Brightness temperature of storm tops

Cloud-top temperature or brightness temperature of tops of convective storms represent probably the most important information source used to infer storm type, structure, its activity and possible severity, widely used since the early days of weather satellites. Before proceeding to a discussion of these applications, lets first briefly summarize what the "brightness temperature" (BT) actually means, what is the difference between BT and a more general term "cloud top temperature" (CTT).

The term "brightness temperature" originates from the physics of radiative processes. Strictly speaking, it represents a virtual temperature at which an object (solid surface, cloud top, ...) would emit the radiance detected by the satellite, provided that the observed emitting object behaves as an idealized black body. As most of the real opaque surfaces (such as optically thick cloud top layers) emit somewhat less than they would emit if they behaved as a perfect black body, their BT derived from the observed radiance data is somewhat lower than their real (thermodynamic) temperature. As an alternative or synonym to BT, a term "black body temperature" (usually abbreviated as TBB) is also being used.

The significance of BT or CTT is in their relatively close link to cloud top height (CTH). The higher the cloud, the lower its BT (CTT).  Therefore, the coldest IR pixels typically represent the highest tops of clouds, such as "overshooting tops" of convective storms (discussed further below). However, one should keep in mind that this link is not quite unambiguous, there are certain storm types (e.g. storms exhibiting cold U or V, or cold ring features, also discussed in further sections) for which the CTT can not be related to the CTH that easily. Nevertheless, in most cases one can conclude that the colder the storm (either its BT minimum, or average BT of the storm top, or a total cloud-top area colder than a certain BT threshold), the higher is its cloud top. As the storm height is closely related to strength of its updrafts, the highest tops (and lowest BT) can be found above the most vigorous storms.

The typical range of cloud-top temperature of convective storms depends not only on the storm itself (on its internal properties), but also on the tropopause height and on the geographical region where the storm forms. The lower is the tropopause, the warmer storm tops will be observed, and vice versa - the higher tropopause heights result in colder storm tops. For this reason the storms which form at high latitudes or in a cold polar airmass, are typically much warmer than storms forming closer to the equator; and the convective storms (thunderstorms) in tropics and sub-tropics are usually very cold, much colder than the ones forming in mid-latitudes. As we will see, this has to be considered when applying color enhancement of the IR BT imagery (discussed in the next section).

 Cloud-top emissivity

To make the BT interpretation even less straightforward, we have to keep in mind that the way a real emitting body differs from an idealized black body (in physics called emissivity, being equal to 1 for a black body, and lower for real objects) is not only a function of the physical properties of the given object, but also of the actual wavelength at which we observe the object (e.g. the clouds). This means that the BT for a certain cloud can be different in different bands, e.g. when comparing the cloud-top BT retrieved from the IR3.9 and IR10.8 SEVIRI bands. Dependence of the emissivity on the cloud top composition (on its microphysics) is the basis of automated cloud type retrieval techniques, and also of the RGB image composites.

You can learn more about the brightness temperature, emissivity, and other radiative properties of clouds e.g. in this material from Paul Menzel: Remote Sensing Applications with Meteorological Satellites (2006), or the Basics of Remote Sensing (1997).