Flow fields on and around a semi-arid Sky Island

Do semi-arid sub-alpine forests in the southwestern United States make an important contribution to the regional carbon budget? This research is one step in providing necessary flow field parameters for quantifying the exchange of carbon and water fluxes between the forest and the atmosphere. The semi-arid southwestern forest ecosystem is located on sky islands, which are isolated mountain ranges surrounded by desert. It is only in these elevated areas that, on average, precipitation exceeds evapotranspiration to the extent that a forest ecosystem can survive.

The topography of these sky islands are made up of a series of complex ridges and valleys. There is a strong summer monsoon season, where there are seasonal reversals of synoptic flow superimposed on the local valley/mountain flow regime. In order to accurately estimate the carbon contribution of this type of forest, the flow patterns affecting surface-atmosphere exchange must be better understood. Existing observation from an above canopy flux tower located on a sky island in the US southwest indicates distinct seasonality in the dominant wind direction. During the winter months (January to March), wind directions in the NW to SW quadrant had a frequency of 60%, during the pre-monsoon months of April to June, wind directions from N to NE had a frequency of 95%, and during the post monsoon period, wind directions from the SW to SE quadrant had a frequency of 58%. During the monsoon season (July and August) there were two dominant wind directions. Wind directions from the NW to SW quadrant had a frequency of 49% and N to NE winds had a frequency of 38%. There is also a distinct diurnal pattern in the flow fields.

Micrometeorological observations in the semi-arid southwest are usually confined to the more easily accessible desert floor and riparian areas, while sky island observations are typically sparse. Determination of the significance of the contribution of sky islands in terms of the environmental water and carbon budgets is limited by the lack of data for model validation. These areas are usually subsumed within large scaled regional models/datasets, as individually they cover a small area, but regionally make up significant vegetated land coverage in the southwest US and northern Mexico. The characterization of the flow fields of a semi-arid sky island is therefore not only of interest to the modeling of the regional carbon budget in terms of flux components such as advection and night-time flows, but also for the regional environmental water budgets, and the development of monsoon cumulus clouds, as these sky islands also coincide with the dominant source areas for the regions' water resource. This study uses a micrometeorological based model and observations to analyze and describe the flow characteristics on and around the Santa Catalina Sky Island located NE of Tucson Arizona, the results of which can serve as a bench mark for numerical models and the improvement of regional modeling datasets for the US southwest and northern Mexico.