Can the Day Cloud Type RGB (1.37, 0.64, 1.61) be a useful RGB when monitoring for convective initiation? This blog post will attempt to share some observations regarding the Day Cloud Type RGB when monitoring for convective development, particularly in the transition from the developing to mature stages of the convective lifecycle.

Firstly, let’s talk about the components that make up the Day Cloud Type RGB. In your best Michael Buffer voice: “In the red corner, we have the 1.37 µm channel; the green corner, we have the 0.64 µm channel; and in the blue corner is the 1.61 µm channel.” Let’s focus on the 1.37 µm channel, and what it is actually measuring. Onboard NOAA’s GOES-R series of geostationary satellites, the 1.37 µm channel corresponds to Band 4 within the Advanced Baseline Imager (ABI), with a given name of the ‘Cirrus Band.’ Per the CIMSS Day Cloud Type RGB quick guide, compared to the Day Cloud Phase Distinction RGB, “The use of the Band 4 ‘Cirrus Channel’ at 1.38 µm (rather than the 10.3 µm Clean Window Channel) allows for better detection and discrimination of thin and thick cirrus clouds.” Additionally, one of the RGB’s composers, Andrew Heidinger of NESDIS (other composer: Jochen Kerkmann from EUMETSAT), provided insights on the original intent of the RGB to aid in viewing thin cirrus, and can be viewed here.

However (spoiler alert), the ‘Cirrus Band’ can observe clouds besides cirrus. Is your mind blown? Don’t worry, mine was, too, after discovering this fact.

Rather, reflectance back to ABI within this channel is dependent upon water vapor content, and to an extent how it is vertically arranged within the atmosphere. The 1.37 µm channel sits within an atmospheric water vapor absorption region of the electromagnetic spectrum. This means given a sufficient amount of water vapor, incoming solar radiation in the 1.37 µm wavelength travelling into the atmosphere will get absorbed by the atmosphere rather than reflected back to space (and satellite). Since most of the atmosphere’s water vapor is within the lowest portions of the troposphere, there is still opportunity for clouds/aerosols above the absorbing amount of water vapor to reflect solar radiation back toward the satellite. Typically these reflective clouds are high enough in altitude to fall within the “cirrus” category, and hence the given “Cirrus Band” title. But should the atmosphere *not* hold a sufficient amount of water vapor to absorb radiation, reflectance from non-cirrus clouds, as well as other meteorological and non-meteorological features, can be detected.

The other two components of the Day Cloud Type RGB, the 0.64 µm channel and the 1.61 µm, are great for monitoring and tracking cloud motion and texture as well as distinguishing cloud particle phase at the top of clouds.

So how does all of this pertain to monitoring convection? Imagine a very unstable environment ripe for deep convection. It may be comprised of rich moisture within the boundary layer underneath very dry air aloft. So while any clouds within this lowest level moisture may not be reflective in the 1.37 µm channel, once clouds vertically grow into the dry layer aloft, it becomes reflective. In this example, the satellite sensor would only see reflectance from clouds growing above the moist boundary layer, such as the case when Cu “breaks the cap” into a relatively dry portion of the atmosphere, such as an elevated mixed layer.

With its inclusion of the 1.37 µm channel, the Day Cloud Type RGB can be an excellent tool to monitor when clouds grow vertically high enough, dependent upon moisture content and its orientation within the troposphere. Couple this with the power of cloud top phase information from the 1.61 µm and general cloud motion and texture information from the 0.64 µm, and you have a tool for monitoring the convective lifecycle.

“But wait, why would I use this RGB when I have the Day Cloud Phase Distinction RGB which that has a proven track record of utility in monitoring the convective lifecycle?” one might ask themselves.

It is true, the Day Cloud Phase Distinction RGB does have this utility, especially when monitoring for convective initiation, or during the transition from the developing to mature stage of the convective lifecycle. However, the Day Cloud Type RGB is starting to show some striking contrast right at the moment of this transition, at least in some cases. Let’s look at an example to compare the two RGBs at time of convective initiation.

In this example near the Tennessee and Kentucky border on September 25, 2022, a cold front forced sustained convection to initiate (here is the associated SPC Severe Weather Event Review from this day). The Day Cloud Phase Distinction RGB is on the top, and Day Cloud Type RGB on the bottom. Both RGBs show convective initiation, but the Day Cloud Type RGB really highlights these vertically growing clouds with its red/orange color starkly contrasted to neighboring shallower cyan clouds. This contrast makes it easy to see to moment of convective initiation. And in this case, the Day Cloud Type RGB can more clearly denote this transition compared to the Day Cloud Phase Distinction RGB where its contrast may be harder to distinguish the vertically growing and glaciating clouds turning green compared to a blue background and cyan clouds.

