Simulation of HIRS observations from the nature run.

The file format for the HIRS nature run is that of the NESDIS level 1B (Kidwell). Since HIRS level 1B data for NOAA 11 and 12 are available for the period of the nature run, these were acquired. This step minimized the level of effort required to define and fill the 1B files. The only values that needed to be changed within the files themselves are the instrument counts.

The nature run was provided at six hour intervals. It was decided to process the satellite data in blocks of plus and minus three hours about the nature run times. The level 1B files are based upon orbital node times, and do not correspond to synoptic times. Thus those 1B files that covered two nature run intervals were split into two files. For example, a 1B file named

NSS.HIRS.NH.D93044.S0752.E0938.B2261819.WI begins at 0752 and ends 0938 UTC. This is broken apart into two output files:

NSS.HIRS.NH.D93044.S0752.E0900.B2261819.WI

NSS.HIRS.NH.D93044.S0900.E0938.B2261819.WI,

the first of which uses data from the nature run day 93044 0600UTC, and the second from 1200UTC.

The basic radiative transfer code is RTTOV version 5 (Saunders et al., 1998). This code can produce both clear and cloudy radiances, and permits user specified emissivities. This code is used because it has different design philosophy and uses different spectroscopy than the operational EMC code, which is based upon OPTRAN (McMillin et al., 1995).

The level 1B files were processed one record at a time, with the three calibration records in each superswath copied to the output file. Each record contains information for one scan line, which is 56 observations of 20 channels (19 infrared and one visible). The visible data for each spot are copied untouched to the output record. For each spot the latitude and longitude are extracted. The nature run temperature, mixing ratio, pressure profile, cloud amount, surface wind speed and surface skin temperature are interpolated to the spot location. The historical ozone data was provided by Mel Gelman of NCEP/CPC, and these are also interpolated to the spot location.

The RTTOV code does not permit multiple cloud layers, so the following logic was used to simulate clouds. A maximum of two cloud layers is permitted. The possible options are: clear, completely cloudy with one layer, partly cloudy with one or two cloud layers. Maximum overlap is assumed for the case of two cloud layers. First the nature run cloud profile was scanned to determine if the spot was completely cloudy, which is defined here as a cloud fraction threshold greater than 0.8 . Otherwise the cloud profile was scanned to see if the spot was clear, which is defined as a cloud fraction of less than 0.2 . If these two tests fail, then a search is made for the two topmost local maxima in cloud amounts. Since RTTOV returns both clear and cloudy radiances, a maximum of two calls is required to satisfy these conditions. The cloudy and clear radiances are then combined using a sum weighted by the relative cloud amounts.

The surface emissivity is determined using a scheme from Masuda et al. (1988). This method gives ocean IR emissivity as a function of wind speed and zenith angle. No good source of land emissivity is currently available, so after some discussion with J. Derber, it was decided that this method would be used over both ocean and land. This action is subject to review.

Instrument noise was characterized by collecting statistics from space calibration looks for one day in the middle of the nature run. Correlated gaussian noise was generated by the method of Searle, and added to the computed radiances. Finally the radiances were converted to instrument counts using the 1B calibration coefficients that correspond to that scan line.

Simulation of MSU observations from the nature run.

The methodology for simulating MSU observations is identical to that of the HIRS, with the exception of surface emissivity. Here the microwave surface emissivity model that is included with RTTOV is used (English and Hewison, 1998).

References:

English S. and Hewison T.J. 1998 A fast generic millimetre-wave

emissivity model. SPIE proc.

Kidwell, K. The Polar Orbiter Users Guide, http://www2.ncdc.noaa.gov:80/docs/podug/

Masuda, K., T. Takashima, and Y. Takayama, 1988: Emissivity of pure and sea waters for the model sea surface in the infrared window regions. Remote Sens. Env. 24:313-329.

McMillin, L. M., L. Crone and T. J. Kleespies, 1995: Atmospheric transmittance of an absorbing gas. 5: Improvements to the OPTRAN approach. Appl. Opt. 34 (36) 1995.

Saunders, R.W., Matricardi M and Brunel P, 1998: An improved

radiative transfer model for assimilation of satellite

radiances QJRMS 124.

Searle, 1982: Matrix Algebra Useful for Statistics, Wiley&Sons, Ch 13.