IWTC VI – Costa Rica – November 2006 Summary Session – Jim Davidson

“Quantitative Forecasts of TC Landfall in relation to an Effective Warning

System”

Effective Warning System

EWS = the targetted communication of timely, relevant, understandable and easily accessible warnings - prepared using a mix of the best available model guidance and forecasting skill.

The primary objective of a warning system is to empower individuals and communities to respond appropriately to a threat in order to reduce the risk of death, injury, property loss and damage. - or simpler still – and in fewer words -

Warnings need to engage the people at risk - and stimulate them to take suitable protective action.

Role of the RSMCs/TCWCs

Established Facts

1. All rapporteurs highlighted the fact that landfall parameter prediction is highly dependent on the cyclone’s track. At a particular location, even small deviations in track can result in vastly different impacts from wind, storm tide, precipitation and flooding.

2. Most deaths in cyclones continue to be caused by flooding, landslips and mudslides.

3. With the steady growth in coastal population and infrastructure, we are witnessing what amounts to a quantum leap in vulnerability (and therefore risk) in many cyclone prone areas, irrespective of trends in cyclone numbers and intensities. Importantly, evacuation times have increased accordingly.

4. Tropical cyclone speed at landfall is also critical in terms of potential wind damage and rainfall/flooding. The faster the cyclone is moving, the lower the risk of wind and flood damage.

Landfall Parameters

1. WIND – both temporal and spatial (over varying terrain and topography)

2. RAINFALL – both temporal and spatial

3. FLOODS – both riverine and localised

4. STORM TIDE (storm surge + normal tide + wave setup)

5. WAVES – including setup and runup

The landfall phase is defined as that period of a cyclone’s lifecycle bounded by the immediate approach of the cyclone to the coast through to the early part of its transition over land. “Glancing blows” (by say destructive winds) is a special case.

Relative Strengths

Track forecasting (but small deviations do matter)

Storm surge modelling (but dependent on inputs)

Hydrological modelling (but dependent on inputs)

NWP (including mesoscale/nested/coupled models)

Remote sensing data (eg scatterometer, dopplers)

Dvorak technique (with some reservations)

in the TC landfall forecast process

Relative Weaknesses

Forecasting intensity, structure, structural change

Forecasting spatial and temporal rainfall patterns

Modelling wave action (including small islands)

Modelling “riverine flooding + storm tide + waves”

Wind-Pressure relationships (cyclone dependent)

Forecasting fine scale wind features upon landfall

Forecasting the wind decay rate over land (SHIPS)

Surface & upper air obs networks in many areas

Quality of topographic & bathymetric data

Public understanding of the storm tide threat

in the TC landfall forecast process

Macau Workshop Priorities

The participants gave the following priority ranking in regards to improving tropical cyclone landfall forecasts:

1. Further improvements in track landfall forecasts;

2. Improved predictions of tropical cyclone-related precipitation following landfall;

3. Advances in understanding and predictions of structure and intensity during and following landfall, and from tropical storm stages to extra-tropical transition; and

4. Further applications of storm surge models, including improved specification of the meteorological forcing.

Recommendation Groupings

1. ENSEMBLE PREDICTION SYSTEMS (single model? probabilities)

2. CONSENSUS FORECASTS (multiple-model? probabilities)

3. PARAMETRIC MODELS (probabilities & “what if” scenarios)

4. STORM TIDE : WAVES and RAINFALL : FLOODS

5. STRUCTURE : WIND : PRESSURE

OTHERS:

Capacity Building, Training, Seminars, Workshops, Symposia, Simulator Models (Macau Workshop).

Field Experiments, Demonstration Projects, WWRP & US Landfall Programs, Australian TCCIP, Aircraft Reccies/Aerosonde.

Observations (eg for forecast verification and model calibration)

Ensemble Prediction SystemsWMO (8) The meeting considers small focus workshops to be a useful means to organise research-operational-hydrological interaction on topics of particular interest to the operational community. The meeting endorses the increasing use and application of ensemble prediction systems (EPS) in forecasting tropical cyclones.

