Simulation of Rapid Intensification of Super Typhoon Lekima (2019). Part I: Evolution Characteristics of Asymmetric Convection Under Upper-Level Vertical Wind Shear

The role of the upper-level vertical wind shear (VWS) on the rapid intensification (RI) of super typhoon Lekima (2019) is investigated with a high-resolution numerical simulation. Our simulation shows that under moderate upper-level easterly VWS, the tilting-induced convective asymmetry is transported from the initially downshear quadrant to the upshear quadrant and wrapped around the storm center by the cyclonic flow of the storm while moving inward. This process enhances upward motions at the upshear flank and creates upper-level divergent flow. As such, the establishment of outflow acts against the environmental flow to reduce the VWS, allowing vertical alignment of the storm. The organized outflow plays an important role in sustaining the inner-core deep convection by modulating the environmental upper-level thermal structure. Accompanying deep convective bursts (CBs), cold anomalies are generated in the tropopause layer due to the adiabatic cooling by the upward motion and radiative process associated with the cloud anvil. Physically, cold anomalies at the tropopause locally destabilize the atmosphere and enhance the convections and the secondary circulation. The CBs continue to develop episodically through this process as they wrap around the storm center to form a symmetric eyewall. The results suggest that deep convections are capable of reducing the upper-level VWS, promoting the development of upper-level outflow. Lekima overcame the less favorable environment and eventually intensified to become a super typhoon.

A Comparison of HWRF Six-Hourly 4DEnVar and Hourly 3DEnVar Assimilation of Inner Core Tail Dopper Radar Observations for the Prediction of Hurricane Edouard (2014)

Six-hourly three-dimensional ensemble variational (3DEnVar) (6H-3DEnVar) data assimilation (DA) assumes constant background error covariance (BEC) during a six-hour DA window and is, therefore, unable to account for temporal evolution of the BEC. This study evaluates the one-hourly 3DEnVar (1H-3DEnVar) and six-hourly 4DEnVar (6H-4DEnVar) DA methods for the analyses and forecasts of hurricanes with rapidly evolving BEC. Both methods account for evolving BEC in a hybrid EnVar DA system. In order to compare these methods, experiments are conducted by assimilating inner core Tail Doppler Radar (TDR) wind for Hurricane Edouard (2014) and by running the Hurricane Weather Research and Forecasting (HWRF) model. In most metrics, 1H-3DEnVar and 6H-4DEnVar analyses and forecasts verify better than 6H-3DEnVar. 6H-4DEnVar produces better thermodynamic analyses than 1H-3DEnVar. Radar reflectivity shows that 1H-3DEnVar produces better structure forecasts. For the first 24–48 h of the intensity forecast, 6H-4DEnVar forecast performs better than 1H-3DEnVar verified against the best track. Degraded 1H-3DEnVar forecasts are found to be associated with background storm center location error as a result of underdispersive ensemble storm center spread. Removing location error in the background improves intensity forecasts of 1H-3DEnVar.

MTrack: Improved Center Fix of Tropical Cyclones from SMAP Wind Observations

MTrack is an automated algorithm which determines the center location (latitude and longitude) of a tropical cyclone from a scalar wind field derived from satellite observations. Accurate storm centers are useful for operational forecasting of tropical cyclones and for their reanalysis (e.g. research on storm surge modeling). Currently, storm center fixes have significantly larger errors for weak, disorganized storms. The MTrack algorithm presented here improves storm centers in some of those cases. It is also automated and, therefore, less subjective than manual fixes made by forecasters. The MTrack algorithm, which was originally designed to work with CYGNSS wind speed measurements, is applied to SMAP winds for the first time. The average difference between MTrack and Best Track storm center locations is 21, 36 and 46 km for major hurricanes, category 1-2 hurricanes, and tropical storms, respectively. MTrack is shown to operate successfully when a storm is only partially sampled by the observing satellite and when the eye of the storm is not resolved.

Tropical Cyclone Center Fix Using CYGNSS Winds

A new method is described for determining the center location of a tropical cyclone (TC) using wind speed measurements by the NASA Cyclone Global Navigation Satellite System (CYGNSS). CYGNSS measurements made during TC overpasses are used to constrain a parametric wind speed model in which storm center location is varied. The “MTrack” storm center location is selected to minimize the residual difference between model and measurement. Results of the MTrack center fix are compared to the National Hurricane Center (NHC) Best Track, the Automated Rotational Center Hurricane Eye Retrieval (ARCHER), and aircraft reconnaissance fixes for category 1–category 3 TCs during the 2017 and 2018 hurricane seasons. MTrack produces storm center locations at intermediate times between NHC fixes with a factor of 5.6 overall reduction in sensitivity to uncertainties in the NHC fixes between which it interpolates. The MTrack uncertainty is found to be larger in the cross-track direction than the along-track direction, although this behavior and the absolute accuracy of position estimates require further investigation.

