Simulated Rainband Research Articles

Impacts of Multiple Radiance Data Assimilation on the Simulation of Typhoon Chan-Hom

With the module of assimilating AMSU-A (Advanced Microwave Sounding Unit-A) and AIRS (Atmospheric Infrared Sounder) data in the WRFDA (Weather Research and Forecasting Model Data Assimilation) system, the impacts of joint assimilation of the radiance observations from two satellites on the simulation of typhoon Chan-hom (2015) are addressed. For comparison, experiments with the assimilation of solely GTS (Global Telecommunications System) data, AMSU-A data, or AIRS data are also performed. The results show that, compared to other experiments, the analysis field after assimilating multiple radiance data is closer to the observation. The simulated steering flow in its forecast field is conductive to the northeast twist of the typhoon. In addition, the simulated rainband and the FSS (fraction skill score) calculated from the experiment with assimilating multiple radiance data are better. In the deterministic forecast, better performance is obtained from the simulation with multiple radiance data in the forecast of track, MSLP (minimum sea level pressure), and MSW (maximum surface wind).

A Numerical Study on the Formation and Maintenance of a Long‐Lived Rainband in Typhoon Longwang (2005)

AbstractA 14‐hr long‐lasting spiral rainband of landfalling Typhoon Longwang (2005) was investigated based on a high‐resolution simulation using the Advanced Research version of the Weather Research and Forecasting model. This rainband was previously suggested to be triggered and maintained by vortex‐Rossby wave (VRW) dynamics based on available observations. The simulation results in the present study further suggest that the rainband originated from a previously existing wavenumber‐2 VRW. The wave amplified via a positive feedback between Ekman pumping and diabatic heating. The wavenumber‐1 convective forcing induced by the deep‐layer environmental vertical wind shear also contributed to the amplification of the rainband after its formation. Although both observations and simulation showed the long‐lived (greater than 10 hr) rainband with leading stratiform precipitation, unlike in observations, the rainband in the simulation showed a transition from an inner rainband to an outer rainband when the simulated rainband moved beyond the VRW stagnation radius at about 140 km from the storm center. As a result, in the band's later stages, VRW dynamics could not contribute to the maintenance of the rainband. Instead, both cold‐pool dynamics and vertical wind shear‐induced wavenumber‐1 convective forcing played key roles in the further maintenance of the rainband.

The Intensification of Hurricane Maria 2017 in the Antilles

Environmental influences on Hurricane Maria in the Antilles Islands are analyzed at the large-scale (1–25 September) and at the meso-scale (17–20 September 2017). The storm intensified rapidly prior to landfall in Dominica, going from category 1 to 5 in 15 h. As the storm progressed toward Puerto Rico (PR), its NE flank entrained air from seas cooled by the earlier passage of two hurricanes, and strengthened on its SW flank. Operational model forecasts tended to delay intensification until west of the Antilles Islands, thus motivating two independent weather research and forecasting (WRF) simulations. These gave minimal track errors at 1- to 3-day lead time. The simulation for landfall at Dominica on 19 September 2017 showed that a static nest with 0.8 km resolution using a Holland-type synthetic vortex and Yonsei University (YSU)/Kain-Fritsch schemes performed better; with a track error of 8 km and intensity error of 10 m/s. Our PR-area simulation of central pressure lagged 30 hPa behind observation; and caught up with reality by landfall in PR. The simulated rainband structure corresponded with Cloudsat observations over PR. Maria’s intensification occurred in an area of thermodynamic gradients included cooler SST in the right side of the track, so operational models with right-track bias were late in predicting intensification. Category-2 forecasts prior to 18 September 2017 left many Antilles islanders unprepared for the disaster that ensued.

