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The recent successful space-borne demonstration of a miniaturized CubeSat precipitation radar is highlighted. The low cost of such a radar, together with the availability of small satellite, platforms to carry it, now make it feasible to consider employing a more distributed approach to observe important atmospheric processes that relate to precipitation. An approach to quantify the transport of water and air by deep convection is described based on a clustering of small radar satellites providing measurements seconds apart. This strategy now adds time as a new dimension for observing such processes. A mission concept, referred to as D-train, comprised of a train of three satellites 30, 90, and 120 s apart is described, and the expected performance of it for providing measures of convective transport is examined based on a large ensemble of simulations of convection with an advanced cloud-resolving model.

International audience; The direct measurement of Earth's energy imbalance (EEI) is one of the greatest challenges in climate research. The global mean EEI represents the integrated value of global warming and is tightly linked to changes in hydrological cycle and the habitability of our planet. Current space-born radiometers measure the individual radiative components of the energy balance with unprecedented stability, but with calibration errors too large to determine the absolute magnitude of global mean EEI as the components' residual. Best estimates of long-term EEI are currently derived from temporal changes in ocean heat content at ~0.7 Wm-2. To monitor EEI directly from space, we propose an independent approach based on accelerometry that measures non-gravitational forces, such as radiation pressure, acting on Earth orbiting spacecrafts. The concept of deriving EEI from radiation pressure has been considered in the past, and we provide analysis that shows today's capabilities are sufficiently accurate to answer the question: At what rate is our planet warming? To measure global mean EEI to within at least ±0.3 Wm-2 requires spacecraft(s) of near-spherical shape and well-characterized surface properties to reduce confounding effects. The proposed concept may provide the basis for a data record of global and zonal mean EEI on annual and potentially monthly time scales. It is not meant to replace existing concepts designed to measure energy balance components or ocean heat storage, but to complement these by providing an independent estimate of EEI for comparison and to anchor data products and climate models that lack energy balance closure.; La mesure directe du bilan énergétique de la Terre (EEI : Earth's Energy Imbalance) est l'un des plus grands défis de la recherche climatique. L'EEI moyen mondial représente la valeur intégrée du réchauffement climatique; sa variabilité spatiale et temporelle reflète la nature des transport de chaleur à travers le système climatique et les modes climatiques internes tels que l'ENSO. Ces transports de chaleur contrôlent les circulations atmosphériques et océaniques, et désormais le cycle de l'eau et l'habitabilité de notre planète. Les systèmes spatiaux actuels mesurent les composantes radiatives individuelles du bilan énergétique avec une précision et une stabilité sans précédent, mais les limites d'erreur sont trop grossières pour déterminer l'amplitude absolue de l'EEI moyen global ainsi que les résidus des contributeurs. Les meilleures estimations de l'IEE à long terme sont actuellement dérivées des changements de la teneur en chaleur des océans telle que mesurée in situ, mais sont affectées par des erreurs d'échantillonnage. Des méthodes alternatives sont nécessaires pour éviter l'imprécision de l'étalonnage radiométrique et l'échantillonnage incomplet in situ de l'océan. Pour suivre la valeur intégrée de l'EEI à partir de l'espace, nous proposons une approche basée sur l'accélérométrie qui mesure les forces non gravitationnelles, telles que la pression de radiation, agissant sur les satellites en orbite autour de la Terre. Le concept pour déduire l'EEI de la pression de radiation a été considéré dans le passé, et nous fournissons une analyse qui montre que les instruments d'aujourd'hui sont suffisamment précises pour enfin répondre à la question: à quel rythme notre planète se réchauffe-t-elle? Le concept proposé ne vise pas à remplacer les concepts existants conçus pour mesurer les composantes du bilan énergétique, mais à les compléter en fournissant une mesure directe de l'EEI responsable des changements climatiques de notre planète.

This is the first of two papers that quantify the high added value of frequent 3-D radar observations of the atmosphere to capture the dynamics of weather systems. Recent advances in small-satellite and radar technologies, such as the “Radar in Cubesat” developed at the Jet Propulsion Laboratory, are paving the way for the design of convoys of spaceborne radars that can directly observe the evolution of severe weather at very fine temporal scales. The analyses presented here are to establish the relation between such observations to the underlying cloud variables and processes, and to quantify the sensitivity to the different physical and instrument parameters. In this first part, a robust algorithm is proposed to estimate the horizontal advection from successive radar reflectivity measurements, and use it to compute total time derivatives $d_{t} Z$ of the observed radar reflectivity factors $Z$ . As illustrated using Next-Generation Radar measurements in a blizzard coupled with an atmospheric river in California, the maps of $d_{t} Z$ reveal features about locations of sources and sinks of condensed water, which are, otherwise, not visible in the maps of $Z$ alone. Using numerical simulations of the blizzard and a radiative-transfer model to forward calculate the corresponding reflectivity factors $Z$ in the S-band, we show the robust correlation between $d_{t} Z$ and the moistening of the troposphere.

This is the second of two papers that quantify the high added value of frequent 3-D radar observations of the atmosphere to capture the dynamics of weather systems. Recent advances in small-satellite and radar technologies, such as the “Radar in Cubesat” developed at the Jet Propulsion Laboratory, are paving the way for the design of convoys of spaceborne radars that can directly observe the evolution of severe weather at very fine temporal scales. The analyses presented here are to establish the relation between such observations to the underlying cloud variables and processes, and to quantify the sensitivity to the different physical and instrument parameters. In this paper, we quantify the uncertainty in the relation between the measured radar reflectivities $Z$ and their time derivatives $d_{t} Z$ , on one hand, and the underlying rate of change of the condensed-water mass $M$ , and fluxes of dry and moist air in convection, on the other hand. The uncertainties are due to the variability of the atmospheric parameters as well as the constraints of an observation strategy that would use pairs of spaceborne instruments. We specifically analyze the sensitivities for pairs of satellites, each carrying a Ka-band profiling radar. Our simulations show that, with a convoy of two spacecraft separated by ~90 s, each with a pointing accuracy of ~0.025° in rms error, a sensitivity of 17 dBZ and a precision of 1 dBZ, the proposed observation strategy would capture more than 70% of the tropical convection between 5 and 10 km of altitude and resolve the air-mass and condensed-water fluxes.