The vertical distribution of humidity: a crucial factor in the organization of convection

Deep moist convection can become organized in many different forms: from isolated cumulonimbus cells, to squall lines, to mesoscale convective complexes. Even individual categories of mesoscale organization can contain vast differences in structure. For instance, some squall lines are best described as a collection of individual cumulonimbus cells, while other squall lines overturn as slabs with > 100 km long updrafts. It is natural to ask why convection organizes in such different manners.

The depth and magnitude of vertical wind shear has been studied by many authors as an important factor in the organization of convection. Furthermore, the relationship between environmental wind shear and convective available potential energy (CAPE) has been identified as a strong determinant of convective structure in supercells and squall lines.

An aspect of convective organization that is relatively unexplored is the thermodynamic properties of the local environment. In particular, the vertical distribution of temperature and humidity structure has been largely overlooked. This study focusses on the vertical distribution of environmental relative humidity and its impact on the organization and evolution of convection.

A three-dimensional cloud-scale numerical model was used with both analytic and observed initial conditions. Different humidity profiles were studied with fixed profiles of temperature and wind. Results show that a spectrum of convective organization can be obtained by varying only the depth of a relatively moist layer, or by the placement of relatively dry layers in the mid- and upper-troposphere. Most importantly, it is demonstrated that a variety of long-lived mesoscale convective systems can be obtained in an environment with the same CAPE-shear balance. This result shows that the balance between CAPE and shear is only effective in certain thermodynamic environments. It is suggested that a third parameter - environmental humidity - needs to be added to the CAPE-shear balance arguments.

Finally, it is important to note that the vertical distribution of humidity can be considered as a proxy for synoptic-scale forcing since the local relative humidity often reflects the integrated effects of persistent ascent or descent. The deep moist soundings that are typically used in numerical experiments are only found (at least in mid-latitude continents) in regions of deep, persistent ascent - i.e., ahead of traveling mid-latitude troughs. The effects of mid- and upper-level variations in humidity that were studied here are associated with the introduction of different air masses at different levels by evolving synoptic-scale cyclones or short waves. In other words, although the storm-scale structure of temperature, humidity, and wind shear are the proximate causes of convective organization, the synoptic forcing ultimately determines the local conditions that allow certain storm-scale organizational modes to be realized.