Numerical simulation of a prolonged rainstorm in Hong Kong on 22 - PowerPoint PPT Presentation

Numerical simulation of a prolonged rainstorm in Hong Kong on 22 July 2015 Ira, Chan Yan-chun Wong Wai-kin 27 July 2016, WSN-16

Background  Prolonged intense rainfall on 22 July 2015.  Amber Rainstorm Warning lasted for 11 hours 55 minutes, the longest record in the past decade. >30mm/hr rainfall over widespread area  Daily rainfall 191.3 mm at HKO, highest among 2015.  More than 1000 cloud to ground lightning Hong Kong over the territory.  Waterspout was reported near Kau Yi Chau.

Daily rainfall map on 22 July 2015  Uneven rainfall distribution  Rainfall confined over southern part of Hong Kong.  Weather relatively stable over northwestern part of Hong Kong, with much less rainfall.

Research direction Synoptic and meso-scale analysis Investigation of cause and mechanism contributed to rainstorms Performance of forecasting models Global model Meso-scale model Short-term forecasting products Experiment of high resolution numerical model - WRF Model physics - Cloud microphysics Radar horizontal resolution Orographic effect experiment and boundary layer scheme Summarizing contributed factors in order to help forecaster to predict intense precipitation

Background information on 21 July (1 day before)  Intense thundery showers moved from SW to NE crossing HK and brought heavy showers in the morning.  Mid level winds changing B C A gradually from SW to W/NW. Showers easing off in the afternoon with more stable atmosphere.  Thundery showers advanced to the S of HK. Developments persisted on a meso-scale convergence line over the coastal water of Guangdong in the afternoon and at night.

A broad trough of low pressure moved N, Active southwesterlies continued to affect the towards inland Guangdong progressively south China coastal areas. Convective clouds developing over south China coastal areas

Upper-air anticyclone (A) stayed around western Guangdong. Coupling with another anticyclone (A’) over the Luzon Strait, the westerly trough moved E slowly and HK was located near the tail of the 500hPa westerly trough. Anticyclone (A’) edged W into the northern part of the South China Sea gradually, making the westerly trough stayed between two anticyclones, favorable for development and unsettled weather. 500 hPa A A A A’ A’ A’ 2015-07-21 00:00 UTC 2015-07-21 12:00 UTC 2015-07-22 00:00 UTC 700 hPa

Radar situation on 22 July

Waterspout near Kau Yi Chau at around 8am on 22 July

Favorable conditions in meso-scale During the time with intense echoes, positive vorticity observed from surface winds, matching with the location of the reported waterspout. Analysis of wind field retrieved from dual Doppler radar radial winds even showed local-scale vortex. Kau Kau Yi Chau Yi Chau

Model performance (ECMWF) Model forecast the Based on 2015.07.21 00Z anticyclone at 500hPa over western Guangdong would weaken, and the mid level westerly trough would continue to move E. The westerly trough would be E of HK at 18Z 21 July. Streamline at mid level even showed slight ridging flow. As such, Based on 2015.07.20 12Z Based on 2015.07.20 00Z the forecast rainfall area confined over the eastern coast of Guangdong, and less rainfall in HK. In fact, the forecast position of the westerly trough was more E than the actual.

Model performance (NCEP) With more later model runs, Based on 2015.07.21 00Z model gradually corrected the weakening trend of the anticyclone over western Guangdong and the moving speed of the westerly trough. Forecast of 00UTC 21 July showed that the tail of the westerly trough close to HK, but the strength and the Based on 2015.07.20 12Z Based on 2015.07.20 00Z intensity of rainfall were still relatively weak.

Model performance (JMA) Model run of 00UTC 21 July Based on 2015.07.21 00Z forecast better the evolution of the anticyclone over the western part of Guangdong. Comparing with the model forecast of 12UTC 20 July, the weakening speed of the anticyclone was slower, which is closer to the actual situation. However, the short Based on 2015.07.20 12Z Based on 2015.07.20 00Z wave feature was unstable between model runs and conclusion for heavy rain was not confident.

