Breakup of Temperature Inversions in Colorado Mountain Valleys.
Tethered balloon observations of temperature inversion breakup have been collected in seven deep Colorado mountain valleys on clear undisturbed weather days in all seasons. By sunrise, the nocturnal inversions (for 21 case studies) build to about the level of the surrounding ridgetops. On average, inversions are 604 m deep with a vertical potential temperature gradient of .0295(DEGREES)K m('-1). The inversions are typically destroyed within 3 1/2 to 5 hours following sunrise, except when the valley is snow covered or the ground is moist. Inversions are destroyed by the growth of convective boundary layers (CBLs) over the valley floor and sidewalls, and by the descent of the top of the nocturnal inversion. The descent of the top of the inversion plays a major role in most cases. During inversion destruction, specific local wind systems are associated with layers in the vertical temperature structure profiles, but the wind structure evolution is not as consistent from day to day as the temperature structure evolution. Three patterns of temperature structure evolution have been identified from the data. These patterns have led to a hypothesis to account for inversion destruction, in which heat and mass are entrained from the elevated inversion layer into the CBLs and are carried up the sidewalls in the slope flows. Sensible heat flux from the valley surfaces provides the energy to cause the CBLs to grow and provides the energy required to remove mass from the base of the inversion layer in the upslope flows, allowing the inversion layer to sink and warm. Based on this hypothesis a thermodynamic model of inversion breakup has been developed. The model is composed of two coupled equations which relate the inversion descent rate and CBL ascent rate to the sensible heat flux. The primary inputs to the model are the valley width, sidewall inclination angles, the characteristics of the valley inversion at sunrise, and an estimate of sensible heat flux obtained from solar radiation calculations. The outputs, obtained by a numerical integration of the model equations, are the time-dependent heights of the valley floor CBL and the inversion top, and vertical potential temperature profiles of the valley atmosphere. The model predicts that valley inversions will be more easily destroyed if the initial inversion is shallow or weak, if the sensible heat flux is strong, or if the valley is narrow. Model results are compared with observations of inversion breakup taken in the Eagle and Yampa Valleys in different seasons. Simulations were obtained by fitting two constants in the model (relating to the surface energy budget and energy partitioning) to the data. The model accurately simulates the evolution of vertical potential temperature profiles and predicts the time of inversion destruction. The model indicates that a substantial fraction of the heat flux is used to drive the upslope flows that cause mass to diverge from the valley and result in a sinking of the inversion. This provides a means by which the valley atmosphere is rapidly warmed through its entire depth.
- Pub Date:
- June 1980
- Physics: Atmospheric Science