3D/2D modelling suite for integral water solutions. Delft3D WES. User Manual

Transcription

1 3D/2D modelling suite for integral water solutions Delft3D WES User Manual

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3 WES Wind Enhance Scheme for cyclone modelling User Manual Version: May 2014

4 WES, User Manual Published and printed by: Deltares Boussinesqweg HV Delft P.O. Box MH Delft The Netherlands telephone: fax: info@deltares.nl www: For sales contact: telephone: fax: sales@deltaressystems.nl www: For support contact: telephone: fax: support@deltaressystems.nl www: Copyright 2014 Deltares All rights reserved. No part of this document may be reproduced in any form by print, photo print, photo copy, microfilm or any other means, without written permission from the publisher: Deltares.

5 Contents Contents 1 A Guide to the manual 1 2 Introduction Functions and data flow of WES Definition of the circular or the spiderweb grid Overview of the in- and output files Getting Started How to run WES Files description The main Input file <.inp> file Creating a track file <.trk> Possible input parameters with respect to the tropical cyclone intensity The diagnostic file Conversion Factors Conceptual description Brief description of Holland s model Further improvements to the original model The approach in WES Method 1: computing wind and pressure fields from V max, A and B Method 2: computing wind and pressure fields from V max, R 35, R 50 and R Method 3: computing wind and pressure fields from V max, P drop and R w Method 4: computing wind and pressure fields from V max, P drop (R w is not known) Methods 5 and 6: Computing wind and pressure fields from V max and R w (P drop is not known) Method 5: P drop based on empirical model based on US hurricane statistics Method 6: P drop based on empirical model for Indian tropical cyclones Method 7: computing wind and pressure fields from V max Comparisons with observations Comparison of Radius of Maximum Wind and maximum wind speed Comparison of Wind speed and direction with satellite data QuickSCAT winds ERS winds Comparison of WES Winds with measured ground data Comparison of different methods 33 9 Glossary of terms 35 References 37 A Description of used files 39 A.1 Description of the main input file for WES <.inp> A.2 Description of the cyclone parameters in the track file <.trk> A.3 Spiderweb file Deltares iii

6 WES, User Manual A.4 Conversion Factors for wind speed A.5 Common Errors and Suggested Solutions in WES iv Deltares

7 List of Figures List of Figures 2.1 Tropical cyclone winds on a circular grid Definition of the spiderweb grid Screen shot from WES requesting the name of the main input file Specifying the name of the main input file Screen shot from WES while processing Screen shot from WES showing error in running WES Example of calculated wind speed for given A and B values Asymmetric wind due to translation of the cyclone. Wind vectors a > b A and B value computed using two different methods Left: Central pressure drop depicted agains maximum wind for 13 hurricanes in USA between 2000 and 2005 data; Right: Comparison between the empirical relation to hurricane Ike data (data source: Central pressure vs maximum wind for WES, HURDAT observations and Hollands P W model and Dvorak for the dependent dataset (source: Holland (2008)) B as a function of Maximum wind speed (V max ) for Indian tropical cyclones (from top to bottom) Comparison between the observed and computed Radius of Maximum Wind and maximum wind speed for Vizag, Kakinada, and Orissa Cyclones QuickSCAT wind measured at 28/10/1999 (Orissa Cyclone - 05B). Black coloured wind barbs indicates rain contaminated data Comparison of WES winds and direction with derived winds from QuickSCAT satellite for 4 different sectors (Orissa Cyclone) Comparison of WES winds and direction with derived winds from QuickSCAT satellite for 4 different sectors (Cuddalore Cyclone) Comparison of ERS and parametric wind speeds and directions (Method A) on three radial cross sections on 28/10/1999 at 0400 Z. Triangles represent ERS data, crosses model data Comparison of ERS and parametric wind speeds and directions (Method B) on four radial cross sections on 28/10/1999 at 0400 Z. Triangles represent ERS data, crosses model data Comparison of ERS and parametric wind speeds and directions (Method A) on four radial cross sections on 28/11/2000 at 0400 Z. Triangles represent ERS data, crosses model data Comparison of ERS and parametric wind speeds and directions (Method B) on four radial cross sections on 28/11/2000 at 0400 Z. Triangles represent ERS data, crosses model data Comparison of WES generated wind speed and direction with ground observation Computed Katrina wind speed on the 25th of August 2005 for 6 different methods in WES A.1 Gust factors for cyclone wind speed (Curve C) as a function of time (Krayer and Marshall, 1992) Deltares v

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9 1 A Guide to the manual This user manual provides detailed information on running of WES program, version 3.3. To make this manual more accessible we will briefly describe the contents of each chapter and appendices. Chapter 2: Introduction, provides an overview of the WES functions, area of applications and the software and hardware configurations of WES. Chapter 3: Getting Started, gives a brief overview of the input files required, data flow within the program, the output files of the program and finally, the steps executed by WES. Chapter 4: Files description, provides practical information of the model input files Chapter 5: Conceptual description, describes the equations in WES. Chapter 6: The approach in WES, describes a number of methods available in WES to generate the the wind and pressure fields. Chapter 7: Comparisons with observations, comparison of numerical results and observations of 4 different tropical cyclones. Chapter 8: Comparison of different methods, comparison of wind and pressure fields generated with different methods in WES. Chapter 9: Glossary of terms, contains a list of terms and abbreviations used in this manual and their explanations. References, provides a list of publications referenced by this manual. Appendix A: Description of used files, gives a description of all the files that can be used in WES. This information is required for generating some files manually or by other means of other utility programs. Deltares 1

