Validation of the MODTRAN 6 refracted geometry algorithms in the marine boundary layer and development of EOSPEC modules

Transcription

1 Validation of the MODTRAN 6 refracted geometry algorithms in the marine boundary layer and development of EOSPEC modules Vincent Ross Aerex Avionics Inc. Prepared By: Aerex Avionics Inc. 324, St-Augustin Avenue Breakeyville (Québec), Canada GS 1E1 Contractor's Document Number: AT6-1 Contract Project Manager: Daniel Pomerleau, PWGSC Contract Number: W /1/QCL, Task AT6 Technical Authority: Denis Dion, DRDC Valcartier Research Centre Disclaimer: The scientific or technical validity of this Contract Report is entirely the responsibility of the Contractor and the contents do not necessarily have the approval or endorsement of the Department of National Defence of Canada. Contract Report DRDC-RDDC-216-C141 March 215

2 Her Majesty the Queen in Right of Canada, as represented by the Minister of National Defence, 215 Sa Majesté la Reine (en droit du Canada), telle que représentée par le ministre de la Défense nationale, 215

3 324, Saint-Augustin Avenue, Breakeyville (Québec), Canada, GS IEI Phone: (4I8) 832-I4, Fax: (4I8) I, AEREX Report No: AT6-1 Validation of the MODTRAN 6 refracted geometry algorithms in the marine boundary layer and development of EOSPEC modules CONTRACT LETTER REPORT delivered to Denis Dion, M.Sc. DRDC, Valcartier Research Center 2459, De la Bravoure Road Québec (Québec) Canada, G3J 1X5 in the framework of contract W /1/QCL, Task AT6 authored by : Vincent Ross, M.Sc. March 215 Sa majesté la reine du Canada, représentée par le ministre de la Défense nationale, 215 Her Majesty the Queen, as represented by the Minister of National Defence, 215

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5 Validation of the MODTRAN 6 refracted geometry algorithms in the marine boundary layer and development of EOSPEC modules AEREX Report No: AT6-1 Version: 1. This document was reviewed for Controlled Goods by AEREX Avionics Inc. using the Guide to Canada s Export Controls. The scientific or technical validity of this Contract Report is entirely the responsibility of the contractor and the contents do not necessarily have the approval or endorsement of the Department of National Defence of Canada. L entrepreneur est entièrement responsable de la validité scientifique ou technique de ce rapport de contrat et le contenu de ce rapport n est pas nécessairement approuvé ni entériné par le ministère de la défense du Canada. Principal author: (Original signed by) March 215 Vincent Ross, M.Sc. (date) Senior Physicist, Atmospheric Optics Publication reviewed by: (Original signed by) March 215 Paul Lacasse, Eng, MBA (date) Senior Project Manager Publication approved by: (Original signed by) March 215 Daniel Pomerleau, Eng. (date) President, AEREX Inc.

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7 1 Introduction As MODTRAN evolves it is essential to maintain validation efforts, even in areas that have been previously studied in great detail. One such area of concern for the DRDC involves the behaviour of MODTRAN in the marine boundary layer. These environments are prone to large gradients of temperature and water vapor content, giving rise to large gradients in refraction index. In cases where viewing geometries are near horizontal, optical trajectories can undergo strong bending either upwards or downwards depending on the direction of the refractive gradients. In 22, the DRDC conducted a series of tests of MODTRAN 4 in these conditions and put to light occasional shortcomings of MODTRAN s refractive geometry algorithm[1]. In 212, MODTRAN Beta was submitted to the same series of test cases with similar results[2]. This is an effort to reproduce the same series of test cases with very similar parameters, and to compare MODTRAN version 6... Beta results with those obtained with MODTRAN Input atmospheric profiles In order to study the behavior of MODTRAN calculations with marine boundary layer conditions, typical atmospheric profiles for the first 5 meters from the sea had to be generated for input (although in this study, only the first 3 meters are used). The temperature, pressure and relative humidity profiles had to be physically plausible and representative of conditions one might encounter in the marine boundary layer. In order to obtain these profiles, the WKD model[3] was used. The first profile (referred to as case 1) is one we might encounter during the day when the water temperature is lower than the air temperature at 12m above the surface. In this case, a water temperature of 1 o C and an air temperature of 13 o C are used. The air will cool down quickly as we get closer to the water surface and this will result in a rise in air density and subsequently in refraction index. With the index of refraction rising rapidly as the altitude decreases, rays with a look angle (initial angle) below horizontal will tend to bend quickly towards the earth. The altitude profiles for the different quantities used in the calculations can be seen in Figure 1. AEREX Report No AT6-1 version 1. 1

