Application of Numerical Weather Prediction Prognoses to Operational Weather Forecasting in Hong Kong Pre-CAS Technical Conference on "Environmental Prediction in the Next Decade: Weather, Climate, Water and the Air We Breathe" Incheon, Republic of Korea November 2009 Hilda Lam Hong Kong Observatory Hong Kong, China

Content NWP prognoses available at HKO in the past 30 years Their impact on forecast performance A few recent cases of unsatisfactory forecasts Concluding remarks

YearNWP centres/ products Model resolutionForecast range Via 1985ECMWF, UKMO5 to 2.5 degT+72GTS 1989ECMWF2.5 degT+72GTS 1996JMA GSM2.5 deg, 1.25 degT + 120GTS 1999ECMWF2.5 degT + 120GTS 2002NOAA NCEP1 degT + 96Internet 2004ECMWF EPSgramFull resolutionT+240Internet 2006ECMWF2.5 degT + 24GTS 2007NOAA NCEP1 degT + 240Internet 2009JMA0.5 and 0.25 degT + 216Internet ECMWF1 deg and some o.25 degT + 168Internet Global model prognoses availbale at HKO since 1980’s

Current NWP models used in HKO forecast up to (hours) modelgridruns (Z)run at 12Zothers ECMWF2.5 deg00,12T deg00,12T deg00,12T+42 JMA1.25 deg00,06,12,18T+192T deg00,12T+216T deg00,12T+216T+84 UKMO(EGRR)2.5 deg00,12T+120 NCEP1 deg00,12T+240 ORSM60 km00,06,12,18T+72 ORSM (20km)20 km00,03, …, 21T+42

Objective verification of weather forecasts at HKO Current scheme used since 1984 Each of the following elements is verified against actual and assigned an accuracy score: Max and min temp, rainfall, wind speed, state of sky, visibility Weighted mean of element scores will give the forecast accuracy scores. Different weighting is applied for the elements depending on the season, to reflect its importance in public’s perception

Monthly mean verification score and 12-month running average of local weather forecasts for next day (score range: 0~100)

Monthly mean verification score of 7-day forecasts (score range: 0~100)

Tropical Cyclone Warnings Area of responsibility of HKO is bounded by the white line, 10-30N and E. Warnings issued every 3 hours for 72 hours

HKO tropical cyclone forecast track error

Forecast track of Chanchu – based on 12 UTC 12 May 2006 HKO’sTC information processing system (TIPS) – forecast tracks of various models and the multi- model ensemble Black line : post- analysed track

A prolonged cold spell in early 2008 over south China

For Hong Kong Longest cold spell in 40 yrs (24/1 – 16/2) Daily min temperature below 12 C Mean min temp was 9.9 C, second lowest during the same period since record began in Continuous push of cold air from Siberia into southern China and moist air transported from the South China Sea and even as far as the Indian Ocean, brought continuously cloudy, rainy and cold weather to the region.

72-hrs forecast pressure change in 24 hrs (PC24) Initial : 12 UTC daily 24 Jan - 17 Feb 2008 forecast and actual PC24 ECMWFJMA

Mean temperature error field (in deg) of 72-hr f/c Initial :12 UTC 24 Jan - 16 Feb 2008 daily JMA sfc (left) & ECMWF 850hPa (right)

EPSgram 10-day forecast with base time at 12 UTC on 22 Jan

Observed temperature (°C) EPSgram 10-day forecast with base time at 12 UTC on 29 Jan

EPSgram 10-day forecast with base time at 12 UTC on 7 Feb

Fengshan Best track June 2008

Forecast tracks of Fengshan predicted by global models 00Z 22 June Z 23 June Z 19 June 2008

ECMWF 500hPa GPH 12 UTC on 24 June 2008 T+72 forecast analysis

Combination of Typhoon Ketsana and NE monsoon over the south China coast

Ketsana & monsoon - surface analysis 27, 29 Sep & 1 Oct 2009

Ketsana and monsoon – satellite imageries 27, 29 Sep & 1 Oct 2009

T+12H forecast of 12-hourly rainfall by ECMWF model for the grid closest to HK - actual rainfall recorded at the HKO in purple

surface prognoses valid for 12 UTC on 30 Sep T+24T+48 T+72T+96

Radar imageries at UTC on 30 Sep 2009 Amber rainstorm warning issued

Concluding remarks NWP advancement >more accurate forecast with longer lead time Helped raise public expectation Occasional situations with satisfactory forecasts by models are more difficult to handle It may be beneficial to foster closer cooperation between operational forecasters and model developers

Thank you