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11 2 Introduction Accurate depiction of the inundation requires the accurate computation of the Storm Surge that again depends on the accuracy of the wind forcing it receives. Tropical cyclone winds usually are accounted for in the Numerical Weather Prediction (NWP) data. However, the grid resolution used in these models is usually not sufficient to accurately represent the strong variations on the wind gradients near its centre due to low grid resolution. Furthermore, the deficiencies in the parameterisation of Cumulus Convection in NWP models do not bring out the tropical cyclone features in their true intensity and size. Hence, the wind and pressure in the tropical cyclone that are predicted by these models are generally underestimated. To overcome this difficulty, all meteorological agencies resort to supply some bogus data to bring out the tropical cyclone in the analysis. The methods used to generate winds are by Rankine Vortex or by generating some synthetic vortex or by the technique suggested by Holland (1980). The winds generated with this approach are geostrophic in nature. Asymmetry, which is usually encountered in the observed wind field, is brought out by vectorial addition of the translatory movement of the tropical cyclone. For the detailed description of this scheme we refer to chapter 7 of this document. For storm surge simulations with Delft3D-FLOW, a Wind Enhance Scheme (WES) following Holland (1980) has been devised to generate tropical cyclone wind field. The program computes surface winds and pressure around the specified location of a tropical cyclone centre and given a number of tropical cyclone parameters (track data: i.e. maximum wind speed, pressure drop, radius of maximum wind and positions of the tropical cyclone). The tropical cyclone track data given by any Meteorological Agency can be taken by WES. However, JTWC, is the only agency that predicts sustained maximum winds at regular intervals. The scheme was initially developed by the UK Met Office. Further improvements to the method have been applied to make the program more robust and yield more reliable and consistent results. The program also calculates the radius of the maximum winds offering a possibility to compare the output with Radius of Maximum Winds (R mw ) derived from radar and satellite observations. The output of WES is suitable as input for Delft3D-FLOW, to simulate a storm surge. 2.1 Functions and data flow of WES The main functions and data flow of WES is to synthesize the tropical cyclone wind and pressure drop on a circular or spiderweb type grid (see Figure 2.1). 2.2 Definition of the circular or the spiderweb grid For the synthesising of the tropical cyclone wind, an elegant technique has been adopted requiring data on a polar co-ordinate grid centred on the centre of the tropical cyclone (Stelling, 1999). On this, so-called spiderweb grid, the tropical cyclone wind and pressure fields are generated. The number of grid points in the spiderweb both in the radial and in the tangential direction is user specifiable. The radius of the tropical cyclone must be specified by the user. If the radius is changed, the grid cell size will vary according to number of grid cells specified. Deltares 3

12 WES, User Manual Figure 2.1: Tropical cyclone winds on a circular grid Figure 2.2: Definition of the spiderweb grid 2.3 Overview of the in- and output files Input files The following input files are needed to run WES: <.inp> file: <.trk> file: This is the main input file for WES. The files contain information regarding input and output files and WES run options. This file contains the tropical cyclone track information that is either created manually (by editing it) or by another separate program. The sign above represents arbitrary filename. It is advisable however to use a filename containing date and time stamp for all the files. Output files WES produces the following output files: <name.spw> file: A file containing the synthesised tropical cyclone wind speed and pressure (drop) on a polar co-ordinate grid or the spiderweb grid 4 Deltares

13 Introduction (see detailed description in section A.5). <name.dia> file: A diagnostic or the log file containing the steps carried out by WES + some intermediary results. It may also contain ERROR and WARN- ING messages from the program in case they occurred, so appropriate actions may be taken to correct them. The name above represents the basename that is identical to the input filename contained in the <.inp> file. Deltares 5

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15 3 Getting Started 3.1 How to run WES When invoking wes on the commandline, one may specify the input file <.inp> manually as the program argument. If not specified, it will ask you to specify this file name, see Figure 3.1. Figure 3.1: Screen shot from WES requesting the name of the main input file Type the <name.inp> file name as indicated in Figure 3.2. Figure 3.2: Specifying the name of the main input file Once the input file is given, the details of the input files are read and processed. The screen shots of WES when running is given, Figure 3.3. The detailed information regarding the processed input can also be seen in one of the output files of WES <.dia>. If there are any errors in the input file WES will stop and it will generate the message Abnormal end of the program (see Figure 3.4). If no error is encountered, WES will produce the output files: <name.dia> file and <name.spw>. Deltares 7

16 WES, User Manual Figure 3.3: Screen shot from WES while processing Figure 3.4: Screen shot from WES showing error in running WES 8 Deltares