8 Temperature Profile Pressure Profile Relative Humidity Profile Refractivity Profile x Temperature (K) Pressure (mb) Relative Humidity (%) Refractivity (n-1) Figure 1: Temperature, pressure, relative humidity and resulting refractivity for case 1. The second atmospheric profile chosen for this study, which will be referred to as case 2, nicely complements the first. It represents the situation here the water temperature is higher than the air, as is often the case at night. Here the water temperature has been chosen to be 1 C while the air temperature has been set to 5 C at an altitude of 12m. In this case, the air temperature will tend to increase close to the water surface and its density will therefore decrease. This decrease in air density will have the index of refraction also decrease in the region very close to the water surface. Rays entering this region of decreasing index of refraction will tend to bend upwards while rays with a higher look angle will not. These rays will eventually meet creating a mirage effect (duplication of the target) for an observer. The altitude profiles for the different quantities used in the calculations can be seen in Figure 2. 2 AEREX Report No AT6-1 version 1.

9 28 Temperature Profile 112 Pressure Profile 8 Relative Humidity Profile Refractivity Profile x 1-4 Temperature (K) Pressure (mb) Relative Humidity (%) Refractivity (n-1) Figure 2: Temperature, pressure, relative humidity and resulting refractivity for case 2 3 Source code modifications In the original 22 study, it was found that the resolution of the atmospheric stratification has an impact on the number of MODTRAN 4 failures. In order to push the analysis to the maximum, the NLAYER parameter was changed to a value of 5 and MODTRAN 4 was recompiled. In MODTRAN 5, such a high value is impossible when compiling on a 32bit architecture because of changes to array sizes. The value for LAYDIM was then set to 21, and MODTRAN 5 recompiled with Intel Fortran using default flags. For comparison purposes, the LAYDIM parameter is also changed to 21 in MODTRAN 6. To insure that no additional bugs are introduced, no other source code changes are made to the version of MODTRAN 5 received on the DVD and of MODTRAN 6 retrieved from the Beta testing web site. Additional modifications in MODTRAN 5 to the DISORT multiple scattering treatment as suggested by Alexander Berk (SSI) after reception of the code are not implemented, since they should not affect the refractive geometry calculations. 4 Results For the first set of tests, both cases are run with an atmospheric stratification of.15 and 1. meters in the first 3 meters of the atmosphere. For speed, calculations are done on the narrow 1.95 to 11.5 micron band. A field of view of.34 degrees is used with the center elevation pointing close to the horizon (the actual elevation changes according to path bending strength and direction) with a resolution of.2 degrees. Case 2 runs are also run with an angular resolution ten times finer near the horizon to emphasize MODTRAN s behavior in that region. In all runs, the script records the angles at which MODTRAN failed. AEREX Report No AT6-1 version 1. 3

10 .3 MODTRAN 5 with.15m layering.3 MODTRAN 6 with.15m layering Figure 3: MODTRAN 5 (left) and MODTRAN 6 (right) results for the case 1 atmosphere with.15m stratification..3 MODTRAN 5 with 1m layering.3 MODTRAN 6 with 1m layering Figure 4: MODTRAN 5 (left) and MODTRAN 6 (right) results for the case 1 atmosphere with 1.m stratification. The results for the case 1 atmosphere are shown in Figure 3 for.15 m stratification and in Figure 4 for 1. m stratification. It is obvious that both MODTRAN 5 and MODTRAN 6 suffer from similar problems for a large section of the field of view where the initial elevation is closest to horizontal. For the.15m stratification, while MODTRAN 5 failed outright (crash) for 4 out of 171 elevations (23%), MODTRAN 6 fails with an error message for 49 cases (29%). The 1m stratification yields identical results, with MODTRAN 5 crashing for 48 (28%) cases and MODTRAN 6 printing errors for the exact same angles. In the previous study comparing MODTRAN 4 to MODTRAN 5, it was determined that MODTRAN 5 would crash while attempting to print a proper error message by using a bad format string. MODTRAN 6 has clearly remedied this problem. In MODTRAN 5, some cases are successful but give wrong results. In these cases seen in the.15 m stratification instances, paths are severely truncated and appear in the lower left corner of the figure. These will undoubtedly produce wrong radiance and transmittance results without any warning. These are unaccounted for in the failure rate stated above for MODTRAN 5 and are responsible for the difference between both versions. 4 AEREX Report No AT6-1 version 1.