Diurnal Cycle of Rainfall Associated with Landfalling Tropical Cyclones in China from Rain Gauge Observations

The diurnal variation of rainfall over China associated with landfalling tropical cyclones (TCs) is investigated using hourly rain gauge observations obtained from 2425 conventional meteorological stations in China. Records between 12 h prior to landfall and 12 h after landfall of 450 landfalling TCs in China from 1957 to 2014 are selected as samples. The harmonic analysis shows an obvious diurnal signal in TC rainfall with a rain-rate peak in the early morning and a minimum in the afternoon. The diurnal cycle in the outer region (between 400- and 900-km radii from the storm center) is found to be larger than in the core region (within 400 km of the storm center). This could be attributed to the effect of land on the inner core of the storms as the diurnal cycle is distinct in the core region well before landfall. As the result of this diurnal cycle, TCs making landfall at night tend to have cumulative precipitation, defined as the precipitation cumulated from the time at landfall to 12 h after landfall, about 30% larger than those making landfall around noon or afternoon. Moreover, the radial propagation of the diurnal cycle in TC rain rate, which has been a controversial phenomenon in some previous studies with remote sensing observations, was not present in this study that is based on rain gauge observations. Results also show that the diurnal signal has little dependence on the storm intensity 12 h prior to landfall.

Influence of Mesoscale Topography on Tropical Cyclone Tracks: Further Examination of the Channeling Effect

Observations have documented typhoons experiencing pronounced track deflection before making landfall in Taiwan. In this study, idealized full-physics model experiments are conducted to assess the orographic influence on tropical cyclone (TC) track. An intense and westward-moving TC is simulated to approach the bell-shaped terrain imitating the Taiwan topography. Sensitivity numerical experiments are carried out to evaluate the topographic effect under different flow regimes and parameters, such as TC intensity, terrain height, and incident angle of the TC movement toward the topography. All the presented simulated storms experience southward track deflection prior to landfall. Different from the mechanism related to the channeling-effect-induced low-level northerly jet as suggested in previous studies, this study indicates the leading role of the northerly asymmetric flow in the midtroposphere in causing the southward deflection of the simulated TC tracks. The midtropospheric northerly asymmetric flow forms as a result of the wind speeds restrained east of the storm center and winds enhanced/maintained west of the storm center. In all, this study highlights a new mechanism that contributes to the terrain-induced southward track deflection in addition to the traditional channeling effect.

A Study on the Asymmetric Rapid Intensification of Hurricane Earl (2010) Using the HWRF System

In this study, the results of a forecast from the operational Hurricane Weather Research and Forecast (HWRF) system for Hurricane Earl (2010) are verified against observations and analyzed to understand the asymmetric rapid intensification of a storm in a sheared environment. The forecast verification shows that HWRF captured well Earl’s observed evolution of intensity, convection asymmetry, wind field asymmetry, and vortex tilt in terms of magnitude and direction in the pre rapid and rapid intensification (RI) stages. Examination of the high-resolution forecast data reveals that the tilt was large at the RI onset and decreased quickly once RI commenced, suggesting that vertical alignment is the result instead of the trigger for RI. The RI onset is associated with the development of upper-level warming in the eye, which results from upper-level storm-relative flow advecting the warm air caused by subsidence warming in the upshear-left region toward the low-level storm center. This scenario does not occur until persistent convective bursts (CB) are concentrated in the downshear-left quadrant. The temperature budget calculation indicates that horizontal advection plays an important role in the development of upper-level warming in the early RI stage. The upper-level warming associated with the asymmetric intensification process occurs by means of the cooperative interaction of the convective-scale subsidence, resulting from CBs in favored regions and the shear-induced mesoscale subsidence. When CBs are concentrated in the downshear-left and upshear-left quadrants, the subsidence warming is maximized upshear and then advected toward the low-level storm center by the storm-relative flow at the upper level. Subsequently, the surface pressure falls and RI occurs.

Spiral Rainbands in a Numerical Simulation of Hurricane Bill (2009). Part I: Structures and Comparisons to Observations

This study examines spiral rainbands in a numerical simulation of Hurricane Bill (2009). This paper, the first part of the study, evaluates the structures of spiral rainbands and compares them to previous observations. Four types of spiral rainbands are identified: principal, secondary, distant, and inner rainbands. Principal rainbands tend to be stationary relative to the storm center, while secondary rainbands are more transient and move around the storm center. Both principal and secondary rainbands are tilted radially outward with height and have many of the commonly observed kinematic features, such as overturning secondary circulation and enhanced tangential velocity on their radially outward sides. Principal rainbands are bounded by very dry air on their radially outward sides. Distant rainbands are radially inward-tilting convective features that have dense cold pools near the surface. Inner rainbands are made of shallow convection that appears to have originated from near the eyewall. Differences in the structures of spiral rainbands between observations and the Hurricane Bill simulation are noted. The second part of the study investigates how inner rainbands propagate and makes comparison with previously proposed hypotheses such as vortex Rossby waves.