RESEARCH (13) The meeting recommends that the research community explore the use of ensemble forecasting techniques for tropical cyclone forecasting including track, intensity, Quantitative Precipitation Forecasts (QPF), storm surge, wind waves and flood forecasting.

OPERATIONAL (4) It is recognised that ensemble forecasting techniques may provide an important opportunity to improve tropical cyclone track predictions. To be able to utilize these techniques in an optimum way, it is recommended that:(1) Ensemble-based products be made available to all RSMCs and Tropical Cyclone Warning Centres, who should undertake verification of these techniques, and (2) Training be provided on how to apply these techniques in an operating setting.     

Ensemble Prediction SystemsStorm surge predictions are readily affected by the error in tropical cyclone predictions in terms both of their tracks and of intensities. Taking into account this, ensemble (-like) and probabilistic methods and outputs should be considered to use in operational storm surge forecast.

There is a need to continue in improving operational hydrologic and hydraulic flood modeling, and this includes consideration of a tighter coupling with meteorological, hydrological/hydraulic, and tidal modeling outputs where useful. We need to have a better definition of the track and intensity forecast of tropical cyclone path and quantitative precipitation forecast (QPF) associated to tropical models. The use of ensemble forecast and short term QPF should be considered.

Consensus Forecasts

OPERATIONAL (2) Consensus forecasting techniques have been demonstrated to improve track forecasts provided an adequate number of skillful forecasts is available. It is recommended that all major NWP centres make available track and intensity forecasts, and radius of gale/storm force winds out to 120 hours or beyond with access via GTS and controlled access on a suitable website (e.g., WMO). The WMO should request the European Centre for Medium-range Weather Forecasts to make available forecasts of tropical cyclones in their deterministic model and their tropical ensemble when it is operational.

OPERATIONAL (3) Consensus forecasts should be closely evaluated to identify (1) the minimum number and optimal combination of forecast members that adds value to the forecast process; (2) scenarios that yield small errors; and (3) strategies to deal with situations when the consensus forecast does not work (e.g., bifurcation scenario when members are in two different track types)

Consensus Forecasts- nil so far -

Parametric Models

RESEARCH (7) Parametric wind models form a basis for a range of forecast and diagnostic applications. Yet many such models are kept confidential or have not been adequately tested. The meeting recommends that a public domain parametric wind field model, fully tested and verified by peer review, be developed to provide the standard for comparison purposes.

RESEARCH (15) A parametric model of precipitation associated with a landfalling storm should be developed combining: 1. A short-range track and intensity forecast; 2. Rainfall rates derived from satellite and radar imagery calibrated from a rain gauge network.

Parametric Models

More extensive use of parametric wind field models in operations is recommended for their quality surface wind estimates compared with Dvorak relatively low cost & effort compared with NWP

Storm Tide:Waves & Rainfall:FloodsWMO (2) Storm surge and wave height forecasting is still a significant problem in many tropical cyclone-affected countries. WMO should endorse and encourage the establishment of storm surge techniques and models, including river flooding and wave action (especially for small islands), for regions that do not have this capability. In addition, vulnerability assessments should be conducted for all countries threatened by storm surge.

RESEARCH (6) While considerable progress has been made in storm surge forecasting in the last several years, the meeting recognises that this capability needs to be improved and extended to real-time forecasts of lowland inundation. Thus the meeting recommends that current storm surge and wave setup models should be improved via validation using measurements, through intercomparison studies, and through improvement of parametric wind models.

OPERATIONAL (9) One of the main impacts of tropical cyclones is the inundation of coastal areas. To address this problem, it is recommended that techniques for forecasting inundation areas be considered and applied, including the use of inundation maps with the combined effects of river floods and storm surge.

Storm Tide:Waves

Storm surge predictions are readily affected by the error in tropical cyclone predictions in terms both of their tracks and of intensities. Taking into account this, ensemble (-like) and probabilistic methods and outputs should be considered to use in operational storm surge forecast.

As mesoscale NWP models with high resolution are having ability to solve tropical cyclone fields, the use of the results of these NWP models in tropical storm surge modeling should be investigated.

