Abstract. Daytime clear-sky total column water vapour (TCWV) is
commonly retrieved from visible and shortwave infrared reflectance (VSWIR)
measurements, and modern missions such as the upcoming Earth Surface Mineral
Dust Source Investigation (EMIT) offer unprecedented horizontal resolution
of order 30–80 m. We provide evidence that for convective planetary
boundary layers (PBLs), spatial variability in TCWV corresponds to
variability in PBL water vapour. Using an observing system simulation
experiment (OSSE) applied to large eddy simulation (LES) output, we show that EMIT can retrieve horizontal variability in PBL water vapour, provided
that the domain surface is uniformly composed of either vegetated surfaces
or mineral surfaces. Random retrieval errors are easily quantified and
removed, but biases from −7 % to +34 % remain in retrieved spatial
standard deviation and are primarily related to the retrieval's assumed
atmospheric profiles. Future retrieval development could greatly mitigate
these errors. Finally, we account for changing solar zenith angle (SZA) from
15 to 60∘ and show that the non-vertical solar path destroys the correspondence between footprint-retrieved TCWV and the true TCWV directly
above that footprint. Even at the 250 m horizontal resolution regularly
obtained by current sensors, the derived maps correspond poorly to true TCWV
at the pixel scale, with r2<0.6 at SZA=30∘. However, the derived histograms of TCWV in an area are closely related to
the true histograms of TCWV at the nominal footprint resolution. Upcoming
VSWIR instruments, primarily targeting surface properties, can therefore
offer new information on PBL water vapour spatial statistics to the
atmospheric community.