Cloud-top Structure of Northeast Colorado Thunderstorms May 24, 2005
Published: May 31, 2005
In the early afternoon of 24 May 2005, thunderstorms formed in northeast Colorado ahead of a weak shortwave trough embedded in moderate westerly upper-level flow. Surface dew points were mostly in the 50's F, with slighly higher values near the eastern border. A number of storms showed supercellular characteristics, but were embedded within a larger convective mass. Severe storm reports included high wind, a number of large hail reports, and several tornadoes.
Satellite observations of these storms include GOES-East and GOES-West, in addition to a MODIS (Aqua) pass around 2010 UTC. This allows the comparison of thunderstorm top structure between GOES and MODIS. GOES-12 was in Rapid Scan Operation (RSO), so approximately 8 images per hour are available, but the relatively coarse spatial resolution prevents analysis of the fine-scale structures atop the storm's anvil. MODIS has 36 channels with a 1-km footprint, and several of the shorter wavelength bands have even finer resolution (down to 250-m).
The GOES-12 visible loop shows the explosive development of thunderstorms around 1900 UTC. Morning convection in extreme northeast Colorado left behind several boundaries which are most evident around 1930 UTC; these boundaries no doubt played very important roles in the evolution of the convection, and perhaps in tornadogenesis. The GOES-12 10.7um loop also shows evidence of the boundaries, as well as a rapid cloud-top cooling rate, indicating a very unstable atmosphere.
In addition to the more traditional visible and infrared GOES channels, the 3.9 um channel 2 provides additional information about cloud-top structure. The loop below has a color table such that brighter values correspond to larger 3.9 um radiances. These radiances have 2 contributions for thick clouds: an emitted component and a solar reflected component. For very cold thunderstorm tops, the emitted component is quite small and is relatively constant across a single anvil cloud. This means that horizontal gradients in 3.9 um radiance across an anvil cloud are related to differences in solar reflection. It has been shown that more numerous smaller ice crystals reflect more 3.9 um radiation, so the bright portion of the large storm in northeast Colorado likely has smaller ice crystals than the darker northern portion.
The 3 images below are from Aqua MODIS at 2110 UTC for bands 1, 22, and 31 with a 1-km footprint. These 3 bands closely correspond to GOES channels 1, 2, and 4 respectively. The improved spatial resolution is striking in the infrared bands: gravity waves can be seen atop the storm in band 31 propagating away from the overshooting tops. In addition, the reflective nature of the storm top is even more pronounced in band 22. A ~30 C brightness temperature gradient exists across top of the largest storm. Also interesting are the smaller clouds over the Colorado mountains. Notice the north-south pair of cells which have significantly different reflectivity characteristics even though they're separated by only a few kilometers.
The image below is also from Aqua's band 1, but it has a 250-meter footprint and the enhancement table has been stretched to show the subtle differences in visible reflectance at storm top. Notice that the two areas atop the largest storm having the largest visible reflectance correspond to the bright areas in the band 22 (above middle). Additionally, 2 overshooting tops with gravity waves moving away seem to be centered within these 2 bright areas. This suggests that strong updrafts are injecting smaller ice crystals into the anvil of the storm.
This case shows an example of an intense midlatitude thunderstorm which has significant reflectivity differences across its anvil. These differences are most robust near 3.9 um, but a more subtle difference exists within the visible wavelength range. Both of these observations suggest that smaller ice crystals are present in the southern portion of the anvil cloud. Future work will hopefully explain why some storms, and even portions of some storm tops, contain more numerous small ice crystals than others.