Total storm tide water level is the combined effect of storm surge, wave set-up and high tide, and so accurate prediction of wind waves and tidal height together with their non-linear interaction with the storm surge in the model is essential, but prediction methods for wave setup are not well established yet. Therefore, further studies of wave setup prediction method are needed.

Rainfall:Floods

There is a need to continue in improving operational hydrologic and hydraulic flood modeling, and this includes consideration of a tighter coupling with meteorological, hydrological/hydraulic, and tidal modeling outputs where useful. We need to have a better definition of the track and intensity forecast of tropical cyclone path and quantitative precipitation forecast (QPF) associated to tropical models. The use of ensemble forecast and short term QPF should be considered.

Some other works that need to be considered include;- (ii) revision of current rainfall inputs to hydrologic models to allow for gridded rainfall inputs from improved operational rainfall spatial analysis and forecast rainfall grids from NWP models; (iii) digital representation of hydrologic model sub-areas to enable improved spatial rainfall inputs (iv) use of radar-rainfall estimation, and forecasting for short lead times, for flash flood situations.

Intense rainstorms brought about by tropical cyclones often caused severe landslides and debris flows, which have claimed many lives and properties. It is imperative that the flood forecasting agencies do research into and develop forecasting models for landslides and debris flows. Currently this is an area where the USGS and NWS are partners to develop such forecasting.

www.bom.gov.au

Single Day Forecast

Up to 8 global models are combined using the “probability matched ensemble mean” technique

Day 1 to Day 4 Forecast

Up to 8 global models are combined using the “probability matched ensemble mean” technique

Day 5 to Day 8 Forecast

Up to 8 global models are combined using the “probability matched ensemble mean” technique

Probability Forecasts

% chance is the proportion of available models predicting rain at or above the given threshold

Structure:Wind:PressureWMO (5) Tropical cyclone intensity should not be defined solely by a single parameter such as central pressure or maximum wind. A more detailed structural analysis is required. The WMO should encourage forecast centres to report within the current WMO format structural information such as quadrant gale radii, eye size, and radius of outer closed isobar in international exchanges of both real-time and best track data. In addition, the time of occurrence and the value of the minimum pressure, and an indication of the occurrence of an eye passage should be added to the synoptic code. This information is needed to determine the maximum wind-minimum central pressure relationship associated with tropical cyclones.

RESEARCH (16) The lack of observational data has caused difficulties with the calibration of the Dvorak intensity analysis technique. It is recommended that calibration or re-calibration of the Dvorak technique, and all pressure-wind relationships, be undertaken in all basins.

OPERATIONAL (7) There is a need for a standard conversion chart that enables users to convert between different wind-averaging periods and gust factors. The meeting endorses the RSMC recommendation for updating the conversion chart in the Global Guide on Tropical Cyclone Forecasting and requests that the updated values be distributed to TCWCs as soon as they become available. In addition, the chart should be updated on a regular basis as more research into conversion factors comes to hand.

Doppler Radar image showing powerful wind & damage streaks in RED

ISSUE: Difficulty of applying the Category scale to the more complex cyclones such as LARRY

Structure:Wind:Pressure

Contributions to asymmetry in TC structure due to motion and proximity to land could be comparable. Interaction between them may be important. Full investigation is needed.

Extreme wind gusts induced by convective, coherent or vortex-related features could be damaging at ground level but not necessarily represented by the broad-brush destructive potential scales (e.g. Saffir Simpsons). Better characterization making use of increased observations (Doppler radar, tower surface wind measurements, GPS dropsondes) should become possible.

Refined surface roughness length scheme over land in conjunction with very fine grid could bring more details of microscale structure of terrain-induced downdrafts

Concept of a cyclone “FRONT” 1 dimensional

measure of Size

LARRY had a relatively narrow “FRONT”

Storm Tide Warning Graphical Product

--- a picture is worth a thousand words ---

IWTC VI – Costa Rica – November 2006 Summary Session – Jim Davidson

“Quantitative Forecasts of TC Landfall in relation to an Effective Warning

System”