Causes of Azimuthal Air Temperature Biases on a 100 m Meteorological Gradient Tower

Azimuthal temperature biases in multi-level tower-mounted sensors have long been reported, yet their underlying physical causes remain unclear. In particular, three candidate mechanisms—flow blockage, structural-geometry effects, and instrument-to-instrument differences in radiative sensitivity—have rarely been disentangled within a single observational experiment. Here, we address this gap by using a 100 m lattice tower at the Shouxian National Climate Observatory (32.5° N, 116.8° E), which has base and top widths of 10.0 m and 2.0 m, respectively. East–west symmetric dual temperature sensors and independent bilateral anemometers were installed at 34, 50, 65, and 80 m, with an additional reference anemometer at 95 m. On 12 May 2025, the complete probe-and-shield assembly at 50 m was reversed east–west, whereas the other three levels remained unchanged as negative controls, thereby establishing a difference-in-differences (DID) framework. Using the top-mounted reference wind direction, the bilateral wind-speed difference shows no statistically significant shadow sector at any level, allowing us to reject the flow-blockage (tower-shadow) hypothesis. The structural baseline bias S(z) of the three un-swapped decreases monotonically in magnitude with height (from −0.92 °C at 34 m to −0.28 °C at 80 m) and tracks the height-dependent tower geometry, proxied by the face width W(z), linearly, with stable day–night behavior—pointing to a geometric/orientational origin. After the swap, the 50 m bias amplifies to 3–18 times the baseline and reverses seasonally; the inter-instrument difference in daytime shortwave response is significant (Δα = +46.27 × 10−3 °C/(MJ m−2 d−1), 95% CI [+5.71, +91.30]), whereas the nighttime net-longwave difference is directionally consistent but not significant (Δβ = +38.33 × 10−3 °C/(W m−2), 95% CI [−1.32, +70.78]). We conclude that instrument-swap-induced homogeneity breaks are more likely caused by inter-instrument differences in radiative-response sensitivity than by a constant offset. This finding has important implications for climate-network homogenization and for azimuthal quality control of tower-mounted sensors.