Orographic effects on heavy rainfall events over northeastern Taiwan during the northeasterly monsoon season

The effects of orographic lifting and blocking on a heavy rainfall event with an accumulation of 631.5mm on 11 October 2009 over the Lan-Yang Plain (LYP) in northeastern Taiwan during the northeasterly monsoon season were studied by performing observational data analyses and numerical simulations using the Weather Research and Forecasting (WRF) model. The synoptic environment included a low-level easterly wind over the East China Sea and a southeasterly wind over the western North Pacific Ocean which produced convergence areas leading to the heavy rainfall event. The mesoscale features and the orographic lifting and blocking effects on the production and maintenance of the heavy orographic rainfall without the direct influence of the typhoon's circulations during the northeasterly monsoon season in fall were first investigated here. Due to orographic blocking on the prevailing easterly wind over the western LYP, the induced near-surface northeasterly flow containing moist airstream was lifted over the windward (south) side of the LYP and rainfall was enhanced in situ. Meanwhile, the precipitating system was embedded in a weak middle-level flow with the wind reversing its direction over the windward side of the LYP, resulting in a quasi-stationary system over the slope area. Furthermore, the prevailing easterly wind ascended over the coastal slope south of the LYP and enhanced the rainfall there. In addition, the approaching east–west oriented rainband from southeast Taiwan also strengthened the rainfall intensity over northeastern Taiwan. Two sensitivity tests were performed to examine the effect of the orographic lifting of the moist airstream on the production of heavy rainfall. The sensitivity experiment with Taiwan's topography removed (the NT run) shows that the simulated accumulated rainfall over northeastern Taiwan was less than 50mm in one day, much less than in the control run (CR run). In the NT run, the low-level convergence over northeastern Taiwan produced by the synoptic circulations and the simulated rainband still approaching northeastern Taiwan, are similar to the CR run. Another sensitivity experiment replacing the LYP with a plateau (the PL run) shows that the accumulated daily rainfall over the slope south of the LYP was reduced by ~250mm compared to the CR run. The reduction of rainfall was caused by lifting relatively less moist air over the slope south of the LYP. These sensitivity tests indicate that the amount of low-level moisture and the orographic effects are equally important for the formation and maintenance of heavy rainfall over northeastern Taiwan under a favorable environment.

CORDEX 동아시아 지역에서 HadGEM2-AO를 경계조건으로 처방한 RegCM4의 상세 지역기후 모의성능

In this study, 27-year (1979-2005) regional climate over the CORDEX East Asia domain was reproduced using a regional climate model, RegCM4, driven by HadGEM2-AO output, and the model`s simulation skill was evaluated in terms of surface air temperature and precipitation. The RegCM4 reasonably simulated the spatial distribution and interannual variability and seasonal variability of surface air temperature, while it had systematic biases in the simulation of precipitation. In particular, simulated rainband of East Asian summer monsoon was southward shifted below as compared with the observation, thereby, summer mean precipitation over South Korea was significantly underestimated. Simulated temperature from the RegCM4 driven by the HadGEM2-AO output was comparable to that driven by the reanalysis. However, the RegCM4 driven by the HadGEM2-AO had prominently poor skill in the simulation of precipitation. This can be associated with the distorted monsoon circulations in the driving data (i.e., HadGEM2-AO) such as southward shifted low-level southwesterly, which resulted in the erroneous evolution of East Asian summer monsoon simulated by RegCM4.

A Cloud-Resolving 4DVAR Assimilation Experiment for a Local Heavy Rainfall Event in the Tokyo Metropolitan Area

