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

Faithful completion of images of scenic landmarks using internet images

Abstract—Previous works on image completion typically aim to produce visually plausible results rather than factually correct ones. In this paper, we propose an approach to faithfully complete the missing regions of an image. We assume that the input image is taken at a well-known landmark, so similar images taken at the same location can be easily found on the Internet. We first download thousands of images from the Internet using a text label provided by the user. Next, we apply two-step filtering to reduce them to a small set of candidate images for use as source images for completion. For each candidate image, a co-matching algorithm is used to find correspondences of both points and lines between the candidate image and the input image. These are used to find an optimal warp relating the two images. A completion result is obtained by blending the warped candidate image into the missing region of the input image. The completion results are ranked according to combination score, which considers both warping and blending energy, and the highest ranked ones are shown to the user. Experiments and results demonstrate that our method can faithfully complete images

A Vertically Resolved MSE Framework Highlights the Role of the Boundary Layer in Convective Self-Aggregation

Convective self-aggregation refers to a phenomenon in which random convection can self-organize into large-scale clusters over an ocean surface with uniform temperature in cloud-resolving models. Previous literature studies convective aggregation primarily by analyzing vertically integrated (VI) moist static energy (MSE) variance. That is the global MSE variance, including both the local MSE variance at a given altitude and the covariance of MSE anomalies between different altitudes. Here we present a vertically resolved (VR) MSE framework that focuses on the local MSE variance to study convective self-aggregation. Using a cloud-resolving simulation, we show that the development of self-aggregation is associated with an increase of local MSE variance, and that the diabatic and adiabatic generation of the MSE variance is mainly dominated by the boundary layer (BL). The results agree with recent numerical simulation results and the available potential energy analyses showing that the BL plays a key role in the development of self-aggregation. We further present a detailed comparison between the global and local MSE variance frameworks in their mathematical formulation and diagnostic results, highlighting their differences.Comment: 50 pages, 2 tables, 12 figures, submitted to the Journal of the Atmospheric Science

Effects of Vertical Wind Shear on Intensity and Rainfall Asymmetries of Strong Tropical Storm Bilis (2006)

Abstract The effects of environmental vertical wind shear (VWS) on the intensity and rainfall asymmetries in Tropical Storm (TS) Bilis (2006) have been analyzed based on the TRMM/TMI estimated surface rainfall data, the QuikSCAT wind fields, 850-hPa and 200-hPa winds of the NCEP/NCAR reanalysis, the precipitation data at 5-minute intervals from automatic weather stations over mainland China, and the best track data of TS Bilis. The results show that the simultaneous and 6h-lagged correlation coefficients between VWS and storm intensity (the minimum central sea level pressure) are 0.59145 and 0.57438 (P<0.01), respectively. The averaged VWS was found to be about 11 m s -1 and thus suppressed the intensification of Bilis. Distribution of precipitation in Bilis was highly asymmetric. The azimuthally averaged rainfall rate in the partial eyewall, however, was smaller than that in a major outer rainband. As the storm intensified, the major rainband showed an unusual outward propagation. The VWS had a great impact on the asymmetric distribution of precipitation. Consistent with previous modeling studies, heavy rainfall generally occurred downshear to downshear-left of the VWS vector both near and outside the eyewall, showing a strong wavenumber-one asymmetry, which was amplified as the VWS increased

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

Impact of Indo-Pacific warm pool Hadley circulation on the seasonal forecast performance for summer precipitation over the western North Pacific

The western North Pacific summer climate affects the densely populated east Asian countries, while seasonal forecasting over these regions remains challenging for dynamical models. This study evaluates the effect of the spring Indo-Pacific warm pool (IPWP) Hadley circulation on the forecast performance for summer western North Pacific (WNP) precipitation in the Met Office Global Seasonal Forecast System (GloSea5). GloSea5 skillfully predicts IPWP Hadley circulation, but has moderate forecast skill for summer WNP precipitation. Compared to observations, the significant relationship between the spring IPWP Hadley circulation and the summer WNP precipitation is overestimated in most hindcast members and in the ensemble mean. Furthermore, we confirmed that the forecast ensemble members with a stronger such regional circulation-precipitation relationship have better forecast performance for the summer WNP precipitation, suggesting the importance of such a relationship for the seasonal forecast of WNP precipitation. These results imply the need to further investigate other important factors for the WNP precipitation, the effects of which may be suppressed by the overestimated regional circulation-precipitation relationship in GloSea5

