A Tropical Upper Tropospheric Trough (TUTT) in Florida
The Tropical Upper Tropospheric Trough – TUTT (Sadler, 1976) is a trough or cyclonic circulation in the mid-upper troposphere located in tropical locations. Sadler defined the TUTT as an upper trough that lies between upper ridges in the subtropics and ridges in tropical locations. The TUTT is a complex system that often enhances convection along its periphery and limits it near its center. However, strong TUTTs can also enhance convection near their centers, given destabilization stimulated by the cold air present in their cores. In rare occasions, a strong TUTT that develops weak upper winds and strong convection near its center can yield to a cyclonic circulation at the surface that eventually triggers tropical cyclogenesis.
GOES-19 6.9um channel satellite imagery (link to quick guide) shows a TUTT located over the Bahamas and southeast Florida on the morning of 23 July 2025. The satellite presentation of the system shows a classical TUTT, characterized by strong convection along its periphery and a minima in convection near its cyclonic core. Animating water vapor channels such as the 6.9um one is an effective method to observe systems located in the mid-and upper troposphere, as these channels capture the motion of water vapor in these layers. The following animation shows the cyclonic rotation associated with the TUTT, which centers near Andros Island in the northwest Bahamas. It also shows convective tops in the periphery of the TUTT in shades of light gray and blue (brightness temperatures near and below -30°C), which rotate cyclonically around the center. The orange coloration (warmer temperatures) indicates drier air, which was entering the system from the northeast. When dry air in the mid troposphere is present very close to convection, it can enhance its strength by stimulating evaporation. This yields to cooling and, in turn, enhances vertical motion in the storms. The common presence of dry air near the center of a TUTT is one reason why they are often associated with strong convection that contains lightning, large rainfall rates and sometimes gusty winds.
The TUTT can also be seen on other satellite products. The following animation shows CIRA’s GeoColor product. In order to evaluate the location of thunderstorms, CIRA’s GLM Optical Energy product (link to quick guide) has been overlaid. The GeoColor is able to measure the surface and low-level clouds when higher clouds are not obscuring the view. This capability illustrates that the cyclonic rotation evident in the 6.9um channel is not occurring at the surface. In fact, the movement of low-level clouds is different. Lightning data from the GLM also confirms the presence of thunderstorms. The stronger storms are occurring over and off the coast of southeast Florida, in the periphery of the TUTT, given the interaction of three processes that are interacting to highlight ascending motions: (1) horizontal moisture gradients in the mid troposphere, (2) upper jet dynamics and (3) TUTT propagation. Role of moisture gradients: In the northwest portion of the TUTT, the dry air intrusion from the northeast of the TUTT (described in the previous paragraph) meets the very moist air mass located near southeast Florida, highlighting evaporative cooling processes and vertical motions. Role of upper jet dynamics: stronger upper winds over Florida, evidenced by the fast motion of the higher clouds, produces horizontal wind gradients that highlight ascent. Role of TUTT propagation: rapidly decreasing geopotentials in the mid-troposphere, as the TUTT approaches Florida from the southeast, enhance ascent. A fourth factor (not shown in these animations) is the presence of high values of available moisture over Florida. For this, precipitable water products are recommended. The combination of all of these processes yields to strong thunderstorms with frequent and dense lightning.
Not related to the TUTT but also of interest, CIRA’s GeoColor product shows that a Saharan Air Layer Intrusion is propagating northwestward in eastern portions of the domain. How can we tell? By the presence of a milky or whitish color shade in eastern portions of the image, which contrasts with the dark blue color of the ocean elsewhere in the figure. This contrast is more defined near sunrise. The reason is that the radiation beams returning to the satellite from the earth-ocean-atmosphere system have to travel in a low angle to reach the satellite. This forces them to encounter larger amounts of dust before reaching the satellite. The scattering of solar radiation caused by the dust produces the bright coloration that appears in CIRA’s GeoColor product.
An additional impact of the TUTT is the modulation of vertical wind shear, due to the vertical differences in wind directions and speeds in different quadrants of the TUTT. This has an impact in convection. The Day Cloud Phase Distinction RGB (link to quick guide) product is very useful to understand cloud types, and animating it shows how clouds at different levels move. It can provide a qualitative assessment of wind shear. It also illustrates convection types. Shallow convection in the tropics consists of water clouds, which appear blue and green. Yellows and oranges show ice clouds, which relate to deep convection and cirrus. The following animation shows this product from 12:50 through 18:00 UTC, capturing the diurnal cycle of solar heating. The impacts of the TUTT and how it relates to the diurnal cycle and to the Saharan Air Layer are described in the animation.
Reference:
James C. Sadler, “A Role of the Tropical Upper Tropospheric Trough in Early Season Typhoon Development,” Monthly Weather Review, October 1976, Volume 104, pp. 1266–1278.