Urban Impact on Landfalling Tropical Cyclone Precipitation:A Numerical Study of Typhoon Rumbia(2018)

Coastal urban areas are prone to serious disasters caused by landfalling tropical cyclones(TCs). Despite the crucial role of urban forcing in precipitation, how fine-scale urban features impact landfalling TC precipitation remains poorly understood. In this study, high-resolution ensemble simulations of Typhoon Rumbia(2018), which crossed the Yangtze River Delta urban agglomeration, were conducted to analyze the potential urban impact on TC precipitation. Results show that the inner-core rainfall of Rumbia is strengthened by approximately 10% due to the urban impact near the landfall,whereas minor differences in outer-core rainfall are found when the urban impact is excluded. Further diagnostic analyses indicate that low-level upward motion is crucial for precipitation evolution, as both co-vary during landfall. Moreover, the frictionally induced upward motion plays a decisive role in enhancing the rainfall when the urban impacts are included.Urban surface friction can decelerate the tangential wind and therefore destroy the gradient balance and strengthen the radial wind within the boundary layer and thus can enhance upward motion. This study demonstrates that urban surface friction and related physical processes make the most significant contribution to landfalling TC rainfall enhancement.

Impacts of the Diurnal Cycle of Solar Radiation on Spiral Rainbands

Based on idealized numerical simulations, the impacts of the diurnal cycle of solar radiation on the diurnal variation of outer rainbands in a tropical cyclone are examined. It is found that cold pools associated with precipitation-driven downdrafts are essential for the growth and propagation of spiral rainbands. The downdrafts result in surface outflows, which act as a lifting mechanism to trigger the convection cell along the leading edge of the cold pools. The diurnal cycle of solar radiation may modulate the diurnal behavior of the spiral rainbands. In the daytime, shortwave radiation will suppress the outer convection and thus weaken the cold pools. Meanwhile, the limited cold pool activity leads to a strong modification of the moisture field, which in turn inhibits further convection development.

Variational Quality Control of Non-Gaussian Innovations in the GRAPES m3DVAR System: Mass Field Evaluation of Assimilation Experiments

The existence of outliers can seriously influence the analysis of variational data assimilation.Quality control allows us to effectively eliminate or absorb these outliers to produce better analysis fields.In particular,variational quality control(VarQC) can process gray zone outliers and is thus broadly used in variational data assimilation systems.In this study,governing equations are derived for two VarQC algorithms that utilize different contaminated Gaussian distributions(CGDs): Gaussian plus flat distribution and Huber norm distribution.As such,these VarQC algorithms can handle outliers that have non-Gaussian innovations.Then,these VarQC algorithms are implemented in the Global/Regional Assimilation and PrEdiction System(GRAPES) model-level three-dimensional variational data assimilation(m3 DVAR) system.Tests using artificial observations indicate that the VarQC method using the Huber distribution has stronger robustness for including outliers to improve posterior analysis than the VarQC method using the Gaussian plus flat distribution.Furthermore,real observation experiments show that the distribution of observation analysis weights conform well with theory,indicating that the application of VarQC is effective in the GRAPES m3 DVAR system.Subsequent case study and longperiod data assimilation experiments show that the spatial distribution and amplitude of the observation analysis weights are related to the analysis increments of the mass field(geopotential height and temperature).Compared to the control experiment,VarQC experiments have noticeably better posterior mass fields.Finally,the VarQC method using the Huber distribution is superior to the VarQC method using the Gaussian plus flat distribution,especially at the middle and lower levels.

Simulated Sensitivity of the Tropical Cyclone Eyewall Replacement Cycle to the Ambient Temperature Profile

In this study, the impacts of the environmental temperature profile on the tropical cyclone eyewall replacement cycle are examined using idealized numerical simulations. It is found that the environmental thermal condition can greatly affect the formation and structure of a secondary eyewall and the intensity change during the eyewall replacement cycle. Simulation with a warmer thermal profile produces a larger moat and a prolonged eyewall replacement cycle. It is revealed that the enhanced static stability greatly suppresses convection, and thus causes slow secondary eyewall formation. The possible processes influencing the decay of inner eyewall convection are investigated. It is revealed that the demise of the inner eyewall is related to a choking effect associated with outer eyewall convection, the radial distribution of moist entropy fluxes within the moat region, the enhanced static stability in the inner-core region, and the interaction between the inner and outer eyewalls due to the barotropic instability. This study motivates further research into how environmental conditions influence tropical cyclone dynamics and thermodynamics.

Modulation of High-Latitude Tropical Cyclone Recurvature by Solar Radiation

In this study,idealized simulations are conducted to investigate potential influences of solar radiation on the tropical cyclone(TC) recurvature at higher latitudes.Results indicate that TC track is sensitive to the seasonal variation of radiative forcing at higher latitudes.In the absence of a background flow,TCs at higher latitudes tend to recurve(remain northwestward) in the cold(warm) season.This feature is an additional aspect of the so-called intrinsic recurvature property of TC movement at high latitude.Physically,the greater meridional gradient of temperature in the cold season due to solar radiative forcing would induce a larger thermal wind,which affects the upper-level anticyclonic circulation and associated outflow.The structure changes of TC,mainly at upper-levels,modulate the steering flow for TC,leading to a higher probability of TCs at higher latitudes to recurve in the cold season than in the warm season.