11 Another strange behavior noticed in both MODTRAN 5 and 6 that is not present in MODTRAN 4 is that after the following warning is issued: Comment from routine FINDMN: Tangent path with H1ALT = km and OBSZEN = 9.34 deg intersects the earth. H2ALT has been reset from.3 km to. km, and LENN has been reset from 1 to The following error still occurs: Error in routine GEOINP: ITYPE = 2, but slant path intersects the earth or ground, and cannot reach the final altitude, H2ALT. If the H2ALT and LENN variables have been reset as stated in the warning, the error should not occur. Note that this case is caught by the script used to run the cases here, and does not affect the results (failure rates). For the case 2 atmosphere, results are shown in Figure 5 for a.15 m stratification and in Figure 6 for 1.m layers. Inspection shows that MODTRAN 6 is successful more often than MODTRAN 5 for paths that get closer to the ground, and therefore suffer more upward bending due to refraction. In fact, for the low resolution test MODTRAN 6 only fails at exactly 9 degrees zenith while it does not fail at the higher resolution test (since the horizontal path is absent). This compared with MODTRAN 5 that fails for 7 (4%) and 63 (37%) of cases respectively. At the coarser 1.m stratification (Figure 6) MODTRAN 6 fails for fewer cases (7%) than MODTRAN 5 (1%) in the lower resolution test, and shines with failures compared with 6 (35%) for MODTRAN 5 in the high resolution test. Similarly to the Case 1 atmospheres, MODTRAN 6 generates a descriptive error message hinting to a convergence failure while MODTRAN 5 crashes while attempting to print the message. From the error message in MODTRAN 6, it seems that the convergence algorithm might be improved to prevent the error completely, although we have made no attempts to verify this ourselves. AEREX Report No AT6-1 version 1. 5

12 .3 MODTRAN 5 with.15m layering.3 MODTRAN 6 with.15m layering MODTRAN 5 with.15m layering.3 MODTRAN 6 with.15m layering Figure 5: MODTRAN 5 (left column) and MODTRAN 6 (right column) results for the case 2 atmosphere with.15m stratification. Higher angular resolution plots of the lower field of view are shown in the second row. 6 AEREX Report No AT6-1 version 1.

13 .3 MODTRAN 5 with 1m layering.3 MODTRAN 6 with 1m layering MODTRAN 5 with 1m layering.3 MODTRAN 6 with 1m layering Figure 6: MODTRAN 5 (left column) and MODTRAN 6 (right column) results for the case 2 atmosphere with 1.m stratification. Higher angular resolution plots of the lower field of view are shown in the second row. In the 22 report other strange behavior was observed in MODTRAN 4. For instance, when the RANGE variable was specified, paths that were close to the horizon had a tendency to move past the maximum range as seen in Figure 7. When MODTRAN 5 is run with the same parameters, it simply crashes in cirarc.f around line 54 while attempting to produce an error message. We do however note that the paths are more smoothly distributed with MODTRAN 5 than with MODTRAN 4 where they seem to jump across gaps erratically. MODTRAN 6 produces the same results as MODTRAN 5, but stops with an error message instead of crashing. AEREX Report No AT6-1 version 1. 7

14 5 MODTRAN 4 with 1m layering Figure 7: Behavior of MODTRAN 4 in for the same case as Figure 8. 5 MODTRAN 5 with 1m layering 5 MODTRAN 6 with 1m layering Figure 8: Behavior of MODTRAN 5 (left) and MODTRAN 6 (right) for case 1 atmosphere when RANGE (HRANGE) is specified. 5 New EOSPEC modules to drive MODTRAN 6 In light of the shortcomings discussed in the previous sections it becomes very useful to take advantage of the new input capabilities introduced in MODTRAN 5 and still available in MODTRAN 6, notably of a user defined path but also of the full spectral aerosol profile (SAP). The introduction of MODTRAN 6 also brought forth a new way to interface the application directly from C/C++ code (API). This new API should also eventually include what are called toolboxes to help connect external models and data to MODTRAN. Regretfully, the path and SAP input to MODTRAN are not part of neither API or toolboxes in the current beta2 version. These input are still, somewhat awkwardly, required to be formatted into text based files. Gladly, over the years, many tools have been developed to interface EOSPEC atmospheric profiles, refracted paths and aerosol profiles to MODTRAN. Here, we push this further by doing the preliminary development of 8 AEREX Report No AT6-1 version 1.

