Nocturnal Valley Cold Air Pool Displacement on a Desert Mountain Slope

Abstract:

The Mountain Terrain Atmospheric Modeling and Observations (MATERHORN) program collected a high-resolution observational dataset to test and improve mesoscale model performance and weather prediction in complex terrain. One of the focal points of the MATERHORN field program was a relatively uniform mountain slope where large nocturnal temperature variations due to the displacement of a valley cold air pool (CAP) up the slope have been observed. CAP displacements can significantly impact the near-surface temperature and wind but are poorly represented in mesoscale models. This work aims to identify the spatial and temporal distribution of CAP displacements and the mechanisms causing them.

CAP displacements are identified using data from a transect of 4 automatic weather stations (AWS) and 5 flux towers deployed on the east slope of Granite Peak (GP) in Northwest Utah. The instrument transect spans 130 m of elevation starting on the valley floor and rising up the uniform alluvial fan at the base of GP. 116 additional AWS’s in the vicinity of GP are used to diagnose the mechanisms causing the displacements. The dense grid of observations allows the tracking of coherent features in the pressure, temperature, and wind fields as they interact with the CAP and the topography of GP. Two time periods, October 2012 and May 2013, were investigated for the presence of CAP displacements and their causes.

In October, a typical night exhibits an average of 5 cold-pool displacement events that are evenly distributed between 2100 and 0500 MST when valley CAPs have developed. In May, fewer displacement events are observed (3 per night) and they occur preferentially later in the night, between 0000 and 0400 MST. Displacements are frequently associated with features that can be tracked across the plain surrounding GP. These features include slope and valley flow “fronts” descending the surrounding terrain, seiching, and mesoscale boundaries. In addition to describing the spatial and temporal distribution of CAP displacements, two case study periods will be used to illustrate the spatial and temporal evolution of CAP displacements forced by two different forcing mechanisms, slope and valley flow “fronts” and mesoscale boundaries.