The quality of the AIRS Level 2 temperature profiles TAirSup (100 level support product) and TAirStd (28 level standard product) is described by the PBest and PGood parameters which are in units of atmospheric pressure [hPa]. Any given temperature profile is ``best'' (equivalent to quality = 0 for other parameters) from the top of the atmosphere down to PBest. The profile is ``good'' (equivalent to quality = 1 for other parameters) from PBest down to PGood and ``do not use'' (equivalent to quality = 2 for other parameters) at altitudes below PGood. When PGood is set to 0 the entire temperature profile is rejected and when PBest or PGood equals the surface pressure the temperature profile is of ``best'' or ``good'' quality all the way down to the surface.
The ``AIRS/AMSU/HSB Version 5 Level 2 Quality Control and Error Estimation1'' document describes PBest, PGood, nBestStd, and nGoodStd as follows:
- The TAirStd temperature profile from the top of the atmosphere to this pressure level is of best quality (i.e., quality = 0).
- The TAirStd temperature profile below the level of PBest down to this pressure level is of good quality (i.e., quality = 1). The temperature profile below the level of PGood is assigned quality = 2 (do not use). Note that PBest and PGood may be identical. In this case the temperature profile for altitudes above and including that pressure level is assigned quality = 0 whereas the temperature profile for altitudes below that pressure level is assigned quality = 2.
- The index of the lowest altitude level of the pressStd and TAirStd profiles for which the quality is ``best'' (or ``good''). Levels whose indices are in the range of nBestStd (nGoodStd) to 28 are therefore marked quality = 0 (1). It is set to a value of 29 to indicate that none are ``best'' (``good''). Take note that nBestStd is 1-based (as are arrays in FORTRAN and MATLAB) rather than 0-based (as are arrays in C and IDL). Note that the standard pressures are ordered from the surface up.
Since PBest and PGood are actually derived from the 100 level support product the standard pressures pressStd at nBestStd and nGoodStd may not correspond exactly with PBest and PGood. The 100 level support pressures pressSupp at nBestSup and nGoodSup are exactly equal to PBest and PGood for the cases when PBest and PGood are not equal to 0 or PSurfStd (the surface pressure). The nBestSup and nGoodSup parameters are defined as follows:
- Support level index of highest pressure (i.e. lowest altitude) for which Quality = 0 (1). A value of 0 indicates that no part of the profile passes the test. Note that the support pressures are ordered from the top of the atmosphere down.
For any given temperature profile one of three sets of error estimate thresholds is used to calculate PBest depending on the surface type (i.e., land, sea or ice) and surface temperature error estimate.
Specifically each profile will fall under one of the following 3 cases:
- Non-frozen ocean with liquid water covering 99% of FOV (MWSurfClass = 2) with surface error estimate 1K
- Non-frozen ocean with surface error estimate 1K
- Land, coast, or ice
The error estimate thresholds for any given profile are defined by values at 70 mbar, PSurfStd/2 (representative of the pressure at the middle of the atmosphere), and PSurfStd, interpolated linearly in pressure. The values for the three case are given in Table 1.
Table 1: Error Thresholds for PBest
|Case ||TAirStdErr ||TAirStdErr ||TAirStdErr |
| ||at 70mbar ||at PSurfStd/2 ||at PSurfstd |
|a ||1.75 K ||1.25 K ||1.75 K |
|b ||2.25 K ||1.75 K ||2.75 K |
|c ||2.25 K ||2.0 K ||2.0 K |
A pressure level PBest is then set by comparing the temperature quality profiles with the level-by-level error estimates (at 100 levels) beginning downward from 70 mbar. If the error estimate exceeds the error thresholds for three consecutive levels, PBest is set to the level above the first of those three levels.
Over ocean, PGood is always set equal to PBest. Over land or ice, if PBest is above 300 hPa (in altitude), PGood is set equal to PBest; if PBest is below 300 hPa (in altitude) PGood is set equal to PSurfStd.
The four profiles labeled in Figure 1 provide examples of the three cases listed in Table 1. Figure 2 show how the error estimates are used to arrive at PBest and PGood. The map of PBest in Figure 3 shows that over thick clouds PBest is very high in the atmosphere while it is much closer to the surface over other regions. Also, the difference between PGood and PBest shows that over most regions PGood equals PBest but that they tend to differ over coastlines.
Figure 1: The left panel shows a visible image of the Gulf of Mexico and surrounding region for Granule 192 on September 6, 2002. The right panel shows TSurfStdErr the error estimate for the surface air temperature.
Figure 2: The error estimates are displayed for the 4 regions labeled in Figure 1 of granule 192 on September 6, 2002. The solid circle at the bottom indicates the surface temperature error estimate. Location 1 is an example of case c from Table 1, location 2 is an example of case b, and locations 3 and 4 are examples of case a.
Figure 3: The left panel shows a map of PBest and the right panels shows a map of PGood both are in hectoPascals. The two values mainly differ over coastlines.
- ... Estimation1
- see http://disc.sci.gsfc.nasa.gov/AIRS/documentation