Abstract
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.
Highlights
Thermodynamic information about the planetary boundary layer (PBL), including information about water vapour, is a targeted observable recommended by NASA’s Decadal Survey (National Academies of Science, Engineering, and Medicine, 2018)
Computational limitations forced adoption of an emulator approach, which provides a useful framework to assess error sources. This framework shows that the errors of interest for retrieval of spatial statistics of partial column water vapour in the PBL (PCWVPBL) are the gradient a1, equivalent to dTCWVret/dTCWV, and random error σε
Our method to derive σε allows users to predict spatial correlation; in particular, we found that an r2 > 0.9 requirement requires smoothing to 100–150 m resolution
Summary
Thermodynamic information about the planetary boundary layer (PBL), including information about water vapour (qv), is a targeted observable recommended by NASA’s Decadal Survey (National Academies of Science, Engineering, and Medicine, 2018). We employ a new type of Observing System Simulation Experiment framework and perform simulated VSWIR retrievals of TCWV from high-spatial-resolution LES output to determine whether horizontal spatial variability in PBL qv can be obtained from retrieved TCWV, and conclusions are limited to daytime non-cloudy conditions. The purpose of this is a detailed sensitivity study using retrieval code and tools already developed for EMIT. What is the correlation coefficient between retrieved and true TCWV, and can the spatial standard deviation be estimated? How does this depend on LESs of different convective PBL types?
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