NWS forecasters have the ability to make modifications to the three RGB components (Red, Green, Blue) in AWIPS. These modifications can be made easily by right-clicking (and hold) on the loaded RGB in the product legend, and selecting the “Composite Options” (Fig 1). Slider bars and numerical values for the max and min values, along with the Gamma, for each of the Red, Green, and Blue composition of the RGB will appear. The forecaster can adjust the numbers using the sliders or by editing the numbers, which will adjust the appearance of the RGB on the fly. It is recommended these adjustments be made on the fly as opposed to saving a procedure as desired thresholds will change depending on the time of day, season, and situation. The goal of the adjustments is to enhance the usefulness of the RGB by drawing out relevant features in certain situations.
There are occasions where making these modifications will extend or improve the usefulness of the RGB, particularly for RGBs which utilize VIS or NIR channels, and therefore depend on sunlight. The Day Cloud Phase Distinction (DCPD) RGB is useful for tracking cumulus cloud evolution from early water cloud (cyan), to convective initiation and glaciation (bright green), to mature convection (yellow). Because this process often takes place midday during the summer, the relatively low max thresholds of the reflectance/albedo components (79% for the 0.64 um vis green component, 59% for the 1.61 um snow/ice band blue component), are often exceeded, causing those components to wash out and provide no texture detail. Therefore, it is beneficial to increase the thresholds for those VIS/NIR fields to ensure saturation of the field will not occur. The exact thresholds to set will depend on the situation, including time of day, time of year, location, etc. These changes should only take around 15 seconds to make, and may need to be updated periodically during the day as the sun angle changes. A video from the 2019 Satellite Applications Workshop exemplifies the process of making adjustments to the DCPD RGB is such a situation.
A convective initiation example from 9 August 2019 over Colorado compares the DCPD RGB before (Fig 2) and after (Fig 3) a change to the RGB recipe was made. The forecaster was monitoring the scene for signs of imminent convective initiation, and made the changes to the RGB on the fly. A comparison of the values for the default RGB and modified version, along with the corresponding imagery at 2014Z, is included in Figure 4.
It is also beneficial to modify the DCPD RGB during low light situations, just after sunrise and before sunset, in an effort to extend its usefulness. During these times of day, features will be more difficult to discern due to low albedo from the green (0.64 um) and blue (1.6 um) components relative to the ranges set in the RGB. Therefore, adjusting the upper threshold downward for both will make cloud features apparent slightly earlier in the morning and later in the evening. If doing this in the morning, the user will need to be sure to return thresholds upward as albedo increases.
A common situation where such low light adjustments are helpful is for low cloud monitoring around sunrise or sunset. For example, early morning low clouds in the San Luis Valley of CO/NM on 12 August 2019 were difficult to visualize in the RGB with default settings (Fig 5). Adjusting the upper bounds downward for the VIS and NIR components, the fog (aqua colors) becomes brighter and easier to diagnose (Fig 6). The adjustments and direct comparison are shown in Figure 7.
Alternatively during low light situations, the Gamma values for the reflectance components in an RGB can be lowered in an effort to put more weight on them. An example of valley fog over PA/NY compares decreasing the Max values with decreasing the Gamma values for the VIS (green) and NIR (blue) components in the DCPD RGB (Fig 8). Both adjustment strategies similarly make the fog more apparent when compared to the default settings. Some combination of Max/Gamma reduction would also work to draw out the low level cloud features.
The VIS/IR Sandwich RGB similarly benefits from adjustments during low-light situations. Lowering the max threshold and gamma for each of the three components equally results in a brighter picture with clearer detail in convective cloud tops. An example of the changes made for a likely severe storm in southern Colorado during the evening of 13 September 2019 is shown in Figure 9. Features such as cumulus clouds, overshooting tops, and above anvil cirrus plumes are more easily and quickly detected in the modified/brighter imagery.
Forecasters across the northern US plains have found utility in the Day Snow-Fog RGB for identifying and tracking areas of blowing snow. This RGB includes two VIS/NIR components, and therefore its use can be extended further into the morning and evening by making simple adjustments (Fig 10). Carl Jones (FGF) and Andrew Ansorge (DMX) discussed this task at the 2019 Satellite Applications Workshop, and specifically mention the value of making on-the-fly adjustments to the RGB in an effort to make features clearer during low light situations.
Alternatively, there are also situations where these modifications may change the meaning of the colors in the RGB, and could lead to misinterpretation of important features. Additionally, making adjustments to an RGB and then sharing it with others who are not aware of the specific adjustments and interpretation of the new scheme can be dangerous. Therefore, forecasters making adjustments should be sure to understand how their changes are influencing the meaning of the final RGB, and should probably not save and share modified AWIPS RGBs with others. Rather, the process or practice of modifying an RGB to help in specific situations should be shared.
Bill Line, NWS