Abstract A cloud-resolving nonhydrostatic four-dimensional variational data assimilation system (NHM-4DVAR) was modified to directly assimilate radar reflectivity and applied to a data assimilation experiment using actual observations of a heavy rainfall event. Modifications included development of an adjoint model of the warm rain process, extension of control variables, and development of an observation operator for radar reflectivity. The responses of the modified NHM-4DVAR were confirmed by single-observation assimilation experiments for an isolated deep convection, using pseudo-observations of rainwater at the initial and end times of the data assimilation window. The results showed that the intensity of convection could be adjusted by assimilating appropriate observations of rainwater near the convection and that undesirable convection could be suppressed by assimilating small or no reflectivity. An assimilation experiment using actual observations of a local heavy rainfall in the Tokyo, Japan, metropolitan area was conducted with a horizontal resolution of 2 km. Precipitable water vapor derived from global positioning system data was assimilated at 5-min intervals within 30-min assimilation windows, and surface and wind profiler data were assimilated at 10-min intervals. Doppler radial wind and radar-reflectivity data below the elevation angle of 5.4° were assimilated at 1-min intervals. The 4DVAR assimilation reproduced a line-shaped rainband with a shape and intensity consistent with the observation. Assimilation of radar-reflectivity data intensified the rainband and suppressed false convection. The simulated rainband lasted for 1 h in the extended forecast and then gradually decayed. Sustaining the low-level convergence produced by northerly winds in the western part of the rainband was key to prolonging the predictability of the convective system.

Impacts of Evaporation from Raindrops on Tropical Cyclones. Part II: Features of Rainbands and Asymmetric Structure

Abstract In this study, the impacts of evaporative cooling from raindrops on a tropical cyclone (TC) are examined using cloud-resolving simulations under an idealized condition. Part I of this study showed that evaporative cooling greatly increases the kinetic energy of a TC and its size because rainbands provide a large amount of condensation heating outside the eyewall. Part II investigates characteristics of simulated rainbands in detail. Rainbands are actively formed, even outside the eyewall, in the experiment including evaporative cooling, whereas they are absent in the experiment without evaporative cooling. Rainbands propagate in the counterclockwise and radially outward direction, and such behaviors are closely related to cold pools. New convective cells are successively generated at the upstream end of a cold pool, which is referred to here as the upstream development. The upstream development organizes spiral-shaped rainbands along a low-level streamline that is azimuthally averaged and propagates them radially outward. Asymmetric flows from azimuthally averaged low-level wind advance cold pool fronts in the normal direction to rainbands, which are referred to here as cross-band propagation. The cross-band propagation deflects the movement of each cell away from the low-level streamlines and rotates it in the counterclockwise direction. Cross-band propagation plays an essential role in the maintenance of rainbands. Advancement of cold pool fronts lifts up the warm and moist air mass slantwise and induces heavy precipitation. Evaporative cooling from raindrops induces downdrafts and gives feedback to the enhancement of cold pools.

Mechanisms for the Generation of Mesoscale Vorticity Features in Tropical Cyclone Rainbands

Abstract A high-resolution tropical cyclone model with explicit cloud microphysics has been used to investigate the dynamics and energetics of tropical cyclone rainbands. Analysis of the vorticity interactions that occur within the simulated rainbands demonstrates that couplets of cyclonic–anticyclonic mesovortices can be produced in outer bands. The primary source of this vorticity is the upward tilting of system-generated horizontal vorticity by diabatic heating gradients. The vertical heating gradient in the stratiform cloud also creates a potential vorticity (PV) dipole that accelerates the tangential flow and develops a midlevel jet. The strength of the jet is enhanced by the vortex pair that is oriented radially across the rainband. The Fourier decomposition of the absolute vorticity field shows two counterpropagating vortex Rossby waves associated with the rainband. The wave located on the inner side of the band transports energy toward the vortex center. The outer wave is made up of high wavenumbers and uses the vorticity gradients generated by the rainband. The results support the hypothesis that the heating profile in the stratiform regions of rainbands generates cyclonic PV across the freezing level, which develops a midlevel jet. This mechanism creates a vorticity gradient that enables the propagation of vortex Rossby waves that could allow the rainbands to interact with the mean flow and potentially influence the evolution of the storm by contributing to the symmetric component of vorticity and the development of secondary eyewalls.