DEPENDENCE OF TROPICAL CYCLONE INTENSIFICATION ON THE CORIOLIS PARAMETER

The dependence of tropical cyclone(TC) intensification on the Coriolis parameter was investigated in an idealized hurricane model. By specifying an initial balanced vortex on an f-plane, we observed faster TC development under lower planetary vorticity environment than under higher planetary vorticity environment. The diagnosis of the model outputs indicates that the distinctive evolution characteristics arise from the extent to which the boundary layer imbalance is formed and maintained in the presence of surface friction. Under lower planetary vorticity environment, stronger and deeper subgradient inflow develops due to Ekman pumping effect, which leads to greater boundary layer moisture convergence and condensational heating. The strengthened heating further accelerates the inflow by lowing central pressure further. This positive feedback loop eventually leads to distinctive evolution characteristics.The outer size(represented by the radius of gale-force wind) and the eye of the final TC state also depend on the Coriolis parameter. The TC tends to have larger(smaller) outer size and eye under higher(lower) planetary vorticity environment. Whereas the radius of maximum wind or the eye size in the current setting is primarily determined by inertial stability, the TC outer size is mainly controlled by environmental absolute angular momentum.

Dependence of Tropical Cyclone Intensification on the Latitude under Vertical Shear

The sensitivity of tropical cyclone（TC） intensification to the ambient rotation effect under vertical shear is investigated. The results show that the vortices develop more rapidly with intermediate planetary vorticity, which suggests an optimal latitude for the TC development in the presence of vertical shear. This is different from the previous studies in which no mean flow is considered. It is found that the ambient rotation has two main effects. On the one hand,the boundary layer imbalance is largely controlled by the Coriolis parameter. For TCs at lower latitudes, due to the weaker inertial instability, the boundary inflow is promptly established, which results in a stronger moisture convergence and thus greater diabatic heating in the inner core region. On the other hand, the Coriolis parameter modulates the vertical realignment of the vortex with a higher Coriolis parameter, favoring a quicker vertical realignment and thus a greater potential for TC development. The combination of these two effects results in an optimal latitude for TC intensification in the presence of a vertical shear investigated.

How Does Tropical Cyclone Size Affect the Onset Timing of Secondary Eyewall Formation？

By using idealized numerical simulations, the impact of tropical cyclone size on secondary eyewall formation（SEF） is examined. Both unbalanced boundary layer and balanced processes are examined to reveal the underlying mechanism. The results show that a tropical cyclone（TC） with a larger initial size favors a quicker SEF and a larger outer eyewall. For a TC with a larger initial size, it will lead to a stronger surface entropy flux, and thus more active outer convection. Meanwhile, a greater inertial stability helps the conversion from diabatic heating to kinetic energy.Furthermore, the progressively broadening of the tangential wind field will induce significant boundary layer imbalances. This unbalanced boundary layer process results in a supergradient wind zone that acts as an important mechanism for triggering and maintaining deep convection. In short, different behaviors of balanced and unbalanced processes associated with the initial wind profile lead to different development rates of the secondary eyewall.

Comparison of Controlling Parameters for Near-Equatorial Tropical Cyclone Formation between Western North Pacific and North Atlantic

In this study,the differences in spatial distribution and controlling parameters for the formation of near-equatorial tropical cyclones(NETCs)between the western North Pacific(WNP)and the North Atlantic(NA)are investigated.NETCs exhibit distinctive spatial variabilities in different basins.Over the past few decades,the majority of NETCs took place in WNP while none was observed in NA.The mechanism behind such a distinguishing spatial distribution difference is analyzed by using statistical methods.It is noted that the dynamical variables such as low-level relative vorticity and vertical wind shear(VWS)are likely the primary controlling parameters.Compared with NA,larger low-level vorticity and smaller VWS appear over WNP.The increase of vorticity attributes a lot to the turning of northeast trade wind.NETCs in WNP tend to occur in the areas with VWS less than 9 m s^(-1),while the VWS in NA generally exceeds 10 m s^(-1).On the other hand,the sea surface temperature in the near-equatorial region of both of the two oceans exceeds 26.5℃and the difference of mid-level moisture is not significant;thus,thermal factors have little contribution to the distinction of NETC activities between WNP and NA.Intraseasonal oscillation(ISO)and synoptic-scale disturbances in WNP are also shown to be more favorable for NETC genesis.More NETCs were generated in ISO active phase.Synoptic-scale disturbances in WNP obtain more energy from the mean flows through the barotropic energy conversion process.The overall unfavorable thermal and dynamic conditions lead to the absence of NETCs in NA.