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We present a series of idealized numerical model experiments to investigate aspects of deep convection in tropical depressions, including the effects of boundary-layer wind structure on storm structure, especially on vertical vorticity production and updraught splitting, and the combined effects of horizontal and vertical shear on vertical vorticity production, with and without background rotation. In warm-cored disturbances such as tropical depressions, the vertical shear and horizontal vorticity change sign at some level near the top of the boundary layer so that, unlike in the typical middle-latitude ‘supercell’ storm, the tilting of horizontal vorticity by a convective updraught leads not only to dipole patterns of vertical vorticity, but to a reversal also in sign of the updraught rotation with height. This finding has implications for understanding the merger of convectively induced vorticity anomalies during vortex evolution. Ambient cyclonic horizontal shear and/or cyclonic vertical vorticity favour amplification of the cyclonically rotating gyre of the dipole. Consistent with an earlier study, storm splitting occurs in environments with pure horizontal shear as well as pure vertical shear, but the morphology of splitting is different. In both situations, splitting is found to require a relatively unstable sounding and relatively strong wind shear.

We present idealized numerical model experiments to isolate and quantify the influence of ambient vertical vorticity on the dynamics of deep convection, such as that in the inner-core region of a tropical depression. The vertical vorticity is represented either by a uniform horizontal shear, a uniform solid-body rotation, or a combination of both. We show that the growing convective cells amplify locally the ambient rotation at low levels by more than an order of magnitude and that this vorticity, which is produced by the stretching of existing ambient vorticity, persists long after the initial updraught has decayed. Significant amplification of vorticity occurs even for a background rotation rate typical of the undisturbed tropical atmosphere and even for clouds of only moderate vertical extent. This finding has important implications for tropical cyclogenesis and provides a basis for a unified theory of tropical cyclogenesis and tropical cyclone intensification. The presence of ambient vertical vorticityreduces the updraught strength, slightly more when the vorticity is associated with horizontal shear than when it is associated with solid-body rotation on account of enhanced entrainment. The reduction of the updraught strength can be attributed to the reduction of the lateral inflow by the centrifugal and Coriolis forces. Despite the significant amplification of vorticity, the sum of the centrifugal and Coriolis forces is mostly small compared with the lateral pressure gradient. Thus, at the level of background rotation studied, the Rossby elasticity effects in the updraught postulated by Montgomery et al. are not large, but may be important for higher levels of background rotation. The simulations ignore several processes that are likely to be important in reality, such as ambient vertical shear and surface friction, but they provide important benchmark calculations for interpreting the additional complexity arising from the inclusion of these effects. Copyright c � 2011 Royal Meteorological Society

Idealized three-dimensional numerical simulations are used to investigate the influence of sea breezes on the evolution of multicell thunderstorms in the Tropics. The study is motivated by a desire to understand a particular type of severe storm system that occurs occasionally over the ‘Top End’ region of northern Australia, but the results should have wider applicability. The calculations are carried out using Bryan’s cloud model. The simulations reproduce the main features of the evolution of an observed storm system. New cells develop on the gust front of the initial updraught, behind the sea-breeze front, and subsequently merge to form a multicell thunderstorm. The propagation speed and direction, the orientation, and length of the line of updraught cells are all similar to those observed. The sea breezes in the model play an important role in the evolution of the storm. In general, the low-level convergence at the gust front produced by the initial cell needs to be strong enough and to persist for a sufficiently long time to allow new cells to develop near this front. A strong cold pool occurs if the initial updraught is sufficiently tilted so that the downdraught does not fall into it, but supplies cold air close to the gust front. A generalized form of the Rotunno–Klemp–Weisman criterion was used to test whether the new updraughts developed at locations where the cold pool circulation was largely opposed by that of the environmental shear. The comparison of the shear components normal to the gust front showed that new cells develop even though this criterion is not fulfilled, suggesting that the criterion is not applicable to the more complex flow configurations studied here. Copyright c

The influence of vertical wind shear on storm development within a tropical environment is studied with the aid of two numerical models and compared with that in simulations of midlatitude storms. The simulations show that larger wind shears are required in a tropical environment than in a midlatitude environment for a storm of given updraft velocity to split. This finding is supported by the experience of forecasters at the Australian Bureau of Meteorology Regional Forecasting Centre in Darwin that the operational storm forecasting tools developed for midlatitude storms overforecast supercells within the tropics. That tropical storms require higher shears to split can be attributed either to the larger gust front speed or to the earlier gust front occurrence compared to those in the midlatitudes. A fast gust front cuts off the storm from the warm moist inflow and the updraft has little or no time to split. In the cases where the midtropospheric relative humidity is larger in the tropics or comparable with that in the midlatitudes, the total liquid water and ice content within the deeper tropical storms is larger than in the midlatitude storms, causing a stronger downdraft. In other words, the main contribution to the negative buoyancy of the downdraft is the water loading rather than the evaporative cooling. When a tropical storm is simulated in an environment with smaller midtropospheric relative humidity than in the midlatitudes, the amount of liquid water and ice within the storm is comparable to that within the midlatitude storm. Intense evaporation within the tropical storm then leads to a stronger negative buoyancy than in the midlatitude storm, causing a stronger downdraft and thus an earlier or a faster-spreading gust front. At higher shears in the tropics, entrainment reduces the storm depth and thus water loading, resulting in a delayed gust front initiation and/or reduction of the gust front speed, which then allows storm splitting to occur.

The resolution of numerical weather prediction models is constantly increasing, making it necessary to consider three-dimensional radiative transfer effects such as cloud shadows cast into neighboring grid cells and thus affecting radiative heating. For that purpose, fast approximations are needed since three-dimensional radiative transfer solvers are computationally far too expensive. For the solar spectral range, different approaches of how to consider three-dimensional effects were presented in the past—in particular, the tilted independent column approximation (TICA), which aims at improving the calculation of the direct radiation, and the nonlocal tilted independent column approximation (NTICA), which is used to additionally correct the diffuse radiation. Here a new version of NTICA is presented that—in contrast to earlier approaches—is applicable for a variety of cloud scenes and grid resolutions and for arbitrary solar zenith angles. This new parameterization for the diffuse irradiance is then applied to the two different TICA approaches and the results are compared with a full 3D Monte Carlo calculation. It is shown that both approaches strongly improve the calculation of radiation fluxes if the new parameterization for the diffuse irradiance—what the authors call “parameterized NTICA (paNTICA)”—is applied. It is found that the method in which TICA is only applied to direct radiation yields the better results. The studies show that consideration of three-dimensional effects is inevitable if higher model resolutions are used in the future. This paper proposes ways to consider these effects and, thus, to substantially reduce the errors made with one-dimensional radiative transfer solvers.