A Study of the Mesoscale Structure of Typhoon Flo (T9019): A Case of COMPARE Model Intercomparison

Numerical experiments are performed for Typhoon Flo (T9019), which was the subject of an international model intercomparison (COMPARE III). The model used is a mesoscale-convection-resolving model. A global objective analysis (GANAL) data produced by JMA is used as an initial condition, and the initial time is 00 UTC 14 September 1990. The horizontal grid size is taken to be 5/36 degrees (about 15 km) in the fine-mesh area of a triply-nested grid model.Mesoscale structure of the simulated typhoon is studied in detail. Cloud water and rainwater fields are compared with satellite images. In the first 12 hours, the predicted rainwater field is quite different from that suggested from satellite images, mainly because the initial field is latently stable in the typhoon area. Latent instability is created by strong surface winds. Later in the integration, the rainfall patterns exhibit several important features in the satellite images.The simulated 6-hour accumulated rainfall between 48 and 54 hours is compared with some simulations from COMPARE. Rainfall amounts in the inner area of the typhoon, and in the tropics, appear to be better simulated. It is also shown that the time change of latent instability distribution is an important measure to explain how the rainfall distribution during this period is realized. Comparison of the wind fields at 200 hPa indicates great differences among the models.The structure of simulated rainbands is examined in some detail. It is shown that the model simulates rainbands of four types, with respect to locations of updraft/downdraft and the cold pool relative to the rainwater field, and whether the inflow into the rainband is on the concave edge or the convex edge. Behavior of mesoscale organized convection, which constitutes the rainband, is also examined.These results suggest that even a model with a coarse grid size of about 15 km can describe important features of the mesoscale structure of tropical cyclones.

Numerical simulation of orographic rainbands

This study, based on a statistical analysis of simulated warm rain event and radar data, aims at highlighting the main physical mechanisms that lead to organize shallow convection on the relief. The region of investigation, the Cévennes‐Vivarais, is located in the southeast part of France. Radar images from the Cévennes experiment (fall 1986–1988) reveal a characteristic and repetitive structure of the rain distribution organized in narrow bands or plumes, oriented south‐north in the case of stationary southerly Mediterranean flow. The event of 14 November 1986 has been selected and constitutes the data set of this numerical study. This work is closely associated with the previous work by Miniscloux et al. [2001] which presents in detail the results of a geostatistical analysis of the radar data set extracted from the Cévennes experiment data base. The main results highlight the physical characteristics and the dynamics of the rain patterns. Following the recent work of Cosma et al. [2002], high‐resolution (Δ = 1 km) simulations have been continued with the nonhydrostatic three‐dimensional (3D) atmospheric model MesoNH, in order to reproduce the observed rainbands over the Cévennes region. The numerical model correctly reproduces the structure and the dynamics of the rainbands. The geostatistical analysis has been applied for the simulated rain fields. The model slightly overestimates the northward advection velocity of the rain cells within the bands (75 km h−1 against 60 km h−1 for the observation), and the simulated rainbands are narrower and more organized around the N180° direction than the observed rain field. The comparison allows the qualification and validation of the choice of the numerical methodology and realism of the physical parameterizations. The analysis of the 3D simulated fields confirms the physical mechanisms responsible for the rain organization demonstrated by Cosma et al. [2002] through idealized simulations. The statistical analysis highlights the presence of mean topographic features under low‐level convergence zones composed of a succession of ridges and penetrating valleys orientated east‐west. The rainbands are generated upstream of these topographic features and enhanced on the leeside due to the convergence created by the flow deflection around the obstacle and its penetration into the valleys. The simulated triggering takes place further to the south than the observed one, and the triggering is active as soon as the relief is suitably described in the model.

