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Day Cloud Type RGB – A Tool for Monitoring Convective Initiation

Posted by Carl Jones on 11/07/2022
Posted in: Convection, G16-CH04_1.37_NIR-Cirrus, RGB, Uncategorized. Tagged: , , , , , , , . 2 Comments

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.

Use the conceptual model with the image slider above. Each letter corresponds to the conceptual understanding of how much insolation is being absorbed and reflected based on the associated cloud top height. Image slider: left / right

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.

Image slider showing the Day Cloud Phase Distinction RGB and Day Cloud Type RGB, and their individual compositional contributions in black to white (white representing a higher contribution; black, lower contribution). Clockwise from the upper left: RGB, Red component, Blue component, and Green component. Image slider: left / right

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.

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.

Carl Jones (NWS Grand Forks)

Northern Plains Cool Season Convection: March 2020

Posted by Carl Jones on 03/09/2020
Posted in: Uncategorized. Tagged: , , , , , , , , , , . Leave a comment

March 2020 started off rather windy and unstable in the Northern Plains leading to convection during a month that still sees predominately wintertime stratiform precipitation. There were several instances of showers producing lightning, graupel/snow, and convectively forced severe wind gusts in addition to synoptically driven high winds. Designating convective cells that may be more impactful than others in the cool season (late fall, winter, and early spring) can be difficult using remote sensing instruments like satellite and radar due to their shallow heights and relatively warm cloud tops. Satellite appearance of shallow convection tends to be unimpressive in the infrared using default color schemes made for taller, colder cloud topped warm season convection. Also, due to the shallow depth of the cloud layer, radar can easily overshoot this layer composed of what tends to be marginal returns. Lastly, lightning activity tends to be much lower than that in the warm season, if present at all.

But there are some ways to make subtle features of shallow convection more easily tracked by adjusting the satellite display. Let’s explore..

Figure 1: 04 Mar 2020 GOES-East Day Snow-Fog RGB. Full res

First let’s take a look at an event on March 4, 2020, that produced several convectively induced high wind gusts at times exceeding 75 mph. The Day Snow-Fog RGB has proven useful in monitoring wintertime precipitation, mainly because of its use of near-infrared bands to help delineate snow cover, bare ground, and cloud top microphysical composition. It is also well known that using RGB’s capable of tracking cloud features while color sorting aforementioned properties (looking at you Day Cloud Phase Distinction RGB) can be quite useful for tracking forcing mechanisms, areas of potential convective initiation, and stage in convective life cycle.

Figure 1 shows convective clouds moving through eastern Montana into the Dakotas and northern Wyoming. Convection is already ongoing in the start of the loop with a line of showers attached to a cold front extending from near Great Falls to Glasgow, Montana. The cold front can be tracked on satellite propagating south and east undercutting additional convection in the “warm sector” ahead of the front. Ahead of the front are additional clusters of growing cumulus clouds as the convective temperature was reached, aided by ample insolation over a snow free surface beneath a relatively cool column of air (focus of cloud growth probably had some influence from differential heating and convergence augmented by variations in terrain). These cumulus clouds initially exhibit a highly textured, bubble-like appearance with whiter coloring indicating non-glaciated clouds. They very quickly become bright pink while maintaining their texture signaling glaciation of clouds through ascent. Finally they turn from cumulonimbus to stratocirrus losing texture, acquire more purple coloring, and move west to east in broader upper flow as parcels detrain from their initial paths of low level ascent, thus indicating the majority of additional ascent is likely over. The ability to track these mesoscale forcing mechanisms and stages in storm growth can be important to forecasters as this may give lead time to potential initiation or advection of impactful convection, as well as convective decay.

Figure 2: 04 Mar 2020 4 Panel: Top Left: GOES-East Day Cloud Phase Distinction RGB (adjusted red component)+ surface observations + severe thunderstorm warnings. Top Right: GOES-East Band 13 (adjusted range and color scheme). Bottom Left: GOES-East GLM (adjusted range) + severe thunderstorm warning + significant weather advisories. Bottom Right: MRMS isothermal reflectivity at -20 C. Full res.

Now let’s focus on the tristate region of the Dakotas and Montana, an area where several severe wind gusts were measured. Using Rapid City’s observed sounding as representative of the environment, this area was primed to transfer not only cloud layer winds of around 50 kt towards the surface, but any evaporation of descending precipitation cores may help accelerate these winds from aloft towards the surface (DCAPE ~700 J/kg, inverted V sounding).

