What favors convective aggregation and why?

International audienceThe organization of convection is ubiquitous, but its physical understanding remains limited. One particular type of organization is the spatial self-aggregation of convection, taking the form of cloud clusters, or tropical cyclones in the presence of rotation. We show that several physical processes can give rise to self-aggregation and highlight the key features responsible for it, using idealized simulations. Longwave radiative feedbacks yield a radiative aggregation. In that case, sufficient spatial variability of radiative cooling rates yields a low-level circulation, which induces the upgradient energy transport and radiative-convective instability. Not only do vertically integrated radiative budgets matter but the vertical profile of cooling is also crucial. Convective aggregation is facilitated when downdrafts below clouds are weak (moisture-memory aggregation), and this is sufficient to trigger aggregation in the absence of longwave radiative feedbacks. These results shed some light on the sensitivity of self-aggregation to various parameters, including resolution or domain size

An assessment of the primary sources of spread of global warming estimates from coupled atmosphere-ocean models.

International audienceClimate feedback analysis constitutes a useful framework to compare the global mean surface temperature responses to an external forcing predicted by general circulation models (GCMs). Nevertheless, the contributions of the different radiative feedbacks to global warming (in equilibrium or transient conditions) and their comparison with the contribution of other processes (e.g. the ocean heat uptake) have not been quantified explicitly. Here we define these contributions from the classical feedback analysis framework, and we quantify them for an ensemble of 12 CMIP3/IPCC-AR4 coupled atmosphere-ocean GCMs. In transient simulations, the multi-model mean contributions to global warming associated with the combined water vapor - lapse rate feedback, cloud feedback and ocean heat uptake are comparable. However, inter-model differences in cloud feedbacks constitute by far the primary source of spread of both equilibrium and transient climate responses simulated by GCMs. The spread associated with inter-model differences in cloud feedbacks appears to be roughly three times larger than that associated either with combined water vapor - lapse rate feedback, the ocean heat uptake or the radiative forcing

Processus régissant la sensibilité climatique

National audienceUne modification de la concentration des gaz à effet de serre modifie le bilan énergétique de la Terre. Cette modification, appelée forçage radiatif, entraîne une modification de la température moyenne de la Terre dont l'amplitude dépend non seulement du forçage lui-même mais aussi de la façon dont le système climatique dans son ensemble répond à ce forçage. Par exemple, la vapeur d'eau, la cryosphère et les nuages sont modifiés par un changement de température et ces modifications influencent le changement initial de température. Dans cet article nous montrons tout d'abord que ces phénomènes, dits de rétroaction, jouent un rôle clé dans l'estimation de l'amplitude du réchauffement climatique en réponse à un doublement de la concentration de l'atmosphère en CO2. Nous montrons également que l'amplitude de ce réchauffement est différent selon les modèles et que cette différence a pour principale origine la réponse des nuages, et plus particulièrement des nuages ba

Observational evidence for relationships between the degree of aggregation of deep convection, water vapor, surface fluxes, and radiation

International audienceTropical deep convection exhibits complex organization over a wide range of scales. This study investigates the relationships between the spatial organization of deep convection and the large-scale atmospheric state. By using several satellite datasets and reanalyses, and by defining a simple diagnostic of convective aggregation, relationships between the degree of convective aggregation and the amount of water vapor, turbulent surface fluxes, and radiation are highlighted above tropical oceans. When deep convection is more aggregated, the middle and upper troposphere are drier in the convection-free environment, turbulent surface fluxes are enhanced, and the low-level and midlevel cloudiness is reduced in the environment. Humidity and cloudiness changes lead to a large increase in outgoing longwave radiation. Cloud changes also result in reduced reflected shortwave radiation. Owing to these opposing effects, the sensitivity of the radiative budget at the top of the atmosphere to convective aggregation turns out to be weak, but the distribution of radiative heating throughout the troposphere is affected. These results suggest that feedbacks between convective aggregation and the large-scale atmospheric state might influence large-scale dynamics and the transports of water and energy and, thus, play a role in the climate variability and change. These observational findings are qualitatively consistent with previous cloud-resolvingmodel results, except for the effects on cloudiness and reflected shortwave radiation. The proposed methodology may be useful for assessing the representation of convective aggregation and its interaction with the large-scale atmospheric state in various numerical models. © 2012 American Meteorological Society

Observed relationships between cloud vertical structure and convective aggregation over tropical ocean

