The variation of Tropical cyclone azimuthal wind speed (V) with distance from storm center (r) is a fundamental aspect of storm structure that has important implications for risk and damages. The theoretical model of Emanuel (2004), which applies well outside the rainy core of the storm, matches radiatively-driven subsidence and Ekman suction rates at the top of the boundary layer to obtain a nonlinear differential equation for dV/dr. This model is particularly appealing because of its strong physical foundation, but has no known analytic solution for V(r). In this paper, I obtain an analytic solution to V(r) for the Emanuel (2004) outer wind model. Following previous work, I then use this solution to explore properties of merged wind models that combine the outer model with an inner model that applies to the rainy core of a storm.

A document by Martin Velez Pardo. Click on the document to view its contents.

A document by Timothy Wallace Cronin. Click on the document to view its contents.

Improving understanding of the two-way interactions between clouds and large-scale atmospheric circulations requires modeling set-ups that can resolve cloud-scale processes, while also including representations of the circulations themselves. In this study, we investigate the potential for mock-Walker simulations to help untangle these interactions by assessing their ability to reproduce the observed climate over the equatorial Pacific. Mock-Walker simulations with realistic zonal sea-surface temperature (SST) gradients show qualitative similarities with reanalysis and satellite data, though notable differences include (1) the presence of double-celled overturning circulations, (2) extreme upper tropospheric dryness over the cold pools, and (3) substantially weaker longwave cloud radiative effects. The double-cell circulations are part of a transition from single to double cells as mean SST is increased, with the transition occurring near present day temperatures. The circulation changes dominate the response of mock-Walker simulations to warming, though their effects are smaller for relatively weak zonal SST gradients. Mock-Walker simulations also exhibit a wide range of climate sensitivities, due to cloud feedbacks that are strongly negative for larger SST gradients and strongly positive for weaker SST gradients. Finally, we show that radiative-subsidence balance can be used to explain the development of the double cells, but are unable to further explain the dynamics of the transition given the complex vertical profiles of stability and atmospheric radiative cooling in these simulations. Since Earth’s present-day climate is close to our simulated transition to a double-celled circulation, these dynamics merit further investigation.

There is no consensus on the physical mechanisms controlling the scale at which convective activity organizes near the Equator, where the Coriolis parameter is small. High resolution cloud-permitting simulations of non-rotating convection show the emergence of a dominant length scale, which has been referred to as convective self-aggregation. Furthermore, simulations in an elongated domain of size 12228km x 192km with a 3km horizontal resolution equilibrate to a wave-like pattern in the elongated direction, where the cluster size becomes independent of the domain size. These recent findings suggest that the size of convective aggregation may be regulated by physical mechanisms, rather than artifacts of the model configuration, and thus within the reach of physical understanding. We introduce a diagnostic framework relating the evolution of the length scale of convective aggregation to the net radiative heating, the surface enthalpy flux, and horizontal energy transport. We evaluate these length scale tendencies of convective aggregation in twenty high-resolution cloud-permitting simulations of radiative-convective equilibrium. While both radiative fluxes contribute to convective aggregation, the net longwave radiative flux operates at large scales (1000-5000 km) and stretches the size of moist and dry regions, while the net shortwave flux operates at smaller scales (500-2000 km) and shrinks it. The surface flux length scale tendency is dominated by convective gustiness, which acts to aggregate convective activity at smaller scales (500-3000 km). We further investigate the scale-by-scale radiative tendencies in a suite of nine mechanism denial experiments, in which different aspects of cloud radiation are homogenized or removed across the horizontal domain, and find that liquid and ice cloud radiation can individually aggregate convection. However, only ice cloud radiation can drive the convective cluster to scales exceeding 5000 km, because of the high optical thickness of ice, and the increase in coherence between water vapor and deep convection with horizontal scale. The framework presented here focuses on the length scale tendencies rather than a static aggregated state, which is a step towards diagnosing clustering feedbacks in the real world. Overall, our work underscores the need to observe and simulate surface fluxes, radiative and advective fluxes across the 1km-1000km range of scales to better understand the characteristics of turbulent moist convection.

The science guiding the \EURECA campaign and its measurements are presented. \EURECA comprised roughly five weeks of measurements in the downstream winter trades of the North Atlantic — eastward and south-eastward of Barbados. Through its ability to characterize processes operating across a wide range of scales, \EURECA marked a turning point in our ability to observationally study factors influencing clouds in the trades, how they will respond to warming, and their link to other components of the earth system, such as upper-ocean processes or, or the life-cycle of particulate matter. This characterization was made possible by thousands (2500) of sondes distributed to measure circulations on meso (200 km) and larger (500 km) scales, roughly four hundred hours of flight time by four heavily instrumented research aircraft, four global-ocean class research vessels, an advanced ground-based cloud observatory, a flotilla of autonomous or tethered measurement devices operating in the upper ocean (nearly 10000 profiles), lower atmosphere (continuous profiling), and along the air-sea interface, a network of water stable isotopologue measurements, complemented by special programmes of satellite remote sensing and modeling with a new generation of weather/climate models. In addition to providing an outline of the novel measurements and their composition into a unified and coordinated campaign, the six distinct scientific facets that \EURECA explored — from Brazil Ring Current Eddies to turbulence induced clustering of cloud droplets and its influence on warm-rain formation — are presented along with an overview \EURECA’s outreach activities, environmental impact, and guidelines for scientific practice.