The day cloud phase distinction RB has been discussed as a useful tool for monitoring the cumulus cloud field leading up to and including convective initiation. Pure liquid cumulus clouds appear as cyan in the RGB because they have relatively high reflectance in the 0.64 um (high green) and 1.6 um (high blue) components, but are warm in the IR (low red). As convection begins to initiate and ice develops in the cloud top, the blue component decreases since ice does not reflect as well in the 1.6 um band (low blue), but the green component (0.64 um) stays the same or increases (high green). The red component begins to increase as the temperature of the clouds decreases (mid red). Therefore, for convective initiation, you are left with a transition from cyan to green.

In the 01 May case, you can see a transition from cyan to green in areas along the boundaries, indicating that convective initiation is imminent or occurring. There are quite a few orphan anvils in this case, indicating failed initiation but that the CAP is likely close to breaking. While these features are all apparent in the VIS alone as well, the color detail added to the imagery as a result of the combination of multiple channels makes these features and trends easier to diagnose. The high, 0.5 km, resolution available from the 0.64 um VIS is maintained in the RGB in AWIPS.

Once the convection matures, the color transitions from green to close to yellow since the convection is still highly reflective in the 0.64 um channel (high green), and is also now cold (high red), but still not as reflective in the 1.6 um channel (low green) due to the presence of ice.

Just as convection begins to initiate, attached are the three components to the RGB, plus the RGB, at 1721 UTC

It should be noted that with this case, the RGB was modified slightly. The 0.64 um (green) component had its max increased to 100% (from 78%) to account for the higher reflectance of cumulus clouds and mature convection during the middle of the day. This change is often necessary when using this RGB to monitor for convective initiation, and can be made easily in the “composite options” setting of the product. Without the change, the green component saturates quickly, leading to a loss of storm top detail in the imagery provided by the high resolution 0.64 um channel.

Bill Line, NWS