Comparison of dust-layer heights from active and passive satellite sensors

Abstract

Abstract. Aerosol-layer height is essential for understanding the impact of aerosols on the climate system. As part of the European Space Agency Aerosol_cci project, aerosollayer height as derived from passive thermal and solar satellite sensors measurements have been compared with aerosollayer heights estimated from CALIOP measurements. The Aerosol_cci project targeted dust-type aerosol for this study. This ensures relatively unambiguous aerosol identification by the CALIOP processing chain. Dust-layer height was estimated from thermal IASI measurements using four different algorithms (from BIRA-IASB, DLR, LMD, LISA) and from solar GOME-2 (KNMI) and SCIAMACHY (IUP) measurements. Due to differences in overpass time of the various satellites, a trajectory model was used to move the CALIOPderived dust heights in space and time to the IASI, GOME-2 and SCIAMACHY dust height pixels. It is not possible to construct a unique dust-layer height from the CALIOP data. Thus two CALIOP-derived layer heights were used: the cumulative extinction height defined as the height where the CALIOP extinction column is half of the total extinction column, and the geometric mean height, which is defined as the geometrical mean of the top and bottom heights of the dust layer. In statistical average over all IASI data there is a general tendency to a positive bias of 0.5-0.8 km against CALIOP extinction-weighted height for three of the four algorithms assessed, while the fourth algorithm has almost no bias. When comparing geometric mean height there is a shift of −0.5 km for all algorithms (getting close to zero for the three algorithms and turning negative for the fourth). The standard deviation of all algorithms is quite similar and ranges between 1.0 and 1.3 km. When looking at different conditions (day, night, land, ocean), there is more detail in variabilities (e.g. all algorithms overestimate more at night than during the day). For the solar sensors it is found that on average SCIAMACHY data are lower by −1.097 km (−0.961 km) compared to the CALIOP geometric mean (cumulative extinction) height, and GOME-2 data are lower by −1.393 km (−0.818 km).

The Mineral Aerosol Profiling from Infrared Radiances (MAPIR) algorithm: version 4.1 description and evaluation

“…5.3.3, the operational level 2 extinction profiles between 0 and 5 km from version 3.00 are used, thereby selecting only the dust aerosol types (dust, dust mix-ture and marine mixture). Quality filtering of the CATS data is done similarly to that in Lee et al (2018).…”

“…3.4, while the evaluation is done by comparing single co-located measurements. The comparison was made following the methodology described in Kylling et al (2018). Within the region of interest the closest CALIOP swaths in time and space to the MAPIR dust pixels were identified.…”

“…CALIOP profiles do not provide a unique dust altitude. Here we use the same CALIOP altitudes as Kylling et al (2018), namely the purely geometric mean altitude (mean of the bottom and top altitudes of the dust layer) and the cumulative extinction altitude (dust altitude set to altitude at which the cumulative extinction at 532 nm is half of the total extinction column). The 5 km profile product from CALIOP data version V4-10 was used for the comparison.…”

“…The table includes the 2010 data from the comparison presented in Kylling et al (2018) for MAPIR v3.2 and v3.4.…”

“…The inlay is the percentage of MAPIR altitudes that are within the CALIOP layer. MAPIR 3.2/3.4 results are taken fromKylling et al (2018).…”