Model performance (meso-NHM / rapids-NHM) The forecast rainfall area by Meso-NHM is close to ECMWF and NCEP. Using Meso-NHM as boundary condition, RAPIDS-NHM has a finer resolution. The assimilation included observation such as radar data and AWS from HK and Guangdong. There were some members capturing development of rainband over the southern water of HK. However, intense rainfall still located E of HK in earlier runs. The intensity was also lower HK than actual and persisted in shorter time.

Short-term forecast performance (SWIRLS)

High resolution numerical model experiment - WRF Use forecast from ECMWF 12UTC 20 July as boundary condition WRF parameter Version 3.4.1 Downscale to 2km resolution for initial condition Horizontal resolution 2km 305 × 305 Grid point Vertical layer 51 layers 3DVAR – Assimilation including GTS, AWS, radar and wind profiler. Upper level 21 km Surface to 2km 11 layers Run the model forecast  Purpose  Aim to reproduce the rainfall event  Understand the related dynamical and physical process

Effect of horizontal resolution of radar data and cloud microphysics Comparison of 24-hour rainfall amount between models and estimation from radar (QPE) For model forecast Clou oud mic microphys ysic ics p proc ocess using 4km resolution WSM-6 WDM-6 radar data, low level Res esoluti tion o of moisture flux radar ar d data: a: 4 km 4 k convergence showed more N-S fluctuation near southern waters of HK with time. The intense rainfall region (red region >100mm) covered larger area than using 2km resolution radar data. Res esoluti tion o of radar ar d data: a: 2 k 2 km

Effect of cloud microphysics and boundary layer physics WDM-6 considered the changing in concentration of various hydrometeors. The represented cloud microphysics is more complete. There is positive effect for simulating cloud physics and WSM-6 +YSU PBL WDM-6 +YSU PBL triggering development of convection. It is even more effective to simulate the rainband over coastal waters in early development and also the distribution from W to E. WSM-6 +ACM-2 PBL

Development of meso-scale vortex

WSM-6 +YSU PBL WSM-6 +ACM-2 PBL Terrain height A B C Modified terrain height X Y Z WSM-6 +ACM-2 PBL modif ifie ied t terrain in

Orographic effect q*V w > 0 WSM-6 + WSM 6 + ACM ACM-2 2 PBL PBL X X Y Y S v > 0 WSM-6 + WSM 6 + ACM ACM-2 PBL PBL modi odifie ied t d terra rrain in X X Y In the modified terrain experiment, the northerly component at around 3 to 4 km is more S than the original terrain. It Y also represented more S of the associated mid level westerly trough tail.

Comparing with the evolution of the vertical cross section from initial time, the initial field of the moisture transport with X original terrain matched with the intense echoes over southern waters of HK. Conversely, under the situation without local terrain, the moisture transport was more evenly distributed. Y orig origin inal t l terr rrain in T + 0 min T + 270 min v > 0 X Y X Y modi odifie ied t d terra rain T + 0 min T + 270 min v > 0 X Y X Y

origi original te terrai ain modif modified d te terrai ain

WS WSM-6 + + ACM ACM-2 PBL A B X Y WS WSM-6 + + ACM ACM-2 PBL modif modified d terrain in

Conclusion and outlook  Performance of models Global model could not give a good indication of heavy rain, but meso-scale model with  high enough resolution could give some signature.  Models seem to overestimate the weakening trend of the anticyclone and could not well predict the quasi-stationary feature of the westerly trough.  WRF Model  From the sensitivity test discussed, rainfall pattern is more sensitive to the terrain height.  Relatively less sensitive to model physics, but the result is more matching with the radar observation using the scheme of WSM-6 and ACM-2.  Overall mechanism  The axis of the quasi-stationary westerly trough stayed around HK. Coupling with various favorable factors such as low level moisture flux convergence and jets and enhanced by orographic effect, thundery showers continued to develop S of HK, leading to prolonged intense rainfall.  Hindsight  Bear in mind when heavy rain is highly sensitive to the location of system.  Nowcasting technique and meso-scale model could give an earlier indication to rainstorm.

Thank you!! Dr. Tin HKO’s Mascot