Factors Controlling Rain on Small Tropical Islands: Diurnal Cycle, Large-Scale Wind Speed, and Topography

A set of idealized cloud-permitting simulations is performed to explore the influence of small islands on precipitating convection as a function of large-scale wind speed. The islands are situated in a long narrow ocean domain that is in radiative–convective equilibrium (RCE) as a whole, constraining the domain-average precipitation. The island occupies a small part of the domain, so that significant precipitation variations over the island can occur, compensated by smaller variations over the larger surrounding oceanic area. While the prevailing wind speeds vary over flat islands, three distinct flow regimes occur. Rainfall is greatly enhanced, and a local symmetric circulation is formed in the time mean around the island, when the prevailing large-scale wind speed is small. The rainfall enhancement over the island is much reduced when the wind speed is increased to a moderate value. This difference is characterized by a change in the mechanisms by which convection is forced. A thermally forced sea breeze due to surface heating dominates when the large-scale wind is weak. Mechanically forced convection, on the other hand, is favored when the large-scale wind is moderately strong, and horizontal advection of temperature reduces the land–sea thermal contrast that drives the sea breeze. Further increases of the prevailing wind speed lead to strong asymmetry between the windward and leeward sides of the island, owing to gravity waves that result from the land–sea contrast in surface roughness as well as upward deflection of the horizontal flow by elevated diurnal heating. Small-amplitude topography (up to 800-m elevation is considered) has a quantitative impact but does not qualitatively alter the flow regimes or their dependence on wind speed

Response of convection to relative sea surface temperature: Cloud-resolving simulations in two and three dimensions

The properties of equilibrated tropical convection are studied using a cloud-resolving model with large-scale dynamics parameterized by the weak temperature gradient (WTG) approximation. Model integrations are performed in both 2-D and 3-D geometries. The target profile toward which horizontal mean free tropospheric temperature is relaxed is held fixed, while sea surface temperature (SST) is varied. Consistent with previous studies, large-scale ascent and precipitation increase under WTG as the SST is increased, but more rapidly in two dimensions than in three dimensions. This is related to greater extremes of near-surface buoyancy in two dimensions as well as a lower gross moist stability, and perhaps also to weaker entrainment. In both two and three dimensions, the vertical profiles of large-scale vertical velocity are top heavy and remarkably self-similar in shape as SST is increased. When all integrations are analyzed together, precipitation increases with column-integrated relative humidity once the latter reaches a threshold, as in observations and other models. However, within each integration, the two quantities are correlated negatively, albeit over a very narrow range

Workshop on Tropical Cyclones and Climate, March 27-29, 2006

In March 2006, the International Research Institute for Climate and Society (IRI) hosted a two and a half day workshop on "Tropical Cyclones and Climate". The centerpiece of the workshop was a set of invited lectures, with a modest number of contributed oral presentations and a small poster session. Relatively lengthy discussion periods were built into the schedule, allowing in-depth discussion of the presentations and related issues

Characteristics of Western North Pacific Model Tropical Cyclogenesis

"Tropical cyclogenesis" in a low-resolution Atmospheric General Circulation model is studied, focusing on the Western North Pacific region during the June-October typhoon season. Time-dependent composites of the cyclones are formed and analyzed, with a focus on the temporal evolution of quantities averaged in space around the storm centers. Day zero of each composite corresponds to the time at which the cyclone passes the criteria for detection. Some variables whose magnitude is related to cyclone intensity (such as low-level vorticity and surface wind speed) show similar temporal evolution, with a slight decrease up to a few days before day zero, a weak local minimum at that point, and a strong increase after that for a week or more. The relative humidity at low levels has its minimum somewhat later, at about day zero. The mean composite environmental vertical wind shear lacks a minimum and increases monotonically through the entire genesis period until a week after day zero. This variation is mostly due to the mean cyclone track's moving through regions of different climatological shear, which varies monotonically from easterly to westerly, crossing zero shortly after day zero, and would be consistent with a controlling role of the shear on model cyclogenesis. A signal in the skewness of the lower-level relative humidity distribution over the ensemble suggests that a dry lower troposphere can prevent development of a model cyclone. The local minimum in many variables' time series suggests the presence of an initial disturbance that is suddenly enhanced, becoming a model tropical cyclone, as has been noted in observations

Moisture Modes and the Eastward Propagation of the MJO

The authors discuss modifications to a simple linear model of intraseasonal moisture modes. Wind–evaporation feedbacks were shown in an earlier study to induce westward propagation in an eastward mean low-level flow in this model. Here additional processes, which provide effective sources of moist static energy to the disturbances and which also depend on the low-level wind, are considered. Several processes can act as positive sources in perturbation easterlies: zonal advection (if the mean zonal moisture gradient is eastward), modulation of synoptic eddy drying by the MJO-scale wind perturbations, and frictional convergence. If the sum of these is stronger than the wind–evaporation feedback—as observations suggest may be the case, though with considerable uncertainty—the model produces unstable modes that propagate weakly eastward relative to the mean flow. With a small amount of horizontal diffusion or other scale-selective damping, the growth rate is greatest at the largest horizontal scales and decreases monotonically with wavenumber

The Hadley Circulation and the Weak Temperature Gradient Approximation

The weak temperature gradient (WTG) approximation is applied to simple shallow-water models of the Hadley circulation. While it is difficult to formally justify the use of the WTG approximation for this problem, the derived WTG solutions are shown to agree well with numerical solutions of the full equations and to converge to the traditional angular momentum conserving (AMC) solutions in the inviscid limit. Heuristic arguments are given to explain this. The WTG method also provides semianalytical solutions in the case of nonvanishing viscosity, in contrast to the AMC solutions, which are strictly inviscid