We can utilize procedures meant for tracking severe convection in the warm season while making some minor adjustments to help gear towards a cool season environment. Figure 2 is an example of one such procedure. The range of the Day Cloud Phase Distinction RGB’s red component (10.3 um, Band 13, “Clean IR”) was edited from [Min: 7.5, Max: -53.5] to [Min: -10, Max -70] in an effort to desaturate red coloring by shifting this thermal sensing band towards the colder, better representing the temperatures typical in the cool season. Convective evolution is represented by a transition of colors from cyan to green to yellow/red as clouds glaciate and then cool. The inclusion of the visible spectrum within the Day Cloud Phase Distinction RGB also gave clues on storm development featuring more turbulent cloud texture near the upshear side of continually developing anvils, indicative of sustained updrafts reaching their equilibrium level. A separate, standalone look at Band 13 offered a way to track coldest cloud tops, although adjustments were also made here shortening the range from [Min: 55.0, Max: -109.0] to [Min: 0, Max -70] while making all values warmer than -25 C transparent. This allowed robust convection to be more easily tracked in an area of poor radar coverage with more red colored tops corresponding to relatively taller, stronger updrafts. One particular cluster of showers moved through Glendive, MT, with its AWOS measuring a 47 kt (54 mph) wind gust 18:56 UTC followed by Dickinson, ND, reporting 66 kt (76 mph) wind gust 19:56 UTC. This cluster exhibited relatively rapid cloud top cooling around 40 minutes prior to reaching Glendive and another prior to reaching Dickinson. Lastly, lightning activity can be analogous to updraft strength. Adjustments to the GLM FED appearance were made to shorten the default range of 1-260 to 1-20 helping draw out meager lightning rates. While overall lightning activity was low (as can be typical in cool season convection in the Northern Plains), this adjustment did help bring attention to cells that had relatively high activity compared to neighboring cells. One cell that stands out emanates from the same cluster that moved through Glendive and Dickinson but on its southern flank. Looking at MRMS isothermal reflectivity at -20C, the cell holding highest lightning activity has reflectivity exceeding 40 dBZ. All of these signals, upstream reported severe wind gusts, and our knowledge of the thermodynamic and kinematic environment can steer the forecaster into thinking this particular cell is strong and chances of producing severe downdraft wind gusts are high. This prompted NWS Bismarck’s to issue its earliest severe thunderstorm warning with an eventual 81 mph wind gust being reported at a mesonet site in Hettinger County, North Dakota.

Honorable mentions:

Figure 3: 03 Mar 2020 4 Panel: Top Left: GOES-East Nighttime Microphysics RGB. Top Right: GOES-East Band 13. Bottom Left: GOES-East GLM (adjusted range) + ENTLN lightning products. Bottom Right: MRMS isothermal reflectivity at -20 C. Full res.

A lightning-producing cell precipitating rain, snow, and graupel formed west of Bismarck, North Dakota, moving southeast into northeast South Dakota, during the evening of March 3, 2020. Using a similar procedure as displayed before, Figure 3 shows relatively rapid cooling of an expanding cloud top/anvil as noted in Band 13 (default color scheme shown). Values exceeding 35 dBZ in MRMS isothermal reflectivity at -20 C correlated nicely with the appearance of sensed lightning by GLM and ground based networks.

Figure 4: 03 Mar 2020 GOES-East Day Snow-Fog RGB (adjusted red and green components) + 0.5 degree reflectivity. Full res.

A graupel/snow depositing shower moved through Fargo, North Dakota, near sunrise March 3, 2020. This along with other showers left a trail of accumulated graupel/snow apparent on the Day Snow-Fog RGB (Figure 4). This is seen as more orange shaded streaks atop the background darker red coloring of the older snowpack. The shower moved through the Fargo-Moorhead metro right as sunlight was becoming available. Thus, it was beneficial to adjust the near infrared components of the RGB to account for little visible light to work with. This helped brighten the image revealing the metro was right on the edge of new accumulation (which lined up nicely with radar).

Figure 5: 03 Mar 2020 GOES-East Day Snow-Fog RGB (adjusted red and green components). Full res.

Later in the day, clouds peeled away revealing additional showers had deposited new frozen precip accumulation throughout eastern North Dakota into northwest Minnesota (Figure 5).

The main takeaway of this post is to encourage forecasters to become more comfortable with adjusting satellite products to help draw out convective features. Such adjustments may be especially necessary during the cool season. That’s not to say just blindly make adjustments, rather forecasters need to understand what is being adjusted and how that would affect the appearance and meaning of edited products. After determining an environment is possible for impactful convection, forecasters should think about adjusting satellite display to make subtle convective features typical in the cool season more easily tracked.

Carl Jones, NWS Grand Forks, ND