The effects of precipitation schemes and horizontal resolution on the major rainband in typhoon Flo (1990) predicted by the MRI mesoscale nonhydrostatic model

¶This paper describes a numerical study of the major spiral rainband in typhoon Flo (1990) using the Meteorological Research Institute Mesoscale Nonhydrostatic Model (MRI-NHM). The effects of precipitation schemes and horizontal resolution on the representation of the simulated rainband are discussed. Dynamic and thermodynamic structures of the simulated major rainband to the north of the storm center are well represented in the model with a 5 km horizontal resolution. The structures are consistent with observational results reported for other tropical cyclones. Among the realistic features are: a cold pool and convergence on the inner side of the band; convergence above low-level inflow layers; and the outward slope of the updraft with height. The band is caused by the motion of the storm through its surroundings where horizontal wind has vertical shear. The simulation of the structure and precipitation pattern associated with the major rainband depends on the precipitation scheme rather than the horizontal resolution. The band appears more realistic when using explicit cloud microphysics as a precipitation scheme, rather than moist convective adjustment. This result is attributable to the difference in scheme triggering. In the simulation with moist convective adjustment, the elimination of vertical instability in low-level atmosphere is excessive, suppressing band formation. The overall structure of the band is also more realistic in the simulation using explicit cloud microphysics, because a cold pool exists in the lower layers and the vertical axis of upward flow tilts outward. This result suggests that prediction will partly depend on variables associated with cloud microphysics, such as the mixing ratio of cloud water. The horizontal grid distance, which varied between 5 and 20 km, quantitatively influenced the rainfall amount, although the large-scale band structure remained unchanged. The rainfall amount increased as the grid interval was reduced from 20 to 10-km but decreased as the interval was further reduced from 10 to 5 km.

Formation and Maintenance Processes of a Stationary Band-shaped Heavy Rainfall Observed in Niigata on 4 August 1998.

On 4 August 1998, a torrential rainfall was observed in the Niigata area, in central Japan. The rainfall was induced by a quasi-stationary rainband aligned in the east-southeast direction from the west of Sado Island. The rainband lasted for several hours with the characteristic features of a back-building formation, in which new convective cells were successively generated in an upstream direction. The rainfall area and intensity simulated by a nonhydrostatic mesoscale model with a 5-km horizontal grid agreed well with the observation. The rainband with a back-building nature was reproduced successfully by a nonhydrostatic model with a 2-km grid. The simulation results showed that the heavy rainfall formed as a result of strong convective instability enhanced by low-level humid and middle-level dry inflows from the southwest into the weak convergence region associated with the Baiu frontal zone. Sensitivity experiments indicated that neither the terrain of Sado Island, nor the evaporative cooling of raindrops affected the formation of the rainband. Although moved to the downstream when the low-level wind convergence was not enhanced enough, the simulated rainband stagnated after the convergence became so strong that the whole low-level air could be lifted upward while crossing the convergence zone. Therefore, the quasi-stationary rainband was maintained by the convection-enhanced low-level wind convergence existing upstream of the rainband. Over the convergence zone associated with the quasi-stationary rainband, new convective cells successively formed and separated from the zone after having developed higher. This successive formation of new cells could maintain a back-building formation. Such a back-building formation was found at several points over the wind convergence zone. These meso-β-scale convective systems with a back-building nature produced the precipitation region with a length over 200 km.

Updraft Dynamics within a Numerically Simulated Subtropical Rainband

Abstract In this study a three-dimensional numerical cloud model is used to examine the early evolution of deep convective rainbands that occur in an environment of weak to moderate buoyancy and directionally varying lower-tropospheric vertical wind shear. A simulation based on a case observed on 8 June 1987 during the Taiwan Area Mesoscale Experiment produced a narrow bow-shaped rainband that comprised 1) short-lived updrafts along the downshear portion of the weak rain-induced cold pool and 2) more persistent updrafts along its southern flank, which were highly correlated with vertical vorticity. Trajectory calculations and an analysis of the dynamic portion of the perturbation pressure field are presented to illustrate the hybrid dynamical character of the simulated rainband. The shorter-lived updrafts were associated with weak upward-directed pressure gradient forces at the leading edge of the surface-based cold pool. The more persistent updrafts exhibited much stronger upward-directed pressure gradie...