Status and future of numerical atmospheric aerosol prediction with a focus on data requirements

Numerical prediction of aerosol particle properties
has become an important activity at many research
and operational weather centers. This development is due to
growing interest from a diverse set of stakeholders, such as
air quality regulatory bodies, aviation and military authorities,
solar energy plant managers, climate services providers,
and health professionals. Owing to the complexity of atmospheric
aerosol processes and their sensitivity to the underlying
meteorological conditions, the prediction of aerosol particle
concentrations and properties in the numerical weather
prediction (NWP) framework faces a number of challenges.
The modeling of numerous aerosol-related parameters increases
computational expense. Errors in aerosol prediction
concern all processes involved in the aerosol life cycle including
(a) errors on the source terms (for both anthropogenic
and natural emissions), (b) errors directly dependent
on the meteorology (e.g., mixing, transport, scavenging
by precipitation), and (c) errors related to aerosol chemistry
(e.g., nucleation, gas–aerosol partitioning, chemical transformation
and growth, hygroscopicity). Finally, there are fundamental
uncertainties and significant processing overhead
in the diverse observations used for verification and assimilation
within these systems. Indeed, a significant component
of aerosol forecast development consists in streamlining
aerosol-related observations and reducing the most important
errors through model development and data assimilation.
Aerosol particle observations from satellite- and groundbased
platforms have been crucial to guide model development
of the recent years and have been made more readily
available for model evaluation and assimilation. However, for
the sustainability of the aerosol particle prediction activities
around the globe, it is crucial that quality aerosol observations
continue to be made available from different platforms
(space, near surface, and aircraft) and freely shared. This paper
reviews current requirements for aerosol observations in
the context of the operational activities carried out at various
global and regional centers. While some of the requirements
are equally applicable to aerosol–climate, the focus
here is on global operational prediction of aerosol properties
such as mass concentrations and optical parameters. It is also
recognized that the term “requirements” is loosely used here
given the diversity in global aerosol observing systems and
that utilized data are typically not from operational sources.
Most operational models are based on bulk schemes that do
not predict the size distribution of the aerosol particles. Others
are based on a mix of “bin” and bulk schemes with limited
capability of simulating the size information. However
the next generation of aerosol operational models will output
both mass and number density concentration to provide a
more complete description of the aerosol population. A brief
overview of the state of the art is provided with an introduction
on the importance of aerosol prediction activities. The
criteria on which the requirements for aerosol observations
are based are also outlined. Assimilation and evaluation aspects
are discussed from the perspective of the user requirements.