17 4 Files description 4.1 The main Input file <.inp> file The main input file must be available describing the options for running WES (e.g. the details about the spiderweb grid size, the required extent of the diagnostic file - brief or detailed, etc.). For a detailed description of all the parameters in the <.inp> file, see section A Creating a track file <.trk> This file may be edited manually. This file contains the main information on the tropical cyclone parameters: the initial and predicted positions of the storm centre (in geographical co-ordinates), the direction and speed of movement of the storm centre during the previous 6 hours, the intensity of the storm in terms of associated maximum winds and the corresponding pressure drop in Pascal. For a detailed description of all the parameters in the <.trk> file, see section A Possible input parameters with respect to the tropical cyclone intensity The important input parameters for WES that determines the tropical cyclone wind intensity are: A This parameter along with the parameter B below, determines the radius at which maximum wind speeds occur. It is very unlikely that the value of this parameter is known. B This parameter determines the shape of the wind and pressure profile as a function of distance from the storm centre. In some regions B has been determined from climatological data and you may wish to use this value. In the modified form, B value is calculated from v max, which can vary from day to day in a given tropical cyclone. Hence B value is not kept constant in the initial and predicted intensities of tropical cyclone. v max The maximum sustained winds in the tropical cyclone [kts]. This parameter is compulsory. It is estimated from satellite data and other observed data. p drop Represents the difference between the ambient and central pressures [hpa]. This parameter is estimated from satellite images, or may be computed using the following relation v max = C p drop (see Chapter 7). In some cases p drop and v max will be the only pieces of data available. Radius of 35, 50 & 100 kts wind These parameters are estimated from observations and are reported in JTWC bulletins. These parameters are to be specified in a certain combination in order to allow WES to produce the desired output (see chapter 6). The values for the parameters described above may be obtained from tropical cyclone advisories produced by JTWC, UKMO, IMD, JMA or any other Meteorological institutes in the world. In all advisories, the present and forecast positions of the tropical cyclones are given. Deltares 9

18 WES, User Manual The tropical cyclone intensity parameter however is only specified in the JTWC (Hawaii) advisory in the form of prevailing and forecast maximum winds or in the IMD tropical cyclone bulletins as prevailing tropical cyclone intensity (specified in categories). Specifying the positions, the prevailing and the expected wind speeds and pressure drops information are sufficient to synthesise the tropical cyclone winds. In this case certain assumptions have to be made about the model parameters (see Chapter 7). JTWC advisory also contains information on the present and expected radius of 100 knots wind (R 100 ), 50 knots wind (R 50 ), and 35 knots wind (R 35 ) as a function of position (c.q. time). In this case the assumptions mentioned earlier are not required and WES can use the above radii in order to find an optimal fitting of the synthesised winds to the observed values. When not available or not used the related columns for these parameters in the trk file must be filled with the Missing Index value (= ). A detailed description of the <.trk> file is given in Appendix A.2. Also of importance is the explanation on how WES will treat the different options and when redundant information is specified in this file. 4.4 The diagnostic file The diagnostic file (<.dia>): In the <.inp> file, the option EXTENDED_REPORT = enables the user in the input file to specify whether an extended diagnostics are to be produced or not. Extended diagnostics include process-logging, extensive error and warning messages, etc. Compact diagnostic will only contain process-logging and main error messages. If the user mentions YES for EXTENDED_REPORT, extended diagnostics will be produced else compact diagnostics will be produced. In the extended mode the following information are dumped to the diagnostic file. 1 The contents of the input file, <.inp> file. 2 The contents of tropical cyclone track file, <.trk>, read including the computed constants and parameters such as A, B, and the pressure drop for different time step. 3 Summary of data in the spiderweb file for Delft3D, which consists of date and time of the tropical cyclone position, maximum wind speed in [kts], its direction and radius of the maximum winds and direction in which it occurs. 4.5 Conversion Factors Any wind speed that is mentioned in the advisories and/or computed by the NWP models is defined as wind speed that has been averaged over a certain period of time. For example, JTWC advisory mentions wind speeds that are based on 1-minute average while the IMD uses the wind speed based on 3-minutes average. Because in WES the tropical cyclone wind speed may originate from any sources and in order to enable WES to compute wind speed in consistent manner for your use, a conversion factor is introduced. It is used to convert wind information (y-minute averaged) specified in the (input) track file to the required, x-minute, averaged (output) wind data required by the user. The conversion value can be found by determining the inverse ratio of the Gust factor for those averaging periods respectively. For a detailed overview and treatment of the Gust factor and the conversion factor we refer you to section A Deltares

19 5 Conceptual description Cyclone wind fields are generated around the given centre positions of the storm, following Holland s method in order to obtain the wind and pressure fields (above sea surface) on a high-resolution grid. The parametric model that has been adopted in WES is based upon Holland (1980). And it has been improved slightly by introducing asymmetry. This asymmetry is brought about by applying the translation speed of the cyclone centre displacement as steering current and by introducing rotation of wind speed due to friction. This model has basically five parameters: 1 the location of the cyclone centre, 2 the radius of maximum wind, 3 the maximum wind speed, 4 the central pressure and 5 the current motion vector of the vortex. 5.1 Brief description of Holland s model Following Holland, the geostrophic wind speed V g of a cyclone is expressed as: V g (r) = AB(p n p c ) exp( A/r B )/ρr B + r 2 f 2 /4 rf 2 (5.1) Where r distance from the centre of the cyclone, f Coriolis parameter, ρ density of air (assumed to be constant equal to 1.10 kg m 3 ), p c central pressure, p n ambient pressure (theoretically at infinite radius, however in this model the average pressure over the model domain is used), and A, B parameters. Wind speed as functions of A & B B= 1.75 A= 5.00E+06 B= 1.5 A= 5.00E+06 B= 1.25 A= 5.00E+06 B= 1.25 A= 1.00E+06 B= 1.25 A= 5.00E+05 W i n d s p e e d ( m / s ) Distance (m) Figure 5.1: Example of calculated wind speed for given A and B values Parameters A and B are determined empirically. Physically parameter A determines the relation of the pressure or wind profile relative to the origin, and parameter B defines the shape of the profile, see Figure 5.1. Holland states that for plausible ranges of central and Deltares 11

