The evolution of vortex tilt and vertical motion of tropical cyclones in directional shear flows

Recent studies have demonstrated the importance of moist dynamics on the intensification variability of tropical cyclones (TCs) in directional shear flows. Here, we propose that dry dynamics can account for many aspects of the structure change of TCs in moist simulations. The change of vortex tilt with height and time essentially determines the kinematic and thermodynamic structure of TCs experiencing directional shear flows, depending on how the environmental flow rotates with height; i.e., in a clockwise (CW) or counter-clockwise (CC) fashion. The vortex tilt precesses faster and is closer to the left-of-shear (with respect to the deep-layer shear), with smaller magnitude at equilibrium in CW hodographs than in CC hodographs. The low-level vortex tilt, and accordingly more low-level upward motions are ahead of the overall vortex tilt in CW hodographs, but are behind of the overall vortex tilt in CC hodographs. Such the configuration of vortex tilt in CW hodographs is potentially favorable for the continuous precession of convection into the up-shear region, but in CC hodographs is unfavorable. Most of the upward motions within a TC undergoing CW shear are concentrated in the down-shear-left region, whereas those in the CC shear are located in the down-shear-right region. Moreover, the upward (downward) motions are in-phase with positive (negative) local helicity in both CW and CC hodographs. Here we present an alternative mechanism that is associated with balanced dynamics in response to vortex tilt to explain the coincidence and also the distribution variability of vertical motions, as well as local helicity in directional shear flows. The balanced dynamics could explain the overlap of positive helicity and convection in both moist simulations and observations

Where is the future of China’s biogas? Review, forecast, and policy implications

This paper discusses the history and present status of different categories of biogas production in China, most of which are classified into rural household production, agriculture-based engineering production, and industry-based engineering production. To evaluate the future biogas production of China, five models including the Hubbert model, the Weibull model, the generalized Weng model, the H–C–Z model, and the Grey model are applied to analyze and forecast the biogas production of each province and the entire country. It is proved that those models which originated from oil research can also be applied to other energy sources. The simulation results reveal that China’s total biogas production is unlikely to keep on a fast-growing trend in the next few years, mainly due to a recent decrease in rural household production, and this greatly differs from the previous goal set by the official department. In addition, China’s biogas production will present a more uneven pattern among regions in the future. This paper will give preliminary explanation for the regional difference of the three biogas sectors and propose some recommendations for instituting corresponding policies and strategies to promote the development of the biogas industry in China

Intensification variability of tropical cyclones in directional shear flows: vortex tilt-convection coupling

The coupling of vortex tilt and convection, and their effects on the intensification variability of tropical cyclones (TCs) in directional shear flows is investigated. The height-dependent vortex tilt controls TC structural differences in clockwise (CW) and counter-clockwise (CC) hodographs during their initial stage of development. Moist convection may enhance the coupling between displaced vortices at different levels and thus reduce the vortex tilt amplitude and enhance precession of the overall vortex tilt during the early stage of development. However, differences in the overall vortex tilt between CW and CC hodographs are further amplified by a feedback from convective heating and therefore result in much higher intensification rates for TCs in CW hodographs than in CC hodographs. In CW hodographs, convection organization in the left-of-shear region is favored because the low-level vortex tilt is ahead of the overall vortex tilt and the TC moves to the left side of the deep-layer shear. This results in a more humid mid-troposphere and stronger surface heat flux on the left side (azimuthally downwind) of the overall vortex tilt, thus providing a positive feedback and supporting continuous precession of the vortex tilt into the up-shear-left region. In CC hodographs, convection tends to organize in the right side (azimuthally upwind) of the overall vortex tilt because the low-level vortex tilt is behind the overall vortex tilt and the TC moves to the right side of the deep-layer shear. In addition, convection organizes radially outward near the down-shear-right region, which weakens convection within the inner region. These configurations lead to a drier mid-troposphere and weaker surface heat flux in the downwind region of the overall vortex tilt and also a broader potential vorticity skirt. As a result, a negative feedback is established that prevents continuous precession of the overall vortex tilt