The enhanced contrast within Day Cloud Type RGB compared to the Day Cloud Phase Distinction RGB in this instance is simply due the relatively shallow nature of convection within this environment not acquiring cold enough cloud top temps for the 10.3 µm channel to add a significant contribution within the red component of the Day Cloud Phase Distinction RGB. Conversely, the moisture profile of this environment allowed for a very large contribution of the Day Cloud Type RGB’s red component, the 1.37 µm channel. The contributions of each RGB component is represented in the image above, with each quadrant representing the RGB itself and the channels that make up its components in black and white, with white representing higher contribution. While the blue and green components are nearly identical between these two RGBs, the red component in the upper right quadrant of the image above differs substantially. The 10.3 µm channel in this configuration of the Day Cloud Phase Distinction RGB is not very bright, representing a lack of contribution to the overall RGB (note that one could adjust the range to make it stand out more, however). Again, the converse is illustrated in the 1.37 µm of the Day Cloud Type RGB.

It is important to emphasize the dependency of moisture content and orientation within the troposphere when considering the Day Cloud Type RGB for monitoring the lifecycle of deep convection; and in this case, it just happened to work out nicely. If you look at Nashville’s 18 UTC RAOB within the warm sector just ahead of this convective line (image above), you can see the vast majority of moisture resides below ~600 mb, with a very dry layer aloft. If we associate this ~600 mb layer as the demarcation of sufficiently absorbing water vapor content in the 1.37 µm, the observed environmental temperature at this layer infers cloud tops would only begin to reach the point of glaciation around -10 C. This makes sense as to why we didn’t see a transition of green to red in the Day Cloud Type RGB as the 1.37 µm reflectance likely increased substantially just as cloud top phase was changing, aka glaciating.

Having an RGB to easily diagnose the point of convective initiation can be a powerful tool within a mesoanalyst’s toolbelt. As a mesoanalyst, knowing this point of initiation can help provide confidence when telling the warning met/team, “These clouds are likely initiating, and these are the likely hazards given the near storm environment (blurb on hazards). I suggest you begin radar and storm-scale interrogation in this area.” Additionally as a mesoanalyst, knowing this point can help those involved in messaging, graphic generation, as well as IDSS to locate near-term hazards both in time and space.

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So let’s briefly mention some potential strengths and weakness of using the Day Cloud Type RGB for monitoring the transition of developing to mature stages of the convective lifecycle:

Strengths:

• Can easily illustrate convective initiation where moisture content is mostly confined within the boundary layer beneath very dry layer aloft.
• Won’t succumb to seasonal thermal differences (like the 10.3 µm channel), outside of seasonal and latitudinal differences in moisture content and solar reflectance.

Weaknesses:

• Environments composed of deep moisture throughout the troposphere (like the tropics), may see a delayed signal (the appearance of orange/red coloring) beyond ‘true’ convective initiation.
• Environments composed of very dry air throughout the troposphere (typical in arid, very cold, and/or high elevation climates), early-lifecycle clouds will likely have strong orange/red coloring, even before ‘true’ point of initiation. This may mislead a forecaster to incorrectly signal a transition from infant to maturing clouds.
• Due to dependence of solar radiation, can only be used in daytime.
• Limited use when higher level clouds/aerosols are upstream or above area of interest.

It cannot be stressed enough that having knowledge of the moisture profile in the area of interest is crucial to applying this technique. Knowing how moisture is parsed throughout the troposphere will aid your understanding of when the 1.37 µm component will begin contributing to the RGB.

It is also worth mentioning the Day Cloud Type RGB’s lack of cloud top temperature information. This information can give convective lifecycle details including rate of growth of convection. If such information is desired, it is recommended to utilize the Day Cloud Phase Distinction RGB, or utilize the single band 10.3 µm.

This type of application of the Day Cloud Type RGB is still in its infancy, with much of the content within this blog post still under review by peers within the satellite applications community. Therefore, proceed with caution. That being said, if you happen to notice any cases where this application either excels or fails, please reach out to the author and/or comment within this post.

Many thanks to Patrick Ayd (NWS DLH) for sparking interest in utilizing the 1.37 µm for convective monitoring, Andrew Heidinger (NESDIS) and Jochen Kerkmann from (EUMETSAT) for composing the Day Cloud Type RGB, and the TOWR-S team for including the Day Cloud Type RGB within AWIPS. Additionally, many thanks to all who have contributed to conversations regarding this potential application.

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Carl Jones (NWS Grand Forks)