Definitions
The specification of the Global Climate Observing System (WMO, 2011) lists aerosol layer height as an essential climate variable. It is requested to be reported with an accuracy of 1 km at a horizontal resolution of 5-10 km and temporal resolution of 4 hours. The aerosol layer height is also expected to have a stability of 0.5 km, where stability in this context can be taken as the extent to which the systematic error remains constant over a decade or more. The lack of a GCOS definition of aerosol layer height is impediment to meeting these requirements and is addressed here.
Within the boundary layer aerosol is generally convectively well mixed. In the free troposphere aerosol mass concentration typically shows an exponential decrease with increasing altitude (Gras, 1991). However this can be punctuated by layers of enhanced aerosol mass. Weigum et al. (2012) examined black carbon plumes measured during the HIPPO campaigns. They defined a plume as “an occurrence of elevated mass mixing ratio above the background level”. Mathematically this involved adopting a threshold mass mixing ratio value that defined a plume roughly equivalent to the noise level of the instrument. Hollstein and Filipitsch (2014) analysed Calipso data and concluded that global measurements could best be represented by the sum of two lognormal vertical aerosol distributions.
The quantity used to define the aerosol has an intrinsic influence on what is meant by aerosol height. Three cases are considered:
I: Aerosol Weighted Height
If the aerosol profile is defined in terms of particle number, n(z)
the aerosol layer height could be the aerosol number weighted height,
II: Aerosol Layer Height
If the aerosol profile is defined in terms of particle number the aerosol layer height would describe the position of an enhancement in aerosol number density compared to the bounding altitudes. The layer should be a homogeneous aerosol type such as volcanic ash, biomass burning or Saharan dust. The method of calculating the plume height could be the layer mode, median or mean altitude. This would need to be defined as would the threshold at which the plume is considered to begin (e.g. a number density 10% lower/higher than the altitude above/below). Either a ‘missing’ value or the surface height could be reported in those cases where there is no number enhancement within the profile. Then a description of the reported value would be “aerosol number layer (mode, median or mean) height” where the text also defined the level of enhancement adopted to define a layer.
Approaches I & II could equally be applied to a profile described using aerosol mass.
III: Aerosol Effective Radiating Height
An aerosol effective radiating height could be calculated as the altitude at which all the aerosol in an atmospheric column must be placed to generate the same top of atmosphere radiation as the initial atmosphere. The disadvantages in this metric are that it will be wavelength specific. In addition the shortwave TOA radiance is only weakly dependent on aerosol height so a value derived in this spectral region will have a very large uncertainty. The most useful application of this metric would be in the thermal window around 11-12 μm.
Given the sensitivity to aerosol altitude is mostly in the infrared channels for imaging instruments I would suggest we report aerosol effective radiating height at 11 μm.
Gras, J. L. Southern hemisphere tropospheric aerosol microphysics. Journal of Geophysical Research: Atmosphere, 96, 5345-5356, 1991.
Hollstein A. and F. Filipitsch, Global Representation of Aerosol Vertical Profiles by Sums of Log-normal Modes: Consequences for the Passive Remote Sensing of Aerosol Heights, Journal of Geophysical Research: Atmosphere, in press, 2014.
Weigum, N. M., P. Stier, J. P. Schwarz, D. W. Fahey, and J. R. Spackman, Scales of variability of black carbon plumes over the Pacific Ocean, Geophys. Res. Lett., 39, 2012.
WMO, Global Climate Observing System Systematic Observation Requirements for Satellite-based Data Products for Climate: 2011 Update GCOS-154, World Meteorological Organization, 2011.