Perturbations to the orographic gravity wave parameterization scheme in an idealized general circulation model reveal a remarkable degree of compensation between the parameterized and the resolved wave driving: when the orographic gravity wave driving is changed, the resolved wave driving tends to change in the opposite direction, so there is little impact on the Brewer–Dobson circulation. Building upon earlier observations of such compensation, an analysis based on quasigeostrophic theory suggests that the compensation between the resolved and parameterized waves is inevitable when the stratosphere is driven toward instability by the parameterized gravity wave driving. This instability, however, is quite likely for perturbations of small meridional length scale in comparison with the Rossby radius of deformation. The insight from quasigeostrophic theory is confirmed in a systematic study with an idealized general circulation model and supported by analyses of comprehensive models. The compensation between resolved and unresolved waves suggests that the commonly used linear separation of the Brewer–Dobson circulation into components (i.e., resolved versus parameterized wave driving) may provide a potentially misleading interpretation of the role of different waves. It may also, in part, explain why comprehensive models tend to agree more on the total strength of the Brewer–Dobson circulation than on the flow associated with individual components. This is of particular relevance to diagnosed changes in the Brewer–Dobson circulation in climate scenario integrations as well.

Recent studies have revealed strong interactions between resolved Rossby wave and parameterized gravity wave driving in stratosphere-resolving atmospheric models. Perturbations to the parameterized wave driving are often compensated by opposite changes in the resolved wave driving, leading to ambiguity in the relative roles of these waves in driving the Brewer–Dobson circulation. Building on previous work, this study identifies three mechanisms for these interactions and explores them in an idealized atmospheric model. The three mechanisms are associated with a stability constraint, a potential vorticity mixing constraint, and a nonlocal interaction driven by modifications to the refractive index of planetary wave propagation. While the first mechanism is likely for strong-amplitude and meridionally narrow parameterized torques, the second is most likely for parameterized torques applied inside the winter-hemisphere surf-zone region, a key breaking region for planetary waves. The third mechanism, on the other hand, is most relevant for parameterized torques just outside the surf zone. It is likely for multiple mechanisms to act in concert, and it is largely a matter of the torques' location and the interaction time scale that determines the dominant mechanism. In light of these interactions, the conventional paradigm for separating the relative roles of Rossby and gravity wave driving by downward control is critiqued. A modified approach is suggested, one that explicitly considers the impact of wave driving on the potential vorticity of the stratosphere. While this approach blurs the roles of Rossby and gravity waves, it provides more intuition into how perturbations to each component impact the circulation as a whole.

According to observations of hurricanes located relatively close to the land, intense and persistent lightning takes place within a 250–300-km radius ring around the hurricane center, whereas the lightning activity in the eyewall takes place only during comparatively short periods usually attributed to eyewall replacement. The mechanism responsible for the formation of the maximum flash density at the tropical cyclone (TC) periphery is not well understood as yet. In this study it is hypothesized that lightning at the TC periphery arises under the influence of small continental aerosol particles (APs), which affect the microphysics and the dynamics of clouds at the TC periphery. To show that aerosols change the cloud microstructure and the dynamics to foster lightning formation, the authors use a 2D mixed-phase cloud model with spectral microphysics. It is shown that aerosols that penetrate the cloud base of maritime clouds dramatically increase the amount of supercooled water, as well as the ice contents and vertical velocities. As a result, in clouds developing in the air with high AP concentration, ice crystals, graupel, frozen drops and/or hail, and supercooled water can coexist within a single cloud zone, which allows collisions and charge separation. The simulation of possible aerosol effects on the landfalling tropical cyclone has been carried out using a 3-km-resolution Weather Research and Forecast (WRF) mesoscale model. It is shown that aerosols change the cloud microstructure in a way that permits the attribution of the observed lightning structure to the effects of continental aerosols. It is also shown that aerosols, which invigorate clouds at 250–300 km from the TC center, decrease the convection intensity in the TC center, leading to some TC weakening. The results suggest that aerosols change the intensity and the spatial distribution of precipitation in landfalling TCs and can possibly contribute to the weekly cycle of the intensity and precipitation of landfalling TCs. More detailed investigations of the TC–aerosol interaction are required.

Little is known about the genesis and growth mechanisms of monsoon depressions, despite the great importance of these storms for the hydrological cycle of the Asian–Australian monsoon region. Of the few theoretical studies that have examined this issue, most have attributed the amplification of monsoon depressions to some form of baroclinic instability or stable baroclinic growth, highly modified by the diabatic effects of moist convection. Here, a simple criterion—namely, the upshear tilt of potential vorticity anomalies—is argued be necessary for dry or moist baroclinic growth. Reanalysis data are then used to assess whether a large ensemble of South Asian monsoon depressions has vertical structures consistent with this criterion. The evolution of these monsoon depressions is compared with that of ensembles of hurricanes and diabatic Rossby waves, the latter being prototypical examples of moist baroclinic instability. During their amplification phase, monsoon depressions do not exhibit an upshear tilt of potential vorticity anomalies. Many similarities are found between developing monsoon depressions and hurricanes but few with diabatic Rossby waves. Thus, the mechanism responsible for the intensification of monsoon depressions remains unknown, but these results indicate greater similarity with the general process of tropical depression spinup than with moist convectively coupled baroclinic instability.