Abstract
<jats:p>Abstract. Every Global Navigation Satellite System (GNSS) station estimates a zenith wet delay (ZWD), a direct measure of the water vapour integrated above it. The ZWD is a standard product of GNSS meteorology, yet its small-scale temporal structure is conventionally treated as noise. We ask whether the temporal spectrum of the ZWD reveals horizontal water vapour heterogeneity near the boundary layer top that a vertical sounding cannot see. Using FESSTVaL observations at Lindenberg in summer 2021, we compare the GNSS ZWD with the precipitable water from a microwave radiometer over 28 days. Both are described by a Matérn spectrum whose scale and amplitude are tracked on sliding windows. The radiometer scale follows the boundary layer height in calm conditions and shifts to a wind-driven, advective behaviour when the atmosphere becomes unstable, confirmed by tower turbulence. The GNSS scale stays stable across all conditions, because its oblique geometry ties it to horizontal structure at the boundary layer top, unchanged by the shifts in vertical stability. The strength of the GNSS signal grows with the horizontal heterogeneity measured independently at two scales (ρ = +0.433 and +0.525), while the vertical radiometer shows none and a scintillometer rules out a surface contribution. Accessible from any positioning solution, the GNSS scale measures horizontal coherence at the boundary layer top that is far more stable than the ERA5 boundary layer height (coefficient of variation 0.32 against 0.90) and persists through the night.</jats:p>