amsub_tb2uth

 AMSUB_TB2UTH calculates UTH from AMSU-B channel 18 Tb

 PURPOSE   The function  is based on the scaling method presented by
           Buehler and John (2005) using the relation:
           ln(UTH)= a +b*Tb
           where Tb is the brightness temperature at 183 +/- 1GHz

  IN
         tb18                      brightnesstemperature for channel 18        
         viewangle                 viewing angle (of nadir) under which the
                                   instruments measures (0 = nadir)
         icewater (optional flag)  Set 'w' for UTH with respece to water,
                                   'i' for UTH with respect to ice. If
                                   nothing is given, provide UTH with
                                   respect to water.

  OUT    uth                       calculated by the scaling method
                                   presented by Buehler and John (2005):


 USAGE
         uth=amsub_tb2uth(tb18,viewangle,icewater)

0001 function uth=amsub_tb2uth(tb18,viewangle,icewater)
0002 % AMSUB_TB2UTH calculates UTH from AMSU-B channel 18 Tb
0003 %
0004 % PURPOSE   The function  is based on the scaling method presented by
0005 %           Buehler and John (2005) using the relation:
0006 %           ln(UTH)= a +b*Tb
0007 %           where Tb is the brightness temperature at 183 +/- 1GHz
0008 %
0009 %  IN
0010 %         tb18                      brightnesstemperature for channel 18
0011 %         viewangle                 viewing angle (of nadir) under which the
0012 %                                   instruments measures (0 = nadir)
0013 %         icewater (optional flag)  Set 'w' for UTH with respece to water,
0014 %                                   'i' for UTH with respect to ice. If
0015 %                                   nothing is given, provide UTH with
0016 %                                   respect to water.
0017 %
0018 %  OUT    uth                       calculated by the scaling method
0019 %                                   presented by Buehler and John (2005):
0020 %
0021 %
0022 % USAGE
0023 %         uth=amsub_tb2uth(tb18,viewangle,icewater)
0024 
0025 % $Id: amsub_tb2uth.m 7584 2012-05-09 14:37:10Z seliasson $
0026 % Author: Mathias Milz and Salomon Eliasson
0027 
0028 errId = ['atmlab:' mfilename 'badInput'];
0029 assert(nargin>=2,errId,'At least 2 argumnents required!')
0030 assert(isequal(size(tb18),size(viewangle)),errId,...
0031     '''tb18'' and ''viewangle'' must be the same size!')
0032 if nargin < 3
0033     icewater='w';
0034 end
0035 
0036 
0037 % From Table 1 in Buehler et. al 2005
0038 
0039 % WATER
0040 aw=[16.474 16.472 16.476 16.479 16.479 16.483 16.488 16.490 16.496 ...
0041     16.501 16.503 16.514 16.527 16.540 16.552 16.561 16.572 16.585 ...
0042     16.599 16.612 16.628 16.649 16.665 16.681 16.709 16.740 16.766 ...
0043     16.789 16.806 16.842 16.874 16.907 16.932 16.972 17.003 17.036 ...
0044     17.063 17.105 17.156 17.201 17.252 17.308 17.375 17.439 17.501];
0045 
0046 bw=[-0.0702169 -0.0702106 -0.0702271 -0.0702456 -0.0702506 -0.0702774 ...
0047     -0.0703084 -0.0703243 -0.0703634 -0.0703988 -0.0704219 -0.0704853 ...
0048     -0.0705569 -0.0706315 -0.0707031 -0.0707656 -0.0708374 -0.0709191 ...
0049     -0.0710062 -0.0710919 -0.0711956 -0.0713153 -0.0714210 -0.0715289 ...
0050     -0.0716877 -0.0718609 -0.0720197 -0.0721669 -0.0722922 -0.0724969 ...
0051     -0.0726909 -0.0728922 -0.0730668 -0.0733017 -0.0735100 -0.0737274 ...
0052     -0.0739261 -0.0741909 -0.0745019 -0.0747932 -0.0751160 -0.0754690 ...
0053     -0.0758780 -0.0762869  -0.0766990];
0054 
0055 % ICE
0056 ai=[18.341 18.339 18.342 18.345 18.344 18.348 18.353 18.354 18.359 ...
0057     18.363 18.362 18.371 18.381 18.391 18.401 18.407 18.416 18.426 ...
0058     18.436 18.448 18.462 18.478 18.490 18.503 18.525 18.552 18.575 ...
0059     18.592 18.605 18.637 18.664 18.695 18.715 18.750 18.778 18.805 ...
0060     18.823 18.859 18.901 18.940 18.983 19.031 19.088 19.142 19.195];
0061 
0062 bi=[-0.0764737 -0.0764688 -0.0764834 -0.0764992 -0.0765034 -0.0765274 ...
0063     -0.0765550 -0.0765713 -0.0766039 -0.0766340 -0.0766454 -0.0766984 ...
0064     -0.0767557 -0.0768198 -0.0768812 -0.0769315 -0.0769950 -0.0770628 ...
0065     -0.0771351 -0.0772143 -0.0773052 -0.0774066 -0.0774960 -0.0775902 ...
0066     -0.0777226 -0.0778808 -0.0780199 -0.0781414 -0.0782481 -0.0784375 ...
0067     -0.0786102 -0.0787986 -0.0789501 -0.0791631 -0.0793542 -0.0795464 ...
0068     -0.0797062 -0.0799444 -0.0802151 -0.0804762 -0.0807632 -0.0810812 ...
0069     -0.0814447 -0.0818039 -0.0821763];
0070 
0071 % corresponding angles
0072 ang=[0.5500 1.6500 2.7500 3.8500 4.9500 6.0500 7.1500 8.2500 9.3500 ...
0073     10.4500 11.5500 12.6500 13.7500 14.8500 15.9500 17.0500 18.1500 ...
0074     19.2500 20.3500 21.4500 22.5500 23.6500 24.7500 25.8500 26.9500 ...
0075     28.0500 29.1500 30.2500 31.3500 32.4500 33.5500 34.6500 35.7500 ...
0076     36.8500 37.9500 39.0500 40.1500 41.2500 42.3500 43.4500 44.5500 ...
0077     45.6500 46.7500 47.8500 48.9500];
0078 
0079 
0080 uth = zeros(size(tb18));
0081 % LOOP over the viewing angles
0082 for k = 1:length(viewangle)
0083     % Find the CLOSET ANGLE in the lookup table to the angles given as input.
0084     % Pick closest_angle(1) since sometimes there are two answers. 1e-3 for
0085     % precision error
0086     closest_angle=find(abs(ang-viewangle(k)) - min(abs(ang-viewangle(k))) < 1e-3);
0087     if strcmp(icewater,'i')
0088         uth(k)  =  100.0 * exp( ai(closest_angle(1)) + bi(closest_angle(1)) * tb18(k) );
0089     end
0090     if strcmp(icewater,'w')
0091         uth(k)  =  100.0 * exp(aw(closest_angle(1)) + bw(closest_angle(1)) * tb18(k) );
0092     end
0093 end
0094 
0095 end