While there are many, many, many examples of RGB’s excelling at diagnosing the early stages of convection during the daytime, there seems to be a lack of examples showcasing similar RGB use during the nighttime. This is likely due to the absence of solar reflectance, particularly in the near-infrared. Reflectance within this wavelength spectrum aids in monitoring the stages of early convective growth through easy detection of cloud top glaciation. Such reflectance gives RGB’s like the Day Cloud Phase Distinction its superiority in monitoring the convective lifecycle. But is there an RGB that can perform a similar application without solar reflectance, i.e. at night? This post will attempt to shed light [get it?] on how the Nighttime Microphysics RGB can be utilized in anticipating convective initiation.

On the night of August 18, 2020, forecasters at the National Weather Service in Grand Forks monitored the potential for overnight thunderstorm development, but were unsure if forcing would be sufficient enough to overcome strong capping over the area. There was anticipation of a low level jet to develop somewhere over eastern North Dakota into northwest Minnesota serving as a potential spark to ignite convection through the capping inversion. While questions remained on where exactly this would happen, focus was given to the mesoanalyst role to monitor for this potential.

At around 12:30 am CDT, the Nighttime Microphysics RGB easily picked up on low level stratus developing northeastward over the northern Red River Valley in far northwest Minnesota. This stratus stood out from other nearby clouds with its telltale pale cyan color compared to higher level dark blue and red cousins to the east and south.

The pale cyan color is a result of increased values within the green gun, the Night Fog difference product (10.3 – 3.9 um), as well as a slight dimming in the blue gun, the longwave infrared band (LIR; 10.3 um), while no information was added from the red gun, the Split Window Difference (SWD; 10.3-12.3 um). These were all signs of low level stratus, mainly through the increased values within the Night Fog product indicative of clouds made of up water droplets. While it drew the attention of the forecasters, questions still remained: Does this low stratus represent the seeds to convection? Or is this simply a benign cloud feature?

Over the next hour, characteristics of this cloud feature changed. The low stratus changed from its uniform pale cyan color and ameba-like structure, growing dark red specks that slowly veered and expanded east-southeast.

The change in color is a result of increased values in the red gun (SWD), sharply decreasing values in the green gun (Night Fog product), and further decreasing values in the blue gun (LIR). This indicated parts of the stratus cloud were starting to glaciate as suggested by the sharp decrease in values from the Night Fog product, continued cooling in the LIR, and increasing difference between the “clean” and “dirty” LIR channels (SWD).

The awareness and knowledge of the subtle change in cloud characteristics as illustrated by the Nighttime Microphysics RGB was crucial in realizing the stratus cloud was continuing to grow one or more updrafts that were beginning to glaciate. This is analogous to the Day Cloud Phase Distinction RGB revealing glaciation of water comprised cumulus, a threshold designating convective initiation.

So we proved that the Nighttime Microphysics RGB can be used to assess convective initiation, but we already have other satellite tools to do this for us, particularly the 10.3 um LIR channel. This single LIR band has a long standing legacy as a useful tool in monitoring convective activity at night. But can this application of the Nighttime Microphysics RGB provide additional lead time towards convective initiation compared to monitoring cloud top temperatures on the 10.3 um LIR channel?

The animation above is a time matched side by side comparison of the Nighttime Microphysics RGB and LIR band. In this case, the easily definable signal of stratus becoming glaciated within the RGB gave around 30 minutes to 1 hour of additional lead time in raising awareness toward potential convective initiation compared to typical LIR if using -24 C as a threshold (standard color curve for LIR in AWIPS turns blue at -24 C). This additional lead time allowed forecasters to feel better prepared in messaging and internal warning operations (better preparation =  less surprises and more confidence in warning/no-warning designation).

While the Nighttime Microphysics RGB can provide crucial information of pre-CI development, it lacks valuable cloud top information after CI, an area where LIR still reigns supreme. So why not have both?

The animation above takes the best of both products by overlaying an adjusted LIR color table on top of the Nighttime Microphysics RGB. Simply make values lower than -24 C transparent within the LIR color table and keep it above RGB in the hierarchy of display within AWIPS. The LIR’s bright colors of ongoing convection probably stands out the most displaying details like a sprawling anvil, overshooting top, and warm trench indicative of an above anvil cirrus plume. But the RGB’s input in this same image can help focus attention west and north of ongoing convection. Notice the tight packs of small , discrete but glaciating cells as shown by a reddening color? This should raise awareness towards the potential of additional convection soon to initiate.

This Nighttime Microphysics RGB – LIR “sandwich” yields information that is helpful in both the pre-CI and post-CI environment. The remainder of the loop showed that many cells matured into robust convection. And while not all of these glaciating cells went on to become mature storms (notice the orphan anvils?), it still signaled the potential of additional development outside of ongoing convection. This knowledge directly led to refined messaging of severe threats for targeted locations, bridging the gap between outlook and warning phases.

For those wondering what hazards this event brought: hail. Numerous reports of large hail up to the size of golf balls fell during the early morning hours of August 19, 2020, within the central Red River Valley into northwest Minnesota. More environmental information can be found via SPC’s Event Archive.

Carl Jones
Meteorologist
NWS Grand Forks