Limestone Mountain Experiment (LIMEX)

Limestone Mountain Experiment (LIMEX)

a) Introduction

The Limestone Mountain Experiment (LIMEX-85) was designed to obtain accurate surface and upper-air data of high spatial and temporal resolution over the Alberta foothills during days exhibiting a capping inversion (or 'lid').

The capping inversion is a frequent stable thermodynamic layer which is usually topped at 1-2 km (or 100-200 mb) above ground. It is a common pre-storm signature throughout the High Plains regions from Texas to Alberta, but appears to be neither a necessary nor sufficient condition by itself for a severe convective storm. Its role in the severe convective storm process is to suppress early convective clouds, allowing a build-up of latent instability. The capping inversion should be distinguished from the more common nocturnal inversion. The latter is usually capped at 500 meters (50 mb) or less above ground, and readily forms under strong surface radiation on clear nights. It is also more quickly removed through simple daytime heating, and is therefore not as important in the severe storm process as the capping inversion. Occasionally, both types of inversion can be distinguished on a morning sounding.

The choice of the region around Limestone Mountain for the field experiment, stems from the observation that ARC radars often indicate earliest echoes over or near this or similar foothills mountain ridges. A simple hypothesis suggested that adiabatic cooling resulting from orographic list and an approaching synoptic system could remove the lower stable layer (capping lid) locally, allowing early convection to take place over the foothills first. If the effects of ascent are significant, then they should be measurable at low levels. The effects should also be most apparent on storm days which initially exhibit a capping inversion.

Primary Goals

The immediate scientific objective of LIMEX was to map the spatial and temporal sequence of processes leading to the creation and breakdown of the capping lid, and to test its hypothesized role in the pre-storm environment. A second objective was to test existing forecasting techniques involving the Synoptic Index, specifically to define optimum resolution of surface temperature moisture, and wind fields.

These two major objectives are both part of the broader scientific goal of Storm Forecast research, which is to define the interactions between large-scale weather systems and convective processes. The completeness of the definition of scale interactions is believed essential to the scientific goal of a "total Conceptual Model" of thunderstorms. The formulation of such a model is crucial to the development of techniques for accurate quantitative convective precipitation forecasts (QCPFs), and thereby to the ultimate goal of successful modification of convective phenomena, both for hail suppression and rain augmentation.

The Capping Inversion Hypothesis

The creation and breakdown of the capping inversion or lid involves a combination of synoptic, topographic and surface radiation influences. The processes involved are also related through the wind structures and the mesoscale moisture field within the boundary layer. Before hypothesizing the role of the lid in severe storm formation, it is first important to recognize that, while topographic and surface radiation effects fall into the lower mesoscale (sub-grid) range of processes, both are 'fixed mesoscale effects', and are therefore deterministic to a large degree. Radiation effects on the lid involve surface heating and cooling, which are determined by surface albedo, the amount of insolation, and the degree of boundary layer mixing of moisture. The albedo varies with surface features, including soil types and vegetation, while insolation is dependent on atmospheric humidity, including the degree of cloudiness.

Creation of the Capping Inversion

It is hypothesized that the sequence of events leading to the capping inversion, and subsequently a severe convective storm, are as follows. (i) Synoptic scale subsidence warming precedes an upper short-wave ridge, which may or may not be sufficient to create the initial inversion. A west or southwest flow alone can create the inversion, or enhance it locally near the mountains through (ii) orographically-induced subsidence warming, (iii) overrunning of hot, dry air mixed over an elevated plateau, and (iv) cooler, moist air underrunning any of the above. In Alberta, the first two effects (subsidence warming) are considered to be most effective in the lid creation, though the fourth effect is important to subsequent storm formation.

Breakdown of the Capping Inversion

The destruction of the inversion also results from a combination of four main factors. These are: (i) synoptic scale ascent (and adiabatic cooling) preceding a short-wave trough, (ii) orographic ascent due to (easterly) upslope flow in the boundary layer, particularly near the foothill mountain ridges such as Limestone, (iii) surface heating, and (iv) enhanced (storm vicinity) ascent and cooling once a convective complex has formed. Alberta forecasting experience and evidence in the literature suggests that the latter two are rarely, if ever, the sole cause for the inversion breakdown. The inversion removal is thought to be well underway by the time the first radar echoes are detected. Resolving this question is one of the major goals of LIMEX-85.

Role of the Capping Lid

One role of the lid in severe storm situations is to temporarily suppress the release of convective instability, while latent instability increases due to the build-up of heat and moisture below the inversion. The combination of topography (such as river valleys) and low-level wind jets (or streaks) are responsible for channeling the moisture when surface convergence is strong enough. The capping lid then completes the three-dimensional picture of moisture convergence.

It is hypothesized that mesoscale regions of ascent, resulting from a combination of topographic forcing and the eddy dissipation of energy from the larger synoptic scale flow, start the process of lid breakdown through adiabatic cooling (Strong, 1985). These 'preferential regions of ascent', which are dynamically associated with the low-level jets as well, allow intense convective to readily take place as the moist air flows out from beneath the lid, a process called "underrunning". The regions of ascent and convection are believed to occur most often on the cold side the lid edge zone.