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Chowdhury sazed123@gmail

Collaboration between meteorological and hydrological services in the provision of improved flash flood forecasting. San Jose, Costa Rica, 13-17 March, 2006. Chowdhury sazed123@gmail.com. 23 º 27 ´ N. 23 º 27 ´ S. ttyy. NE. NW. SE. SW. HD. HT.

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Chowdhury sazed123@gmail

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  1. Collaboration between meteorological and hydrological services in the provision of improved flash flood forecasting. San Jose, Costa Rica, 13-17 March, 2006 Chowdhury sazed123@gmail.com

  2. 23º27´N 23º27´S

  3. ttyy NE NW SE SW HD HT

  4. Flash Flood :Rising water occurs during or in a matter of a few hours after the associated rainfall. (Charles A. Doswell , NOAA ) Flash floodis the combination of a meteorological event with a particular hydrological situation.( Charles A. Doswell, Harold E. Brooks, Robert A. Maddox, July 1995, article on weather and forecasting, NoAA) hydrology • Size, shape and topography of the basin including the section of the stream • Forecasting methods meteorology • Flash Flood is mainly a Convective storm-related event (Houze 1982, 1989) • QPF/QPE

  5. Meteorologists are always interested for the rain forecast before it occurs. • Hydrologist alone can go for the forecast only when it commence. Hence without collaboration in between the Meteorologist and hydrologist lead time can never be achieved and this forecast is quite meaningless for flash flood events.

  6. Hydrological Domain

  7. Rainfall-runoff (NAM) Model • Hydrodynamic (HD )model • Advection-Dispersion (AD) • Cohesive Sediment Transport (CST) • Non-cohesive Sediment Transport (NST) • Water Quality (WQ) • Flood Management Model (MIKE11-GIS)

  8. Rainfall Runoff Modelling • Catchment Delineation • Input of Rainfall, Evaporation Data • Data Processing • Setting up (calibration) of Hydro-geological Parameters • Calibration - Groundwater level • - Runoff (Discharge)

  9. NAM Rainfall-Runoff Model Rain Potential Evaporation Routing Irrigation water from river Overland flow Actual Evapotrans- piration Surface Storage Inter flow Irrigation water from ground water Groundwater Pump Capillary Rise Root Zone Simulated River Discharge Routing Groundwater storage Base Flow

  10. Catchment Delineation GWL Station Q + WL Station WL Station Rainfall Stn.

  11. Recharge Mechanism Rain ET Interception Runoff Effective rainfall Surface Storage Rootzone Storage Infiltration Root Depth Percolation Surface balance = Eff. Rain - INF Rootzone balance = INF - ET - DP

  12. ET Rain Interception Runoff Effective rainfall Surface Storage Rootzone Storage Infiltration GWL Root Depth Capillary Flux RWL Percolation Before Groundwater level GWL Now Physical Processes lead time is 4 Hrs.

  13. Meteorological Domain Before falling of the rain drop on the ground numerous process and activities are going on in the space. Meteorologists are looking for these activities in an endless manner keeping any stone un touch specially for the convective precipitation which is a potential storm for the flash flood.

  14. Convective ppt ;-strong vertical velocities>1m/secwith rain rate>5mm/hrStratiformppt;- small vertical velocities < 1m/sec with low rain rate <5mm/hr Across the tropics ( 20ºN-20ºS) , theratio of theconvective rain rate to the stratiform rain rate is 4.1 on average on horizontal resolution of the Precipitation data.-----Courtney Schumacher and Robert A. Houze Jr. Dept. of Atmospheric science , University of Washington in 2002,Tropical Rainfall Measuring Mission . They studied 3-yrs (1998-2000) data and it was found that convective rain amounts are highest (>1.5m/yr) over Central Africa, the Maritime Continent, the Amazon into the Central America and the east Atlantic. So it reveals the maximum possibilities of Flash Flood events in the basins of these areas. Again the convective rain i.e. potential flash flood storm differs from years to years depending on the cool and warm episodes in the tropic zone. 1998 was the strong el nino year where 1999 was la nino. 2000 was neutral. In their study it was found that seasonal distribution of rain all over the year varies in different parts of the world according to the ENSO episodes. According to Mr.Yukari N. Takayabu of Tokyo University in his TRMM study found the wide variation of convective rain in early morning and mid-night.

  15. Global Rain Picture in Tropical zone

  16. Global rain Picture In Tropical zone

  17. Percent of rain in Tropical Zone for a EL nino, LA nino and a Neutral year

  18. For an operational QPF the input from satellite as well radar is widely used. Again the potential flash flood producing storm structure seen from a geostationary satellite may be different from that seen by radar. So hydrologist should know the approach so that a logical choice for the application of forecast may be taken up. Hence Charles A. Dowell, Harrold E. Brooks and Robert A.Maddox,1995 suggested an Ingredients –based Methodology for the flash flood forecast. Ingredients are Precipitation efficiency (E), ascent rate (w) and mixing ratio of the rising air (q).

  19. contd.. Again an intensified long lasting convective precipitation with accurate QPF may not cause a flash flood. Hydrological events including the rain pattern may also be the considerations for the forecaster. Cities have greater precipitation runoff than rural areas. Because of increasing population hydrological and morphological settings in cities are altering in their vicinity. Unpaved locations exposed to the rain in many cities are decreasing dramatically as a result runoff eff is increasing. As such experts has established that all the models or techniques may not give good results for a meaningful forecast for good many reasons. Mr. Plillip L. Spencer and David J.Stensrud of NOAA made a case study with different models in six flash flood events that occurred in central and eastern united states.

  20. Table 3. Maximum total precipitation (mm) for each of the six cases (as well as the average for all cases) based on observations (OBS) and derived from each of the four model simulations. Maxima from model simulations were in the region of the observed maxima rather than over the entire model domain. Percentages indicate percentage of observed. CASE OBS NOMOD MPE NDD DDD AUG86 170 53 (32%) 61 (36%) 107 (63%) 66 (39%) JUL87 254 48 (19%) 53 (21%) 188 (74%) 79 (31%) SEP89 150 76 (51%) 94 (63%) 170 (114%) 97 (64%) JUN90 127 48 (38%) 36 (28%) 142 (112%) 53 (42%) NOV92 236 51 (22%) 61 (26%) 132 (56%) 75 (32%) NOV93 239 124 (52%) 140 (59%) 160 (67%) 130 (54%) AVG 196 67 (34%) 74 (38%) 150 (77%) 83 (42%) Comparison of the MM4(3-D mesoscale model) in Pennsylvania State NOMOD-Unmodified, (deep,moist convection ) MPE-Maximizing PPT. EFF, (ratio of the water mass to the ground to vapor mass entering the cloud)NDD-No down drafts,strong upward motion along convectively induced outflowDDD- Delayed downdraft, No convective downdraft below cloud base.

  21. Meteorologist Hydrologist QPF/QPE/RADAR Satellite Real/prediction HD/RM/ Stream profile Mode of Collaboration in between meteorologist and Hydrologist. Model product Validation Validation Result

  22. For the collaboration of the Meteorologist and Hydrologist there should be • Same institutional Arrangement • They should be under the same umbrella • Working desk of Meteorologist and Hydrologist should be side by side. And thus a strong Collaboration in between the Meteorological and Hydrological services can improve the flash flood forecasting.

  23. Meteorological event Flash flood events

  24. Thank You

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