20 WES, User Manual ambient pressures and radii of maximum wind speeds B is constrained to be between 1 and 2.5. In the region of maximum winds the Coriolis force is small in comparison to the pressure gradient and centrifugal forces, and therefore the air is in cyclostrophic balance. The cyclostrophic wind V c at a distance r in this region is given by: V c (r) = AB(p drop ) exp( A/r B )/ρr B (5.2) For simplicity reason we have introduced a pressure drop term, p drop, equal to p n p c. By setting d V c /dr = 0, the radius of maximum winds (R mw ) can be obtained and is given as follows: R mw = A 1/B (5.3) The R mw is independent of the relative values of ambient and central pressure and it is defined entirely by the scaling parameters A and B. Substituting eq. 5.3 back into eq. 5.2 yields an expression for the maximum wind speed as follows: V max = Bp drop /ρe (5.4) Where e is the base of the natural logarithm (= ). Parameters A and B can now be expressed as functions of measurable quantities as follows: A = R B w B = ρ e V 2 max p drop (5.5) (5.6) and the central pressure drop is given by p drop = ρ e V 2 max B (5.7) By substituting equations 5.5 and 5.6 into equation 5.1 we can also express the geostrophic wind V g as function of R mw : V g (r) = (R w /r) B V 2 max exp(1 (R w /r) B ) + r 2 f 2 /4 rf 2 The equations above are valid for geostrophic winds. Before deriving A and B the wind speed and pressure values are now scaled to their geostrophic values. (5.8) 5.2 Further improvements to the original model After determining the values of parameter A and B, the cyclone winds as a function of distance r and direction θ on a spiderweb like grid can be computed. The computed winds are then adjusted to account for the asymmetry introduced by the interaction of the cyclone with the steering flow (Chan and Gray, 1982) by adding the translatory movement of the cyclone to the existing wind field. On the northern hemisphere this vectorial addition increases the winds on the right hand side of the direction of the cyclone movement and reduces the winds 12 Deltares

21 Conceptual description Cyclone track Cyclone track a a b Northern hemisphere b Southern hemisphere Figure 5.2: Asymmetric wind due to translation of the cyclone. Wind vectors a > b on the left hand side and the other way around on the southern hemisphere, see Figure 5.2, so bringing out an asymmetry in the wind field. The wind direction (northern hemisphere) is rotated 20 in the anti clock-wise (cyclonic) direction so that the wind spirals in towards the centre (Shea and Gray, 1973) to account for the frictional effects. Also for the same reason a reduction factor of 0.7 is applied for the geostrophic wind to generate winds at 10 meters above mean sea level. Deltares 13

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23 6 The approach in WES Equation 5.8 synthesises cyclonic winds assuming that the values of parameters A and B are known. However, in practical situation these parameters are usually not directly available. Consequently, in operational / forecast mode the tropical cyclone winds and pressure field is computed based on (measured/observed/forecast) physical quantities that are reported in the tropical cyclone advisories: V max, R max, Pressure drop and or Radius of 100, 65, 50 and 35 kts wind (if and when the wind exceeds the associated speed). Unfortunately cyclone advisories originating from different meteorological agencies do not contain identical information for which a standard procedure can be developed. Below we summarise the contents of some of the advisories: Cyclone advisories from UK Met Office contain the present and 72 hours forecast of the cyclone positions accompanied by a qualitative description of the changes in the cyclone strength (intensifying, weakening etc.). JTWC advisories contains the present and 48 hours forecast of the cyclone positions accompanied by a quantitative description of the cyclone strength with the help of four parameters: 1 Maximum sustainable winds (V max ) at present positions, 2 Radius of 35 knots wind (R 35 ) at present and future positions, 3 Radius of 50 knots wind (R 50 ) at present and future positions and 4 Radius of 100 knots wind (R 100 ) at present and future positions. IMD advisories contain the present and 36 hours forecast positions of the tropical cyclone and a qualitative description of the changes in its strength (intensifying, weakening etc.) In all advisories the present tropical cyclone translation speed and its direction are mentioned. The method to compute the wind and pressure fields in WES depends on the tropical cyclone parameters specified and is described below. 6.1 Method 1: computing wind and pressure fields from V max, A and B In case the parameters A, B and V max are specified, then the wind and pressure fields can directly be computed using equations Equation 5.7 and Equation Method 2: computing wind and pressure fields from V max, R 35, R 50 and R 100 The available data (wind speed associated with R 35, R 50 and R 100 ), together with V max, can been used to fit Equation 5.1 with these parameters and obtain the appropriate values of A and B. The value of R w is then determined from Equation 5.3. The method described here has been tested on a number of tropical cyclones in India and Vietnam. As an example Figure 6.1 shows the results in one of the case tested namely the Orissa Cyclone (05B) The method is found to produce consistent result when at least 3 out of 4 values of R 35, R 50 and R 100 are available (i.e. in case of a strong tropical cyclone). Otherwise the method is less dependable. Deltares 15