The rubber tree genome reveals new insights into rubber production and species adaptation

The Para rubber tree (Hevea brasiliensis) is an economically important tropical tree species that produces natural rubber, an essential industrial raw material. Here we present a high-quality genome assembly of this species (1.37 Gb, scaffold N50 = 1.28 Mb) that covers 93.8% of the genome (1.47 Gb) and harbours 43,792 predicted protein-coding genes. A striking expansion of the REF/SRPP (rubber elongation factor/small rubber particle protein) gene family and its divergence into several laticifer-specific isoforms seem crucial for rubber biosynthesis. The REF/SRPP family has isoforms with sizes similar to or larger than SRPP1 (204 amino acids) in 17 other plants examined, but no isoforms with similar sizes to REF1 (138 amino acids), the predominant molecular variant. A pivotal point in Hevea evolution was the emergence of REF1, which is located on the surface of large rubber particles that account for 93% of rubber in the latex (despite constituting only 6% of total rubber particles, large and small). The stringent control of ethylene synthesis under active ethylene signalling and response in laticifers resolves a longstanding mystery of ethylene stimulation in rubber production. Our study, which includes the re-sequencing of five other Hevea cultivars and extensive RNA-seq data, provides a valuable resource for functional genomics and tools for breeding elite Hevea cultivars. The rubber tree (Hevea brasiliensis, hereafter referred to as Hevea) is a member of the spurge family (Euphorbiaceae), along with several other economically important species such as cassava (Manihot esculenta) and the castor oil plant (Ricinus communis). Natural rubber (cis-1, 4-polyisoprene) makes up about one-third of the volume of latex that is essentially cytoplasm of the articulated laticifers in Hevea. The latex is extracted by tapping the bark, a non-destructive method of harvesting that facilitates continual production. As an industrial commodity, natural rubber is an elastomer with physical and chemical properties that cannot be fully matched by synthetic rubber1. In contrast to synthetics, the production of natural rubber is sustainable and environment friendly2. The commercial cultivation of Hevea, a native to the Amazon Basin, began in 1896 on a plantation scale in Malaya (now Malaysia) and expanded to other Southeast Asian countries that lead in world natural rubber production today3. Decades of selective breeding have resulted in a gradual improvement in rubber productivity, from 650 kg ha–1 derived from unselected seedlings during the 1920s to 2,500 kg ha–1 yielded by elite cultivars by the 1990s4. Nevertheless, the field production achieved so far is still well below the theoretical yield of 7,000–12,000 kg ha–1, as has been suggested for the rubber tree5. Meanwhile, conventional rubber breeding has been stagnating in the introduction of high-yield cultivars. The reasons include a narrow genetic basis for exploiting breeding potential and difficulty in introducing wild germplasms because of the genetic burden in removing unfavourable alleles6. The incorporation of marker-assisted selection and transgenic techniques offers promise to improve breeding efficiency for latex yield, and sequencing of the Hevea genome would uncover even more avenues leading to this end. The first draft Hevea genome was released by a Malaysian team7 that was participant to the recent boom in transcriptomic and proteomic studies of the species8,9,10,11. However, its low sequence coverage (∼13×) and a lack of large insert libraries (such as fosmid- or BAC-based clone libraries) have limited the success of genome assembly (a scaffold N50 size of 2,972 bp), precluding its application for furthering quality research in the field. Here, we report a high-quality genome assembly of Hevea Reyan7-33-97, an elite cultivar widely planted in China12,13 based on sequence data from both whole-genome shotgun (WGS) and pooled BAC clones. This assembly contains 7,453 scaffolds (N50 = 1.28 Mb), has a length of 1.37 Gb and covers ∼94% of the predicted genome size (1.46 Gb). Together with analysis of data from re-sequencing five other cultivars and comprehensive transcriptomic surveys, we aim to obtain new insights into the physiology of laticifers and molecular details of rubber biosynthesis, especially in relation to ethylene-stimulated rubber production. (Résumé d'auteur