15 a true MODTRAN toolbox meant to bring the EOSPEC models into MODTRAN by working side-by-side with the API. For this, a new class modtranpathformat was added to EOSPEC that interpolates an EOSPEC path onto the internal MODTRAN atmospheric layering. This class uses the MODTRAN algorithms to determine the internal layering. It also accounts for the numerical roundoff issues that are introduced when passing atmospheric layer altitude data trough the MODTRAN in single presision floating point precision as dictated by the API. These measures are necessary because MODTRAN is quite stringent in enforcing an exact match between its internal altitude layering and the layering found in the ASCII input files. An example code was also produced that demonstrates how to setup MODTRAN with EOSPEC meteorological data trough the new API, how to produce compatible.pth and.sap files with the new EOSPEC modules, how to launch MODTRAN trough the API and finally how to retrieve the results, also trough the API. This example code is distributed as auxiliary material along with this report (see Annex A). 6 Conclusion Similar verifications have been performed on MODTRAN 6 that had put to light limitations in the refracted geometry calculation of MODTRAN 4 in 22 and of MODTRAN 5 in 212. The problems found in MODTRAN 4 have largely not been corrected in both MODTRAN 5 and 6 apart from some seemingly smoother variations from one elevation to the next. On the other hand, crashes in error printing routines in MODTRAN 5 were corrected in MODTRAN 6, and in some cases the algorithm now seems slightly more robust with MODTRAN 6 having higher success rates than MODTRAN 4 and 5 in some cases. One major advantage of MODTRAN 5 and 6 versus MODTRAN 4 is that the problems found in the refracted path calculations are far less critical, since they can be bypassed using the user defined.pth files. For this reason new classes as well as an example code was put into place to show how EOSPEC can be used to circumvent the shortcomings of MODTRAN. AEREX Report No AT6-1 version 1. 9

16 Annex A Provided material Material for the raytracing tests In order to help in the debugging process, we provide a number of complementary documents along with this report. These are located in directory Test Material, in sub directories Test1 trough Test3. Test1 corresponds to Figure 3 and Figure 4 in this report, Test2 to Figure 5 and Figure 6, Test3 to Figure 7 and Figure 8. In each directory you will find files with the following nomenclature MODV_S_R.xxx Where V denotes the MODTRAN version (4, 5 or 6), S is the stratification resolution (.15 or 1) and R is the angular resolution of the ray pattern (.2 or.2). The R is not present for test3 files. Finally, xxx denotes the file extention, which are:.emf Windows enhanced metafile containing the corresponding result graphic from this report..txt.tp5 Log file with failure rate and a list of zenith angles that produce a failure. Only available for Test1 and Test2. The tape5 MODTRAN file for the final run (path) of each field of view. To reproduce the errors, simply change the zenith look angle parameter to any logged into the.txt file. Material for the preliminary MODTRAN toolboxes As mentioned in section 5, the example code that demonstrates how to use MODTRAN with EOSPEC the new EOSPEC capabilities is also distributed as auxiliary material. The source code as well as a Visual Studio 28 solution can be found in directory API_EOSPEC_example_solution distributed along with this report. The solution should enable the example to built on any computer provided that: - EOSPEC (build 1373 or later) is installed and compiled in both Release and Debug configurations; - The EOSPEC_DIR environment variable is set to point to the root EOSPEC directory, and; - The PATH environment variable contains the EOSPEC binary directories (release and debug). 1 AEREX Report No AT6-1 version 1.

17 References/Bibliography... [1] V.Ross and D. Dion, Validation of MODTRAN 4 Output within the Marine Boundary Layer, DRDC report, October 22. [2] V.Ross and D. Dion, Validation of the MODTRAN 6 refracted geometry algorithms in the marine boundary layer, DRDC report, May 212. [3] J. L. Forand, The L(W)WKD Marine Boundary Layer Model - Version 7.9, Technical Report , Defence Research Establishment of Valcartier (DREV), Valcartier, Quebec, Canada (1999). AEREX Report No AT6-1 version 1. 11

18 Distribution list AEREX Report No AT6-1 version 1. Internal distribution Internal distribution Vincent Ross (AEREX) Paul Lacasse (AEREX) Denis Dion (DRDC) Total internal copies:3 Total copies: 3 12 AEREX Report No AT6-1 version 1.