24 WES, User Manual Figure 6.1: A and B value computed using two different methods 6.3 Method 3: computing wind and pressure fields from V max, P drop and R w If all the three parameters V max, P drop and R w are known then the tropical cyclone wind field can be easily computed using Equations 5.5, 5.6 and 5.8. However, usually these parameters are not available all at the same time. So the following solutions are available in WES (see next sections). 6.4 Method 4: computing wind and pressure fields from V max, P drop (R w is not known) If the value of R w is not known/specified then a default value of 25 km for R w will be assumed. 6.5 Methods 5 and 6: Computing wind and pressure fields from V max and R w (P drop is not known) When pressure drop is not specified two methods can be applied: Method 5: P drop based on empirical model based on US hurricane statistics Based on data of 13 hurricanes (Ida, Bill, Hannah, Gustav, Dolly, Dean, Dennis, Emily, Katrina, Rita, Wilma, Charley and Ivan 1 ); that occurred in USA between the year 2000 and 2005, we have derived an empirical relation between the pressure drop and quadrate of maximum wind speed (as suggested by Equation 5.7). This empirical relation reads (See Figure 6.2 left): P drop = 2V 2 max (6.1) 1 source: 16 Deltares

25 The approach in WES Substituting this to Equation 5.7 yields a B value equal to: B = 1 ρ e = (6.2) Wind speed and Pdrop relation EMPIRICAL RELATION ALL (excl. IKE) Power (EMPIRICAL RELATION) y = 2x 2 P ressure drop (P a) Wind Speed (m/s) Wind speed and Pdrop relation WES derived relation IKE Power (WES derived relation) y = 2x 2 Pressure drop (Pa) Wind speed (m/s) Figure 6.2: Left: Central pressure drop depicted agains maximum wind for 13 hurricanes in USA between 2000 and 2005 data; Right: Comparison between the empirical relation to hurricane Ike data (data source: This relation is then compared to data from Hurricane Ike (See Figure 6.2 right). For this hurricane the applied relation seems to systematically underestimate the pressure drop for winds < 50 m/s and slightly overestimate the pressure (drop) for wind speed > 50 m/s. Lower wind speed in this graph depicts the winds after the 8th of September. The systematic bias during this period may be caused by the fact that the ambient pressure during the last stages of Ike is slightly lower than 1010 mbar. Once the value of B is determined, subsequently, the value of P drop can be determined. Holland (2008) devised a new empirical relation for relating maximum winds to central pressure in tropical cyclones. He determined a derivative of the Holland B parameter, B s, which relates the pressure drop directly to surface winds. This parameter B s is a function of pressure drop at the centre of the tropical cyclone, intensification rate, latitude, and translation speed. To compare the empirical relation given by Equation 6.1 to Hollands findings, Figure 6.3 is presented. Deltares 17

26 WES, User Manual Figure 6.3: Central pressure vs maximum wind for WES, HURDAT observations and Hollands P W model and Dvorak for the dependent dataset (source: Holland (2008)) Method 6: P drop based on empirical model for Indian tropical cyclones The following empirical method has been devised after investigating a number of tropical cyclones in the Bay of Bengal as an option after the observation that method 2 often fails to produce a reliable result, due to limited number of parameters.?e / pd B derived directly form hindcast data B as a function of wind speed used in WES B Wind speed (m/s) Figure 6.4: B as a function of Maximum wind speed (V max ) for Indian tropical cyclones The method is based on practice applied by IMD that uses a constant value to relate the maximum sustained wind speed with the pressure drop. The value of B is subsequently determined by fitting the data from a number of tropical cyclone occurring in India. This relation is described by a linear function where B is set equal to 1.18 for 20 knot winds (approximately 10 m/s). It linearly increase towards a value of 1.55 for wind speed equals 150 knots (approximately 77 m/s; see Figure 6.4). Once the value of B is determined, subsequently, the value of P drop can be determined. For A, a value between 4.1E+06 and 4.2E+06 has been appled. An example of the result is 18 Deltares

27 The approach in WES shown in Figure 6.1 for the Orissa Cyclone (05B) 1999 test case. 6.6 Method 7: computing wind and pressure fields from V max The only data used in this method is the maximum wind speed V max. Through some error minimisation procedure the value of A and R w is determined. However this method is proven not to be robust and is only maintained in WES for backward compatibility reason. The use of this method is no longer recommended. Deltares 19

