. The associated dynamical response would then act to maintain convection at the intraseasonal timescale by enhancing surface heat fluxes. These SST perturbations are primarily linked with mixed layer temperature ones and so depend, as the available energy to maintain the convection, on the mixed layer depth. Observations support this thermodynamical conception of the physical origin of intraseasonal variability that enables to explain its climatological link with the seasonal march of the monsoons. The impact of SST intraseasonal variations on the organisation of intraseasonal convective events is then further explored using ensembles of LMDZ global circulation model simulations. An important part of the SST intraseasonal variability is moreover associated with the formation of diurnal warm-layers as shown by results using a simple diagnostic algorithm forced by ERA-40 reanalysis data. The validation of this approach using in situ measurements of the global SVP drifter network makes possible to use such an algorithm as a parametrisation of diurnal warm-layers in a climate model such as LMDZ. The ability of the LMD model to produce realistic diurnal warm-layers is shown in a first simulation before discussing the effect of such phenomenon on the simulated intraseasonal variability of the convection.Dans les tropiques, la convection atmosphérique est fortement modulée à l'échelle intrasaisonnière (périodes de 20 à 90 jours). L'océan jouerait un rôle important dans l'activité atmosphérique à cette échelle. Les variations de Températures de Surface de l'Océan (TSO) perturberaient en effet les flux de surface et déstabiliseraient l'atmosphère pour déclencher un événement convectif organisé à grande échelle. La réponse dynamique associée permettrait alors l'entretien de la convection à l'échelle intrasaisonnière par augmentation des flux de chaleur à la surface. Ces perturbations de TSO sont principalement liées aux perturbations de la température de la couche de mélange et dépendent donc, comme l'énergie disponible pour l'entretien de la convection, de la profondeur de cette dernière. Les résultats d'observations, présentés ici, étayent cette conception thermodynamique de l'origine de la variabilité intrasaisonnière qui permet d'expliquer son lien climatologique avec le cycle saisonnier des moussons. L'impact des perturbations intrasaisonnières de la TSO sur l'organisation des événements convectifs est ensuite étudié au moyen de simulations effectuées avec le modèle de climat LMDZ. Une part importante de la variabilité intrasaisonnière de la TSO est de plus liée à la formation de Couches de Réchauffement Diurne comme le montre le diagnostic effectué à partir d'un algorithme simple forcé par les données ERA-40. La validation de cette démarche au moyen des mesures du réseau global de bouées SVP autorise l'inclusion de cet algorithme dans LMDZ. On vérifie enfin la capacité du modèle à reproduire des CRD réalistes avant de discuter leur effet sur la variabilité intrasaisonnière simulée