Convection - Vertical Moisture Transport

As mentioned earlier, TPW products are often derived together with stability parameters. The vertical distribution of humidity and temperature needed to derive the TPW content also serves for deriving atmospheric stability.

Traditionally, stability indices are derived from temperature and humidity soundings by radiosondes. Vertical profiles provided by satellite instruments can fill the temporal and spatial gaps between radiosonde measurements.


Stability parameters and TPW products to forecast convection

TPW and stability parameters from geostationary satellite data over land and seas can be derived in cloud-free regions only. Satellite-derived stability indices are used to predict convection. To learn more about the GEO satellite stability product GII see: Global Instability Index.

Dry convection may start as soon as the ground heats up during the day and temperature stratification permits the vertical shift of air masses. Dry convection may also lead to strong winds caused by the relocation of air, but it seldom results in severe weather conditions. This changes drastically when humidity and condensation processes are involved as the release of condensation heat boosts the tropospheric processes.

Therefore, the intensity of severe thunderstorms depends on the moisture supply available to sustain convection. The preconditions for thunderstorms are given by tropospheric stability parameters, while the magnitude or severity of the event largely depends on TPW.

Water vapour imagery often provides a good estimate for convective initiation (see CAL on WV imagery). In cases with strong moist advection from the south, a sharp gradient of TPW on the western edge of the band with the highest moisture coinciding with a moisture gradient in a water vapour image can be a strong indicator of potential for heavy precipitation. Sharp or tight gradients make for particularly strong indicators. This gradient can also be applied to other GOES Sounder instability products like LI for pointing out areas with potential for severe weather. The gradient is typically located in an area between a baroclinic boundary and an upper level trough. It lies between the minimum and maximum values of TPW, possibly slightly toward the highest values of TPW and/or the most unstable area (highest negative LI's, for example).

Note
Neither stability nor TPW can predict the exact location of a thunderstorm. Convective initiation depends on several factors that either favor or inhibit lifting processes, such as orography, the general synoptic situation, wind and wind shear, capping inversions, etc.