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29 7 Comparisons with observations For comparison with observations (or derived parameters from observations) the following tropical cyclones and parameters have been selected. Tropical cyclone Parameter(s) compared Name Id Year Month Radius of Max. wind and Max. Wind speed Wind speed & direction profile (QuickSCAT data) Ground observation (wind speed & direction) Visakhapatnam (Vizag) 05B 1994 Nov Kakinada 05B 1996 Nov Orissa 05B 1999 Oct Cuddalore 03B 2000 Nov Wind speed & direction (statistic ERS data) When comparing the computed wind with observation data, it is important to bear in mind that the tropical cyclone centre is usually given in latitude and longitude up to a tenth of a degree. So due to round off error the actual positions may actually lie ± 5 km from the specified point. This might introduce discrepancies in the results when compared to the observed values, especially in the region where wind gradients are high. 7.1 Comparison of Radius of Maximum Wind and maximum wind speed The radius of maximum wind (R mw ) and the maximum wind speed data originates from JTWC tropical cyclone bulletins and IMD RSMC report. Method 6 (see chapter 6) was subsequently adopted to synthesise the tropical cyclone winds. The resulting wind speed was then compared with the data. R mw can also be compared against the radius of maximum reflectivity (R mr ) which can be measured by radar (R mr and is a measure of R mw ). The JTWC provides two values for radii for 35, 50 and 100 knots winds in their bulletin. One number represents the value for the north-eastern semicircle and the other one for remaining areas. In the computation of the wind an average of these two values have been used. Furthermore the bulletins are issued every 12 hours. To compute the wind field for every 6 hours interpolation of these values have been applied. Figures 7.1a to 7.1c depict the plots of radius of maximum wind and the maximum wind speed during the life cycle of the tropical cyclone from the Vizag-1998, Kakinada-1996 and Orissa cyclones. In Figure 7.1c, the Orissa Cyclone case, the radius of maximum wind (R mw ) gradually decreased from a value of 42 km on the 26th October to a value of 7.5 km on the 29th October when the tropical cyclone reached its peak intensity. The radius of maximum wind increases after the tropical cyclone made landfall and became weaker. The triangles shown in the diagram represents the R mr values measured by the radar at Paradip and the squares in the diagram represents 0.5 the eye diameter (radius of the eye). The reported accuracy of the observed data is represented in the figures by an error bar. The agreement with R mr and R mw in this case is remarkably good. Similar holds for the computed maximum wind speed, especially for the winds computed using method A. Figures 7.1a and 7.1b represent the diagrams Deltares 21

30 WES, User Manual Figure 7.1: (from top to bottom) Comparison between the observed and computed Radius of Maximum Wind and maximum wind speed for Vizag, Kakinada, and Orissa Cyclones for Vizag and Kakinada cyclones. Similar conclusion can be drawn for the Vizag cyclone case. The R mw values in the Kakinada cyclone case however, are slightly larger than the reported R mr value. 22 Deltares

31 Comparisons with observations 7.2 Comparison of Wind speed and direction with satellite data As long as the tropical cyclone is at the ocean comparison of model result with observations has to rely on winds derived from the satellite data, as conventional observations are hardly available over the oceans. Fortunately, ERS satellite and more recently QuickSCAT derives wind data from scatterometer data, despite some limitations. The accuracy of the winds is claimed to be 1.4 m/s up to the speeds of 20 m/s and an accuracy of 10% beyond that. The maximum measurable speeds with the scatterometer are 50 kts ( 25 m/s). Hence, at higher wind speeds comparison cannot be made. This QuickSCAT data from the year 1999 onwards is available at the site QuickSCAT winds WES computed winds has been compared with QuickSCAT data for the Orissa cyclone (26-30 Oct, 1999) and Cuddalore Cyclone (27 29 Nov, 2000). The comparison is made by printing out the graphs that have been downloaded from the above referred internet site and reading the wind speed and direction visually. Figure 7.2: QuickSCAT wind measured at 28/10/1999 (Orissa Cyclone - 05B). Black coloured wind barbs indicates rain contaminated data. Despite the fact that this method of comparison is imperfect and very error prone, it is nevertheless useful to check the main characteristics of the wind fields produced. An example plot of QuickSCAT observed data is depicted in Figure 7.2c for the Orissa Cyclone case. The following figures depicts 4 cross-sections of the wind derived with WES. The wind speeds and direction of the winds from QuickSCAT data are shown in each diagram at the appropriate points. Figures 7.3 and 7.4 shows the wind speed and direction at 28th October, UTC 1 Deltares 23

32 WES, User Manual Figure 7.3: Comparison of WES winds and direction with derived winds from QuickSCAT satellite for 4 different sectors (Orissa Cyclone) (Orissa Cyclone) and 28th November, UTC (Cuddalore Cyclone). There is a very good agreement for wind speeds and wind directions in most of the cross-sections. The results of WES, especially the wind speed in the Orissa case and the wind direction in the Cuddalore case show good agreements with the satellite observations. Especially when we bear in mind that the comparison has been done by estimating the wind speed and direction visually from a printed graph and the fact in some occasion the wind measured was contaminated due to rainfall. 24 Deltares

33 Comparisons with observations Figure 7.4: Comparison of WES winds and direction with derived winds from QuickSCAT satellite for 4 different sectors (Cuddalore Cyclone) Deltares 25

34 WES, User Manual ERS winds Comparison with ERS winds are carried out in a different manner as we had direct access to the measured data (provided by the UK Met. Office). Therefore we could compare the computed results with observation points at the exact locations. Furthermore, statistical computations were possible. In the following figures and tables the comparison of the WES results, both using method A and B, with the observed ERS derived winds are presented. When interpreting the figures and tables below please note that the maximum measurable speeds with the ERS are 50 kts ( 25 m/s). Hence, winds at higher speeds were not included in the analysis. Orissa case Method 2 (using JTWC advisories containing R 35, R 50 and R 100 ) A = B = 1.46 (7.1) v max = 39 m/s p drop = 31.2 hp a R mw = 46.5 km (7.2) Number of observation points Max distance from tropical cyclone centre (km) Mean speed (m/s) wind error RMS speed (m/s) wind error Mean wind direction error (degrees) RMS wind direction error (degrees) n.a n.a n.a n.a Orissa case Method 6 (using IMD data; v max and p drop ) A = B = 1.33 (7.3) v max = 41.5 m/s p drop = 38.7 hp a R mw = 16.5 km (7.4) Number of observation points Max distance from tropical cyclone centre (km) Mean speed (m/s) wind error RMS speed (m/s) wind error Mean wind direction error (degrees) RMS wind direction error (degrees) n.a n.a n.a n.a Cuddalore case Method 2 (using JTWC advisories containing R 35, R 50 and R 100 ) A = B = 1.44 (7.5) 26 Deltares

