Channel Selection

Simulated ice water path (IWP) and particle size (Dme) retrievals were generated using a variety of submm channelcombinations. The three base channels are: 22.0, 28.2, 45.3 cm-1. To these we added, individually, 4th channel candidates: 15.15, 22.3, 28.2, 49.9, 66.1, 84.45, 115.0 cm-1.

The following 2 figures show the IWP error as a function of cloud base height. Since the normal CIRRUS operation will include IR from the GOES, these errors from the retrievals also include the IR channels (832, 937, 1480 cm-1).

 

Fig. 1. Tropical retrievals. Cloud base height vs IWP median error

 

Figure 2. Mid-latitude retrievals. Cloud base height vs IWP median error.

 

For the tropics (Fig. 1) the 115 cm-1 results in the lowest IWP error for cloud bases above 10km. Note, as lower wavenumber channels are substituted, the error increases.

In the mid-latitude winter case (Fig. 2), the addition of the 15.15 gives the lowest error. The improvement over the other channels appears to be less than dB.

 

No specific channel combination seems significantly better than any other when the IR is included.


The next 2 figures show the same errors as above but only for some select channels without the IR. The errors resulting from using IR only is also shown.

For the tropics (Fig. 3) the 115 results in the lowest errors for the highest level cloud bases. For cloud bases 12-14 km high, there is no significant difference in the errors between the various channel combinations.

In the mid-latitudes (Fig. 4) the 15.15 channel reduces the error for cloud bases in the 5-10km range. This is the most notable improvement in terms of the amount the retrieved IWP error is reduced over a large range of cloud base heights.

 

Figure 3. Tropical retrievals. Cloud base height vs IWP error for selected channel combinations without IR and also error for IR only.

  

 

Figure 4. Mid-latitude retrievals. Cloud base height vs IWP error for selected channel combinations without IR and also error for IR only