Vol.19 No.3 JOURNAL OF TROPICAL METEOROLOGY September 2013 Article ID: 1006-8775(2013) 03-0284-13 NUMERICAL SIMULATION OF CLOUD MICROPHYSICAL CHARACTERISTICS OF LANDFALL TYPHOON KROSA HUA Cong (花 丛), LIU Qi-jun (刘奇俊) (National Meteorological Center, Beijing 100081 China) Abstract: In this study, the super typhoon KROSA (2007) was simulated using a mesoscale numerical model Global and Regional Assimilation and Prediction System (GRAPES) with a two-moment mixed-phase microphysics scheme. Local rainfall observations, radar and satellite data were also used to analyze the precipitation structure and microphysical features. It was shown that low-level jets and unstable temperature stratification provided this precipitation process with favorable weather condition. Heavy rainfall centers were located in the north and east part of KROSA with the maxima of 6-hourly total rainfall during the simulation more than 100 mm. The quantities of column solid water and column liquid water were generally equivalent, indicating the important role of ice phase in precipitation formation. Results of CloudSat showed that strong convection occurred in the eyewall around the cyclonic center. According to the simulation results, heavy precipitation in the northeast part of the typhoon was mainly triggered by convective clouds, accompanied by the strongest updraft under the melting level. In the southwest part of KROSA, precipitation intensity was rather homogeneous. The ascending center occurred in high-level cold clouds, favoring the formation and growth of ice particles. Key words: typhoon heavy rainfall; KROSA; GRAPES model; two-moment mixed-phase microphysics scheme; cloud microphysics CLC number: P444 Document code: A 1 INTRODUCTION with little rainwater takes place without ice microphysics. Wang[6] reported that both evaporation China is one of the countries in the world heavily of rain and melting of snow and graupel are influenced by landfall tropical cyclones (TCs). The responsible for the generation of downdrafts and annual mean direct economic losses caused by rainbands. Franklin et al.[7] showed that rain rates in landfall TCs are 29 billion yuan (equivalent to about [1] the inner core of the storm become higher with US$6.19 billion) . Heavy rainfall brought by TCs is increasing speeds of graupel fall. a major contributor to disasters. Significant progress Super typhoon Krosa (2007) first appeared over has been made in research associated with typhoon the ocean east of Luzon in the Philippines at 0000 storms in the past decade, such as the formation of Coordinated Universal Time (UTC) 2 October 2007, spiral rainbands, the function of underlying surface, and subsequently moved northwestward. At 1800 and the interactions between TCs and mesoscale [2-4] UTC 4 October, Krosa strengthened into a super systems . Meanwhile, studies also pointed out that typhoon with its central pressure down to 935 hPa. At cloud microphysical processes have a significant 0730 UTC 7 October, Krosa made landfall at Xiaguan impact on the intensity, structure and precipitation of town located at the junction of Fujian and Zhejiang TCs. provinces, with a central pressure of 975 hPa. After In recent years, cloud-resolving simulation with that, Krosa weakened into a strong tropical storm and mesoscale numerical models with detailed cloud moved northeastward along the Zhejiang coastline. microphysical processes has become more and more Krosa returned to the East China Sea at 0930 UTC 8 attractive in TC research. It is suggested that cloud October and damped into a tropical depression at microphysical processes are critical to the realistic 1500 UTC 8 October. simulations of TC clouds and precipitation. Zhu and [5] Krosa was strong and stayed over the mainland of Zhang found that the weakest and shallowest storm China for 26 hours, which was rarely seen in the same Received 2012-04-24; Revised 2013-05-08; Accepted 2013-07-15 Foundation item: “Abnormal Changes and Mechanism Study Before and After Typhoon Landing” (2009CB421500) from the National Key Basic Research Program (973 Program) Biography: HUA Cong, M.S., primarily undertaking research on cloud model and cloud microphysics. Corresponding author: HUA Cong, e-mail: [email protected] No.3 HUA Cong (花 丛) and LIU Qi-jun (刘奇俊) 285 period of history. Its long-term effects brought heavy equation for hydrometeor mixing ratios, hydrometeor rainfall to north Fujian and most of Zhejiang province. number concentration ratios and spectrum broaden [8, 9] There have been studies considering the mesoscale functions in this scheme is written as:. cloud clusters in spiral cloud bands as the main reason for precipitation during early landfall. Spiral cloud ∂F()mFm⎛⎞→ ∂ δ () x x =−∇⋅⎜⎟UFxmxxx() m +∇⋅() K ∇⋅ F () m +() UF () m + bands provided favorable moisture conditions to the ∂∂tzt⎝⎠ δ heavy rainfall. An inverted trough resulted in the where Fx (m) stands for hydrometeor mixing ratios of coupling of upper divergence and lower convergence, Q , Q , Q , Q , Q and Q , hydrometeor number enhancing precipitation in the north of Krosa. Cold air v c r i s g concentration ratios Nr, Ni, Ns, Ng, ice spectrum invaded into the cyclone to increase atmospheric broaden function F and snow spectrum broaden baroclinic instability, assisting the maintenance of i function Fs. The right-hand-side terms of the equation heavy rainfall. are advection transport, turbulent diffusion, In this study, super typhoon Krosa (2007) was sedimentation and cloud microphysical sources and simulated using a mesoscale numerical model, Global sinks, respectively. 29 kinds of microphysical and Regional Assimilation and Prediction System processes are included in the scheme (Figure 1), such (GRAPES), which is independently developed in as condensation or evaporation of cloud droplets and China. Local rainfall observations, radar and satellite raindrops, deposition or sublimation of ice crystals, data were also used to analyze the precipitation snow crystals and graupel, automatic conversion of structure and microphysical features. cloud droplets to raindrops, ice crystals to snow crystals, ice crystals to graupel, snow crystals to 2 THE MICROPHYSICAL SCHEME graupel; collection of cloud droplets and raindrops, cloud droplets and ice crystals, cloud droplets and [10] GRAPES is a numerical forecasting model graupel, ice crystals and ice crystals, rain drops and designed for both scientific research and operation. Its ice crystals, raindrops and snow crystals, raindrops physics schemes, which are used to describe the and raindrops, ice crystals and graupel, ice crystals atmosphere and underlying surface processes, include and snow crystals, snow crystals and graupel, snow radiation transfer, boundary layer processes, sub-grid crystals and snow crystals; nucleation of ice crystals; moist convection, grid-scale cloud and precipitation multiplication of ice crystals; melting of ice crystals, processes, surface-layer physics process and sub-grid snow crystals and graupel; congelation of raindrops scale terrain gravity wave drag. Here we used the and cloud droplets, et al. GRAPES-Meso model with a two-moment mixed-phase microphysics scheme[11]. The continuity Condensation, evaporation Condensation, evaporation Water Vapor (Qv) Sublimation, deposition and nucleation Sublimation, condensation Sublimation, Sublimation, deposition Auto-conversion Cloud droplet (Qc) Raindrop (Qr, Nr) and condensation and Multiplication, collision Melting Melting Freezing Collision Collision, melting Collision Collision Ice crystal (Qi, Ni) Auto-conversion Snow (Qs, Ns) Auto-conversion Graupel (Qg, Ng) Collision Collision Auto-conversion, collision Figure 1. Microphysics processes in the two-moment mixed-phase scheme. 285 286 Journal of Tropical Meteorology Vol.19 Except for the mutual transformation of the mode is used in the simulation, indicating the inner microphysics, the scheme also took traction on the grid is run with the initial and boundary conditions dynamic field and the latent heat release into provided by outer coarse grids. The T213 forecast consideration. The temperature equation is given by field incorporated with a bogus vortex is adopted as ∂∂TT⎛⎞→ δ the initial field. All three nested domains have the =−∇⋅⎜⎟UT +∇⋅() K ∇⋅ T +() UT + ∂∂tzt⎝⎠ δ same initial physical quantities, including horizontal wind speed u and v, temperature t, geopotential height δT where is latent heat induced by phase change. h, water vapor content Qv, sea level pressure ps and δt sea level temperature ts. Figure 2 shows the model domains. Domain 3, with a horizontal resolution of 3 EXPERIMENT DESIGN 3.3 km, covers the five provinces in East China severely affected by Krosa, and only an explicit The physics options adopted in the simulation are microphysical scheme is applied. See Table 2 for listed in Table 1, and the two-moment mixed-phase more details. microphysics scheme introduced above is chosen as the microphysics scheme. A triply nested, one-way Table 1. Summary of the physics scheme. Physics Scheme Microphysics Ice scheme Longwave radiation physics RRTM scheme Shortwave radiation physics Dudhia scheme Surface-layer physics Monin-Obukhov scheme Land-surface physics Thermal diffusion scheme Boundary-layer physics MRF scheme Cumulus physics Kain-Fritsch (new Eta) scheme (only for domain 1&2) Table 2. Summary of the experiment design. Domain Simulation time Resolution /km Time step/s Vertical levels 1200 UTC October 6 to 1 17.6 60 17 1200 UTC October 8, 2007 0000 UCT October 7 to 2 6.0 30 17 1200 October