35 Comparisons with observations Figure 7.5: Comparison of ERS and parametric wind speeds and directions (Method A) on three radial cross sections on 28/10/1999 at 0400 Z. Triangles represent ERS data, crosses model data Deltares 27

36 WES, User Manual Figure 7.6: Comparison of ERS and parametric wind speeds and directions (Method B) on four radial cross sections on 28/10/1999 at 0400 Z. Triangles represent ERS data, crosses model data 28 Deltares

37 Comparisons with observations v max = 26.0 m/s p drop = 14.0 hp a R mw = 54.5 km (7.6) Number of observation points Max distance from tropical cyclone centre (km) Mean speed (m/s) wind error RMS speed (m/s) wind error Mean wind direction error (degrees) RMS wind direction error (degrees) Cuddalore case Method 6 (using IMD data; v max and p drop ) A = B = 1.37 (7.7) v max = 50.4 m/s p drop = 55.4 hp a R mw = 12.5 km (7.8) Number of observation points Max distance from tropical cyclone centre (km) Mean speed (m/s) wind error RMS speed (m/s) wind error Mean wind direction error (degrees) RMS wind direction error (degrees) Based on the figures above one might conclude that both the methods used by WES to derive the tropical cyclone winds are comparable. However, when the maximum winds are compared than a slightly better result is obtained by applying Method 6. The agreement between the derived wind directions with the observed data is reasonably accurate. The derived wind speed is quite accurate up to approximately 12 m/s. Deviations along some cross-sections then increase up to 7 m/s on average for the wind speed values between 12 and 25 m/s. Similar to the QuickSCAT case we were not able to determine the rain contamination in the measured data Comparison of WES Winds with measured ground data Some surface wind speeds during Orissa cyclone (05B) 1999 originating from ground observations has been collected from IMD Hyderabad Met. Office for comparison purposes. The results are shown in Figure 7.9. From the figures it is clear that the WES winds are able to reproduce the surface wind measurements with sufficient degree of accuracy. One exception forms the results at Puri. However when a location slightly North of Puri is selected then the results are again quite good. Deltares 29

38 WES, User Manual Figure 7.7: Comparison of ERS and parametric wind speeds and directions (Method A) on four radial cross sections on 28/11/2000 at 0400 Z. Triangles represent ERS data, crosses model data 30 Deltares

39 Comparisons with observations Figure 7.8: Comparison of ERS and parametric wind speeds and directions (Method B) on four radial cross sections on 28/11/2000 at 0400 Z. Triangles represent ERS data, crosses model data Deltares 31

40 WES, User Manual Figure 7.9: Comparison of WES generated wind speed and direction with ground observation 32 Deltares

41 8 Comparison of different methods The different methods to compute the wind and pressure fields in WES that is made available in WES will produce results that differs marginally from each other. For instance, when reproducing the Katrina hurricane winds on 28th of August UTC we have applied the 7 different methods with the following parameter values: Method Vmax Rmax [nmi] R_100 [nmi] R_65 [nmi] R_50 [nmi] R_35 [nmi] B A Pd [Pa] E The results are depicted in Figure 8.1, along with H*winds model output 1 and observed data 2. Based on the results we can conclude that for this specific case, method 7 shouldn t be used at all 3. For this specific case, Method 3 (or 4 where R max is set to a default value of 12.5 nautical miles) performs the best. Furthermore, as expected, the asymmetry feature in WES due to interaction with land mass is poorly represented. WES is, by design, (not yet) capable of including the effect of land mass. 1 See 2 See 3 As mentioned earlier only available in WES for backward compatibility reason Deltares 33

42 WES, User Manual Figure 8.1: Computed Katrina wind speed on the 25th of August 2005 for 6 different methods in WES 34 Deltares

43 9 Glossary of terms IMD JMA JTWC NWP NOAA AOML PC QuickSCAT ERS UKMO WES dia hpa inp kts spw trk H*wind India Meteorological Department Japanese Meteorological Agency Joint Typhoon Warning Centre Numerical Weather Prediction National Oceanic and Atmospheric Administration of teh United States Atlantic Oceanographic and Meteorological Laboratory - on eof the facilities of NOAA Personal Computer A polar orbiting satellite carrying a scaterometer; the data from this satellite can measured winds up to 40 kts. European Remote Sensing satellite United Kingdom Meteorological Office Wind Enhance Scheme Standard file name extension used for WES Diagnostic file unit for pressure (hecto)pascal Standard file name extension used for WES input file unit for wind magnitude in Knots Standard file name extension used for WES Spiderweb Standard file name extension used for WES Track file Tropical cyclone wind and pressure data obtained from H*Wind Project (Hurricane Research Division of AOML/NOAA). The data is based on an integrated tropical cyclone observing system in which wind measurements from a variety of observation platforms are used to develop an objective analysis of the distribution of wind speeds in a hurricane. Deltares 35

