The Weather and Research and Forecasting Model (WRF) is used to simulate secondary eyewall formation (SEF) in a tropical cyclone (TC) on the β plane. The simulated SEF process is accompanied by an outward expansion of kinetic energy and the TC warm core. An absolute angular momentum budget demonstrates that this outward expansion is predominantly a symmetric response to the azimuthal-mean and wavenumber-1 components of the transverse circulation. As the kinetic energy expands outward, the kinetic energy efficiency in which latent heating can be retained as local kinetic energy increases near the developing outer eyewall. The kinetic energy efficiency associated with SEF is examined further using a symmetric linearized, nonhydrostatic vortex model that is configured as a balanced vortex model. Given the symmetric tangential wind and temperature structure from WRF, which is close to a state of thermal wind balance above the boundary layer, the idealized model provides the transverse circulation associated with the symmetric latent heating and friction prescribed from WRF. In a number of ways, this vortex response matches the azimuthal-mean secondary circulation in WRF. These calculations suggest that sustained azimuthal-mean latent heating outside of the primary eyewall will eventually lead to SEF. Sensitivity experiments with the balanced vortex model show that, for a fixed amount of heating, SEF is facilitated by a broadening TC wind field.

Based on the daily maximum temperatures (Tmax) from 587 surface observation stations in China during 1959–2013, heat waves are detected using both absolute and relative definitions. The spatiotemporal variations of heat wave occurrence/duration/amplitude are compared between the two definitions. Considering the significant differences in regional climatology, relative threshold is more meaningful to detect the local extremes. By utilizing the empirical orthogonal function, the integral index heat wave total intensity is decomposed into three dominant modes: interdecadal (ID), interannual‐tripole (IA‐TR), and interannual‐dipole (IA‐DP) modes. The ID mode shows uniform anomalies over the whole China, with the maximum in north, and its corresponding time series depict notable interdecadal variations with a turning point around mid‐1990s. The IA‐DP mode exhibits opposite‐signed anomalies over north and south China. The IA‐TR mode shows an anomalous tripole pattern with negative anomalies over central China and positive anomalies over north and south China in its positive phase. Both the IA‐DP and IA‐TR patterns are more obvious since mid‐1990s with mainly year‐to‐year variations before that. All the three modes are controlled by anomalous high‐pressure systems, which are accompanied by local‐scale dry land conditions. The diabatic heating associated with anomalous convective activities over tropical western Pacific triggers Rossby wave trains propagating northward along the East Asia, which causes abnormal heat waves through descending motion over the high‐pressure nodes. In turn, the severe convections are generated by enhanced Walker circulation in the tropical Pacific due to warming and/or cooling sea surface temperature (SST) anomalies in the tropical western and eastern Pacific, respectively.

Based on a successful cloud-resolving simulation with the Weather Research and Forecasting Model, this study examines key processes that led to the early development of Hurricane Dolly (2008). The initial development of Dolly consisted of three stages: (i) an initial burst of convection; (ii) stratiform development, dry intrusion, and thermodynamic recovery; and (iii) reinvigoration of moist convection and rapid intensification. Advanced diagnosis of the simulation-including the use of vorticity budget analysis, contour frequency analysis diagrams, and two-dimensional spectral decomposition and filtering-suggests that the genesis of Dolly is essentially a ''bottom-up'' process. The enhancement of the low-level vorticity is mainly ascribed to the stretching effect, which converges the ambient vorticity through stretching enhanced by moist convection. In the rapid intensification stage, smaller-scale positive vorticity anomalies resulting from moist convection are wrapped into the storm center area under the influence of background convergent flow. The convergence and accompanying aggregation of vorticity anomalies project the vorticity into larger scales and finally lead to the spinup of the system-scale vortex. On the other hand, although there is apparent stratiform development in the inner-core areas of incipient storm after the initial burst of convection, little evidence is found to support the genesis of Dolly through downward extension of the midlevel vorticity, a key process in the ''topdown'' thinking.

Through cloud-resolving simulations, this study examines the effect of ß on the evolution of tropical cyclones (TCs). It is found that the TC simulated on a /3 plane with variable Coriolis parameter/is weaker in intensity but larger in size and strength than the TC simulated on an/plane with constant/. Such differences result mainly from the effect of the ß shear rather than from the variation of/due to the latitudinal change of the TC position, as illustrated in a three-stage conceptual model developed herein. The first stage begins with the establishment of the ß shear and the emergence of asymmetries as the TC intensifies. The ß shear peaks in value during the second stage that subsequently leads to the formation of an extensive stratiform region outside of the primary eyewall. The evaporative cooling associated with the stratiform precipitation acts to sharpen the low-level equivalent potential temperature gradient into a frontlike zone outside of the eyewall region, which leads to the burst of convection outside of the primary eyewall. The third stage is characterized by a weakening ß shear and the corresponding TC vortex axisymmetrization and expansion. The convection on the inner edge of the stratiform region becomes more organized in the azimuthal direction and eventually causes the TC structure to evolve in a manner similar to the secondary eyewall formation and eyewall replacement usually observed in TCs. It is the active convection outside of the primary eyewall that contributes to a relatively weaker but larger TC on the ß plane than that on the /plane.

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