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Tropical Cyclone Research and Review  
  Tropical Cyclone Research and Review--2020, 9 (1)   Published: 2020-03-15
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Third assessment on impacts of climate change on tropical cyclones in the Typhoon Committee Region - Part I: Observed changes, detection and attribution

Tsz-Cheung Lee, Thomas R. Knutson, Toshiyuki Nakaegawa, Ming Ying, et al.
Tropical Cyclone Research and Review. 2020, 9 (1): 1;  doi: 10.1016/j.tcrr.2020.03.001
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Published findings on climate change impacts on tropical cyclones (TCs) in the ESCAP/WMO Typhoon Committee Region are assessed. We focus on observed TC changes in the western North Pacific (WNP) basin, including frequency, intensity, precipitation, track pattern, and storm surge. Results from an updated survey of impacts of past TC activity on various Members of the Typhoon Committee are also reported. Existing TC datasets continue to show substantial interdecadal variations in basin-wide TC frequency and intensity in the WNP. There has been encouraging progress in improving the consensus between different datasets concerning intensity trends. A statistically significant northwestward shift in WNP TC tracks since the 1980s has been documented. There is low-to-medium confidence in a detectable poleward shift since the 1940s in the average latitude where TCs reach their peak intensity in the WNP. A worsening of storm inundation levels is believed to be occurring due to sea level rise-due in part to anthropogenic influence-assuming all other factors equal. However, we are not aware that any TC climate change signal has been convincingly detected in WNP sea level extremes data. We also consider detection and attribution of observed changes based on an alternative Type II error avoidance perspective.

Towards an objective historical tropical cyclone dataset for the Australian region

Joseph B. Courtney, Andrew D. Burton, Christopher S. Velden, et al.
Tropical Cyclone Research and Review. 2020, 9 (1): 23;  doi: 10.1016/j.tcrr.2020.03.003
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The appropriate design of infrastructure in tropical cyclone (TC) prone regions requires an understanding of the hazard risk profile underpinned by an accurate, homogenous long-term TC dataset. The existing Australian region TC archive, or ‘best track’ (BT), suffers from inhomogeneities and an incomplete long-term record of key TC parameters. This study assesses mostly satellite-based objective techniques for 1981-2016, the period of a geostationary satellite imagery dataset corrected for navigation and calibration issues. The satellite-based estimates of Australian-region TCs suffer from a general degradation in the 1981-1988 period owing to lower quality and availability of satellite imagery.
The quality of the objective techniques for both intensity and structure is compared to the reference BT 2003-2016 estimates. For intensity the Advanced Dvorak Technique algorithm corresponds well with the BT 2003-2016, when the algorithm can use passive microwave data (PMW) as an input. For the period prior to 2003 when PMW data is unavailable, the intensity algorithm has a low bias. Systematic corrections were made to the non-PMW objective estimates to produce an extended (1989-2016) homogeneous dataset of maximum wind that has sufficient accuracy to be considered for use where a larger homogeneous sample size is valued over a shorter more accurate period of record. An associated record of central pressure using the Courtney-Knaff-Zehr wind pressure relationship was created.
For size estimates, three techniques were investigated: the Deviation Angle Variance and the ‘Knaff’ techniques (IR-based), while the ‘Lok’ technique used model information (ECMWF reanalysis dataset and TC vortex specification from ACCESS-TC). However, results lacked sufficient skill to enable extension of the reliable period of record. The availability of scatterometer data makes the BT 2003-2016 dataset the most reliable and accurate. Recommendations regarding the best data source for each parameter for different periods of the record are summarised.

Finite-time circulation changes from topological rearrangement of distinguished curves and non-advective fluxes