44 WES, User Manual 36 Deltares

45 References Chan, J. and W. Gray, Tropical cyclone movement and surrounding flow relationships. Monthly Weather Review 110: Delft3D-FLOW, Delft3D-FLOW User Manual. Deltares, 3.14 ed. 41 Holland, G., An Analytical Model of the Wind and Pressure Profiles in Hurricanes. Monthly Weather Review 108: , 11, 40, A Revised Hurricane Pressure Wind Model. Monthly Weather Review 136: v, 17, 18 Krayer, W. and R. Marshall, Gust Factors Applied to Hurricane Winds. Bulletin American Met. Society 73:5. v, 42 Shea, D. and W. Gray, The HurricaneâĂŹs Inner Core Region I. Symmetric and Asymmetric Structure. Journal of the Atmospheric Sciences 30: Stelling, G. S., Internal note on Cyclone modelling. Tech. rep., WL Delft Hydraulics, Delft, The Netherlands. 3 Deltares 37

46 WES, User Manual 38 Deltares

47 A Description of used files A.1 Description of the main input file for WES <.inp> Sample Input file for WES (<name.inp>). COMMENT COMMENT = WES run = This file is created on UTC COMMENT = Grid Delft3D file 259 by 129 CYCLONE_PAR._FILE SPIDERS_WEB_DIMENS. = RADIUS_OF_CYCLONE = WIND CONV. FAC (TRK)= NO._OF_HIS._DATA = 0 HIS._DATA_FILE_NAME = OBS._DATA_FILE_NAME = EXTENDED_REPORT = d:\wes\orissa_cyclone.trk = YES COMMENT All lines proceeded by the keyword COMMENT will be treated as comments. There can be any number of comments. CYCLONE_PAR._FILE The name of the track parameter file (ASCII file containing positions and the cyclone parameters). Advised extension for the file name is: <trk>. The path of the existing file may be included otherwise the current working directory will be assumed. SPIDERS_WEB_DIMENS. The dimension of the spider-web grid as well as the maximum radius of the cyclone. The grid resolution in the radial direction (i.e. ratio of cyclone radius/number of points) must be sufficient to resolve the expected wind gradients. From our experience, the following values may be prescribed for the dimensions and cyclone radius (R cyc ): 400 by 16 points and 600,000 meters respectively. The resolution in the radial direction is then equal to 1500 meters. A resolution between meters is required especially when dealing with compact cyclones with high wind gradients. RADIUS_OF_CYCLONE Radius of the cyclone, see also description of keyword SPIDERS_WEB_DIMENS.. WIND CONV. FAC (TRK) When measuring wind force, small (temporal) scale fluctuations are averaged during a certain period. Each country defines its own standard and defines a different averaging period. To produce a consistent output and to allow you to use input data from different sources, separate conversion factors for the cyclone wind magnitude has been defined, the WIND CONV. FAC (TRK). This factor will be applied to convert the values from the averaging period used in the cyclone advisories to the, required, x-minute average by the user, see also section A.4. For example, cyclone wind data from JTWC is usually based on 1 minute average. If the output is to be based on 3 minutes average as per IMD convection, then a multiplication factor equal to to the cyclone parameters (WIND CONV. FAC (TRK)) should be specified. NO._OF_HIS._DATA Number of history data station (maximum 9). When specified, then at the end of the diagnostic file a time series of wind data (magnitude + direction) for these stations will be produced. HIS._DATA_FILE_NAME File name containing station locations. OBS._DATA_FILE_NAME File name containing observation data (not yet active). Deltares 39

48 WES, User Manual EXTENDED_REPORT This option enables you to specify whether extended diagnostics are to be produced or not. Extended diagnostics include e.g. processlogging, extensive error and warning messages, etc. Compact diagnostic will only contain process-logging and main error messages. A.2 Description of the cyclone parameters in the track file <.trk> File description: File contents: File Format: Description of observed features of the cyclone and predicted future positions and characteristics. ASCII free formatted data. Record description: record 1-N record N+1 record N+2 A text header (comments) defining the file and the content of the following records (to be superseded by an asterisk in the first column). Year (integer), month (integer), day (integer), and hour of prediction (integer), Latitude of centre in decimal degrees (real), longitude of centre in decimal degrees (real), Direction of movement over past 6 hrs in degrees (real), speed over past 6 hrs in kts (real), Maximum sustained velocity in kts (real), Radius of 100, 50 and 35 Kt winds in Nautical miles (real), Constants B and A (reals), as defined in Holland (1980) Central pressure drop (background pressure - central pressure) in Pa (real). Year (integer), month (integer), day (integer), and hours (integer), Latitude and Longitude of eye centre in decimal degrees (real), Maximum sustained velocity in kts (real), Radius of 100, 50 and 35 kts winds in Nautical miles (real), Constants B and A (reals), as defined in Holland (1980) Central pressure drop (background - central pressure) in Pa (real). Record N+2 may be repeated as many times as required. Restriction: Any undefined values (or values that are not used) should be indicated using the missing data value PMDI (Parameter missing data Index = 1.e30) Example: Cyclone track for 13 positions: * * EXAMPLE file for cyclone tracks that is required by WES * Pressure drop is in Pascal. * *yyyy mm dd hh lat long Dm V VMax R100 R50 R35 B A Pdrop * (UTC) deg. (kts) (kts) (nm) (nm) (nm) (Pa) e30 1e30 1e30 1e30 1e e30 1e30 1e30 1e30 1e e30 1e30 1e30 1e30 1e Deltares