The Role of Mountain Flows in Making Clouds

Abstract

Mountains disrupt basic airflows to force ascending air currents and initiate clouds. Clouds are produced when air becomes saturated and water condenses. Air can be brought to saturation by evaporation, by cooling, or by expansion from adiabatic lifting—adiabatic lifting being the most important process in the atmosphere. Mountains produce rising motion and adiabatic ascent in two ways, forced ascent and convergence caused by the heating of elevated topography.

Two factors must be considered in assessing the ability of a mountain range to produce clouds under a given flow regime: 1) The properties of the atmosphere, and 2 ) the characteristics of the flow disturbance; i.e., the lifting created by the mountain. The properties of the atmosphere include two important vertical levels; the lifting condensation level (LCL), and the level of free convection (LFC). The LCL is where air first reaches saturation by lifting, and the LFC is where saturated air becomes positively buoyant. The properties of the atmosphere thus determine the amount of lifting needed to produce clouds, and the flow characteristics determine the amount of lifting provided. If air is lifted to its LCL but not to its LFC, stable or stratiform clouds will be produced. If air is lifted to its LFC, unstable or cumuliform clouds will be produced. It is important to remember, however, that the two factors are not independent because characteristics of the atmosphere (especially its stability) affect how much lifting the flow will be able to produce—and lifting destabilizes the atmosphere.

Stable clouds are formed by forced lifting as flow ascends a mountain barrier. A conceptually simple case is considered that seems to apply when moisture is confined to the lowest 1 or 2 km above the terrain and the flow is nearly parallel to the topography. The main issues in this case are physical rather than dynamic the ability of these dynamically simple clouds to produce precipitation on the ground is governed by microphysical factors and may be influenced by radiation. In general, the dynamics of stable orographic flow are more complex, however, and mountain wave and blocking effects provide regions of enhanced rising motions that determine the occurrence and distribution of precipitation.

The major focus of this chapter is on unstable mountain clouds (especially mountain thunderstorms) that result from a mixture of large- and small-scale effects. Whereas larger-scale processes control whether or not thunderstorms will be able to form, the role of the mountain (i.e., smaller-scale) circulations is in the initiation or triggering of thunderstorms. Thus mountains have an important role in the temporal and spatial distribution of storms. Three major kinds of lifting that lead to thunderstorm initiation are direct orographic forced lifting to the LFC, convergence of thermally forced circulations, and aerodynamic or obstacle effects.

Warm season convection in relatively arid mountainous regions such as the western United States is generally triggered by convergence and updrafts produced by thermally forced, daytime upslope circulations. Temporally, such clouds and thunderstorms form first over the mountains, later moving downwind onto or reforming on the plains. It is conceptually useful to define two types of thunderstorm, primary and secondary. During its initiation, secondary convection contains a feature generated by a preexisting thunderstorm, such as a gust front or a gravity wave, whereas primary convection does not. Thus primary convection may be forced, for example, by orographically or thermally forced mountain flows. Spatially, thunderstorms develop in preferred areas of the topography, depending on external factors such as ridgetop winds and moisture distribution.

Forecasters traditionally recognize three ingredients necessary for thunderstorms. Applied to mountain storms of the arid western United States these are 1) a conditionally unstable temperature lapse rate, which is generally the case in the warm season, 2) an initiation or triggering mechanism, which is provided daily by mountain circulations, and 3) adequate moisture, which is generally advected in by the larger-scale flow, although local sources can be important.

Two other considerations that are important in mountain thunderstorm initiation are the roles of larger-scale processes and surface moisture. Larger-scale processes can be prohibitive, permissive, or actively forcing in their effect on thun­derstorms. Soil moisture reduces surface heat flux by partitioning increased amounts of incoming energy during the day into latent heat. The reduction in heat flux reduces the strength of thermally forced circulations and retards the rate of growth of the daytime boundary layer.