Surface temperature patterns in complex terrain: daily variations and long-term change in the central Sierra Nevada,

An important new paper that further documents the difficulty of monitoring multi-decadal surface air temperature trends has been accepted. It is

Lundquist, J. D. and D. R. Cayan, 2007. Surface temperature patterns in complex terrain: daily variations and long-term change in the central Sierra Nevada, California. J. Geophys. Res., in press.

The abstract reads

“A realistic description of how temperatures vary with elevation is crucial for ecosystem studies and for models of basin-scale snowmelt and spring streamflow. This paper explores surface temperature variability using temperature data from an array of 37 sensors, called the Yosemite Network, which traverses both slopes of the Sierra Nevada in the vicinity of Yosemite National Park, California. These data indicate that a simple lapse rate is often a poor description of the spatial temperature structure. Rather, the spatial pattern of temperature over the Yosemite Network varies considerably with synoptic conditions. Empirical orthogonal functions (EOFs)were used to identify the dominant spatial temperature patterns and how they vary in time. Temporal variations of these surface temperature patterns were correlated with large-scale weather conditions, as described by NCEP-NCAR Reanalysis data. Regression equations were used to downscale larger-scale weather parameters, such as Reanalysis winds and pressure, to the surface temperature structure over the Yosemite Network. These relationships demonstrate that strong westerly winds are associated with relatively warmer temperatures on the east slope and cooler temperatures on the west slope of the Sierra, and weaker westerly winds are associated with the opposite pattern. Reanalysis data from 1948 to 2005 indicate weakening westerlies over this time period, a trend leading to relatively cooler temperatures on the east slope over decadal time scales. This trend also appears in long-term observations and demonstrates the need to consider topographic effects when examining long-term changes in mountain regions.”

Climate Science has discussed the difficulty of using point observations of surface air temperature to construct a global average surface temperature to diagnosis global warming and cooling; e.g. see

Science Questions on the Global Surface Temperature Trends

The paper by Lundquist and Cayanfurther document how even a systematic change of wind direction over time, which could occur without any area average surface air temperature change, can be misinterpreted with sparse surface temperature data is a available. The issue they raise is not only true for mountainous terrain but anywhere that there are heterogeneous landscapes.

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