Blake Rutherford, Timothy J. Dunkerton
Tropical Cyclone Research and Review. 2020, 9 (1): 37;  doi: 10.1016/j.tcrr.2019.05.001
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A vorticity budget diagnoses the growth or decay of a vortex from advective transport, or non-advective fluxes such as those due to friction or vortex tilting. However, when a budget calculation is performed with respect to a fixed coordinate, errors may result from time-dependence of the flow, leading to disagreement between the vorticity tendency and the observed vorticity field. An adaptive Lagrangian coordinate resolves this problem, provided that the resulting Lagrangian structure does not become too complicated.
In this study, a numerical simulation of Hurricane Nate (2011), the vorticity tendency is evaluated along distinguished material curves. There can be no net advective flux along a closed material curve, therefore, the total circulation tendency for a material region includes only the non-advective uxes acting along its boundary. A distinguished set of material curves (DMCs) associated with a distinguished hyperbolic trajectory (DHT) form a Lagrangian topology similar to that of a cat’s eye flow or “pouch” at each Eulerian snapshot. The time-dependence of velocities allows additional regions called lobes, which are formed by the intersections of DMCs, to exchange fluid across the vortex boundary by redefining the boundary.
Because the vortex boundary changes, we refer to this redefinition of material boundary as “topological rearrangement”. The method is useful for unsteady cat’s-eye flows and more complex interactions of multiple waves, vortices and background shear. All advective changes of the vortex circulation are identified by exchanges of the lobes, and all non-advective uxes act between the vortex and either the lobes or environmental flow. The Lagrangian topology and combination of advective and non-advective uxes relative to the topology is used to describe the evolution of the circulation of Nate during its time of formation.

Improvement of wind field forecasts for tropical cyclones over the North Indian Ocean

S.D. Kotal, S.K. Bhattacharya
Tropical Cyclone Research and Review. 2020, 9 (1): 53;  doi: 10.1016/j.tcrr.2020.03.004
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This paper demonstrates a modification method for real-time improvement of wind field forecasts for a typical cyclone VARDAH, which formed over the Bay of Bengal (BoB) in 2016. The proposed method to improve the wind field forecasts associated with tropical cyclone consists of two components. The first one is the relocation method, which relocates the wind field forecasts obtained from the Global Forecast System (GFS) data of National Centres for Environmental Prediction(NCEP). The relocation of the model forecasts wind field is made on forecast locations generated by Multi Model Ensemble (MME) track forecast of India Meteorological Department(IMD). The second one is the modification of wind speed, which directly modifies the NCEP GFS wind speed forecasts based on intensity forecasts by Statistical Cyclone Intensity Prediction(SCIP) model of IMD. Applying these two methods, the displacement of wind field and underestimation/overestimation of wind speed in the model forecast field can be improved. Both modification methods show considerable improvements in the displacement and speed of wind field forecasts. The displacement error of wind field is found to have improved by about 51% at 48 h and about 80% at 72 h
forecast. Overestimation of maximum wind speed in the forecast field is found to be improved by about 88% at 48 h and about 38% at 72 h forecast. The spatial distributions of corrected wind speed forecasts are also found to be more analogous than direct model forecasts with the corresponding analysis wind at all forecast hours. Two proposed modification methods could provide improved wind field forecast associated with tropical cyclones in real-time.

Tropical cyclone track prediction using a large-area WRF model at the Hong Kong Observatory

Kai-Kwong Hon
Tropical Cyclone Research and Review. 2020, 9 (1): 67;  doi: 10.1016/j.tcrr.2020.03.002
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Tropical cyclone (TC) track predictions of the 10-km resolution WRF (provisionally named "AAMC-WRF") of the Hong Kong Observatory (HKO), spanning (20?S - 60?N, 45?E  160?E) is studied for a 1-year period from April 2018 to Mar 2019. Real-time predictions, up to 4 times a day and Tþ48 h ahead, are verified against operational analysis positions of HKO for storms over the South China Sea (SCS) and Western North Pacific (WNP); and of the New Delhi Regional Specialised Meteorological Centre (RSMC) for storms over the North Indian Ocean basin (NIO; including the Bay of Bengal). Out of 21 named TCs over SCS and WNP, mean positional errors of the AAMC-WRF are 33 km (Tþ0), 63 km (Tþ24), and 107 km (Tþ48) based on 209, 178 and 142 forecasts. The AAMC-WRF outperformed Meso-NHM, also run in real-time at HKO, with mean error reduction up to 34 km or 24%. Mean positional errors for 13 NIO storms are 38 km (Tþ0), 69 km (Tþ24) and 107 km (Tþ48) based on 183, 131 and 85 forecasts. This is the first study in which TC predictions of a regional model are simultaneously examined over the SCS, WNP and NIO basins through real-time experiments.

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