Using the satellite-infrared-based Simple Convective Aggregation Index (SCAI) to determine the degree of aggregation, 5 years of CloudSat-CALIPSO cloud profiles are composited at a spatial scale of 10 degrees to study the relationship between cloud vertical structure and aggregation. For a given large-scale vertical motion and domain-averaged precipitation rate, there is a large decrease in anvil cloud (and in cloudiness as a whole) and an increase in clear sky and low cloud as aggregation increases. The changes in thick anvil cloud are proportional to the changes in total areal cover of brightness temperatures below 240 K (cold cloud area, CCA), which is negatively correlated with SCAI. Optically thin anvil cover decreases significantly when aggregation increases, even for a fixed CCA, supporting previous findings of a higher precipitation efficiency for aggregated convection. Cirrus, congestus, and mid-level clouds do not display a consistent relationship with the degree of aggregation. We present the lidar-observed low-level cloud cover (where the lidar is not attenuated) as our best estimate of the true low-level cloud cover and show that it increases as aggregation increases. Qualitatively, the relationships between cloud distribution and SCAI do not change with sea-surface temperature, while cirrus clouds are more abundant and low-level clouds less at higher sea-surface temperatures. For the observed regimes, the vertical cloud profile varies more evidently with SCAI than with mean precipitation rate. These results confirm that convective scenes with similar vertical motion and rainfall can be associated with vastly different cloudiness (both high and low cloud) and humidity depending on the degree of convective aggregation

Comprender las nubes para anticipar el clima futuro

La humanidad siente una fascinación instintiva por las nubes. Las comunidades meteorológica e hidrológica han llegado a comprender tras décadas de observación e investigación que los procesos nubosos, desde la microfísica de la nucleación inicial a las supertormentas visibles desde los satélites, proporcionan información fundamental para la predicción meteorológica y, en particular, para la de la precipitación. Mirar las nubes desde un punto de vista climático presenta nuevos y difíciles interrogantes que cuestionan nuestras premisas generales sobre el modo en el que funciona realmente nuestra húmeda y nubosa atmósfera

Understanding the O-17 excess glacial-interglacial variations in Vostok precipitation

International audienceCombined measurements of delta O-18, delta O-17, and delta D in ice cores, leading to d excess and O-17 excess, are expected to provide new constraints on the water cycle and past climates. We explore different processes, both in the source regions and during the poleward transport, that could explain the O-17 excess increase by 20 per meg observed from the Last Glacial Maximum (LGM) to Early Holocene (EH) at the Vostok station. Using a single-column model over tropical and subtropical oceans, we show that the relative humidity at the surface is the main factor controlling O-17 excess in source regions. Then, using a Rayleigh-type model, we show that the O-17 excess signal from the source region is preserved in the polar snowfall, contrary to d excess. Evaporative recharge over mid and high latitudes and delta O-18 seasonality in polar regions can also affect the Vostok O-17 excess but cannot account for most of the 20 per meg deglacial increase from LGM to EH. On the other hand, a decrease of the relative humidity at the surface (rh(s)) by 8 to 22% would explain the observed change in O-17 excess. Such a change would not necessarily be incompatible with a nearly unchanged boundary layer relative humidity, if the surface thermodynamic disequilibrium decreased by 4 degrees C. Such a change in rh(s) would affect source and polar temperatures reconstructions from delta O-18 and d excess measurements, strengthening the interest of O-17 excess measurements to better constrain such changes

Thermodynamic constraint on the depth of the global tropospheric circulation

The troposphere is the region of the atmosphere characterized by low static stability, vigorous diabatic mixing, and widespread condensational heating in clouds. Previous research has argued that in the tropics, the upper bound on tropospheric mixing and clouds is constrained by the rapid decrease with height of the saturation water vapor pressure and hence radiative cooling by water vapor in clear-sky regions. Here the authors contend that the same basic physics play a key role in constraining the vertical structure of tropospheric mixing, tropopause temperature, and cloud-top temperature throughout the globe. It is argued that radiative cooling by water vapor plays an important role in governing the depth and amplitude of large-scale dynamics at extratropical latitudes

Preface to the Special Issue “ISSI Workshop on Shallow Clouds and Water Vapor, Circulation and Climate Sensitivity”

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The Influence of Convective Aggregation on the Stable Isotopic Composition of Water Vapor

Remote sensing datasets of water vapor isotopic composition are used along with objective measures of convective aggregation to better understand the impact of convective aggregation on the atmospheric hydrologic cycle in the global tropics (30°N to 30°S) for the period 2015–2020. When convection is unaggregated, vertical velocity profiles are top-heavy, mixing ratios increase and water vapor δD decreases as the mean precipitation rate increases, consistent with partial hydrometeor evaporation below anvils into a relatively humid atmospheric column. Aggregated convection is associated with bottom-heavy vertical velocity profiles and a positive correlation between mixing ratio and δD, a result that is consistent with isotopic enrichment from detrainment of shallow convection near the observation level. Intermediate degrees of aggregation do not display significant variation in δD with mixing ratio or precipitation rate. Convective aggregation provides a useful paradigm for understanding the relationships between mixing ratio and isotopic composition across a range of convective settings. The results presented here may have utility for a variety of applications including the interpretation of paleoclimate archives and the evaluation of numerical simulations of convection