Uncertainty in the Mobile Observation of Wind

Abstract

Air quality and greenhouse gas sampling from mobile platforms enables local to regional analyses of pollutant exposure, atmospheric chemistry, and emission sources. Simultaneous meteorological observations, particularly wind speed and direction, are often used to interpret measurements and construct emission fluxes. However, the wind arising from a moving platform contributes to the observed wind speed and direction, and this artifact requires adequate removal to best apply wind observations. Here, we calculate the theoretical limitations to the measurement of wind from a moving vehicle, assess the accompanying uncertainty, and apply these methods to an example transect across a plume of methane. The angle of the wind relative to the moving vehicle is a crucial determinant of the ability to distinguish a true wind and defines its uncertainty. Unlike a stationary wind measurement, the wind speed and direction contain complementary information that broadens the capability of the mobile anemometer. We find that the isolation of a true wind depends on the anemometer wind speed accuracy for true winds moving with or against the vehicle, while the anemometer directional accuracy is more important for crosswinds, such as is experienced when observing across a plume. The uncertainty in estimated wind speed has similar geometry, but the uncertainty in estimating true wind direction is the opposite: the accuracy of measured wind speed most greatly impacts crosswind direction. Exact values are determined by the specific accuracy limitations of the anemometer and vehicle speed, and the geometrical distributions vary. As a result, the characteristics of each mobile lab setup should be assessed individually to best inform meteorological analyses and observation route planning.