Dust Radiative Effect Estimations

Project title: An ESM-Free Approach for Dust Direct Radiative Effect Estimations Based on EMIT, CALIPSO, and Mineralogy-Resolved Dust Optical Property Models

Dust aerosol is one of the most dominant aerosol species by mass and is ubiquitous globally due to the intercontinental transport of its plume. Airborne dust plumes affect the radiation fields directly through scattering and absorbing solar radiation, emitting infrared radiation, and indirectly through modifying cloud microphysical and thermodynamic properties via ice-dust interactions. Additionally, dust plumes fertilize iron-rich dust particles for oceanic phytoplankton growth. The optical properties of dust aerosols vary substantially with their particle sizes, shapes, and mineralogical compositions, as suggested by rigorous electromagnetic theory and confirmed by laboratory measurements. These variations in dust optical properties lead to significant uncertainty in the estimation of the dust direct radiative effect (DDRE).

Many previous studies have attempted to estimate DDRE using spaceborne observations and Earth System Model (ESM) simulations. In general, DDRE estimations obtained from these studies rely on significantly simplified assumptions made for dust optical property models and dust plumes, such as the spherical particle assumption, the regionally invariant dust refractive index, a universal particle size distribution (PSD) across the globe, and/or a single homogeneous dust layer assumption. These assumptions are often inappropriate for typical dust plumes in the atmosphere. The latter two factors can be improved by an ESM-based approach. However, ESM-based approach may involve uncertainty due to dust emission/transport/microphysics schemes, and therefore the estimated DDRE could be ESM-dependent. Thus, the conventional assumptions made in dust aerosol studies create a substantial gap in our in-depth understanding of the role of mineralogical and microphysical properties in DDRE estimation, making the accurate estimation of climatological DDRE challenging based on observations. To tackle this long-standing challenge, we propose a three-year project focused on better observational constraints of DDRE over arid and semi-arid regions using EMIT observations, CALIPSO observations, and state-of-the-art dust optical property models.

Our proposed three-year research project includes the following tasks: 1) Developing a mineralogically resolved dust optical property model and a dust radiative parameterization for EMIT Science and Application Team, as well as the numerical modeling communities.

2) Validating the dust optical property model with EMIT observations and other measurement datasets, with a focus on mineralogical compositions and PSDs. 3) Estimating DDRE over the Saharan desert region using EMIT, CALIOP, and the dust optical property model through an ESM-free approach.

One striking feature of our proposed project is that the dust optical property model will be developed based on realistic particle shape assumptions, a rigorous electromagnetic framework for various internal mixtures of mineralogical compositions, and various PSDs of dust aerosol particles. Another feature of this project is that the DDRE

evaluation is solely based on spaceborne observations, without the uncertainty introduced by ESM numerical schemes. With these proposed efforts, we will provide a 'benchmark' dust optical property model and observational DDRE estimations.

Project Team

Masanori Saito

University of Wyoming