The Direct Radiative Forcing of Biomass Burning Aerosols: Investigations During SCAR-B and ZIBBEE
Sundar A. Christopher, PI
Brent N. Holben, Collaborator
Thomas Eck, Collaborator
Abstract:
Biomass burning aerosols have a large spatial distribution that is
governed by the geographic distribution of their source regions and
tropospheric circulation. Annually, more than 114 teragrams of smoke aerosols
are produced in the tropics. Depending upon their size and chemical
composition, these aerosols have atmospheric residence times ranging from days
to weeks. They reflect the incoming solar directly which is called the "direct
effect" and they also modify the shortwave reflective properties of clouds,
which is called "indirect effect". The global top of atmosphere (TOA) direct
radiative forcing (DRF) of aerosols range between -0.2 to -1.1 W/m^2. The
uncertainties in this estimate are due to the various assumptions involved in
the calculations. While the TOA radiative forcing estimates provide valuable
information on the effects of aerosols on climate, equally important are the
downward shortwave irradiances (DSWI) at the surface and the associated
atmospheric impacts. The DSWI is an important component of the surface
radiation balance and the atmospheric heating rates due to absorption by
aerosols govern both local and regional circulation patterns. A recent study
(Konzelmann et al. 1997) has shown large differences on the order of 40-80
W/m^2 between measured and calculated DSWI values in the presence of biomass
burning aerosols. These differences are attributed to the inaccurate
characterization of aerosols in radiative transfer models. Most satellite
detection algorithms classify aerosols as clouds, which could lead to errors in
radiative forcing calculations. Very few studies have attempted to estimate
the surface and atmospheric effects of biomass burning aerosols largely due to
the lack of information on the microphsical properties and chemical
composition.
In 1995 a major field program called the Smoke Clouds and
Radiation-Brazil (SCAR-B) was conducted in Central Brazil to understand the
effects of biomass burning aerosols on radiation and climate. In 1997 another
field experiment was conducted over Zambia called the Zambian International
Biomass Burning Experiment (ZIBBEE) that also studied the effects of biomass
burning on climate. A wide range of insitu and surface measurements are
available from both of these field projects that will help reduce the
uncertainties in the DRF estimates of biomass burning aerosols. The major
focus of this proposed effort is to estimate the direct TOA, surface and
atmospheric effects of biomass burning aerosols during SCAR-B and ZIBBEE in
order to reduce the uncertainties in the aerosol radiative forcing values.
Using near-coincident coupled measurements from satellites, aircraft,
and ground-based measurements from SCAR-B and ZIBBEE, a radiative transfer
model will be used to estimate the DRF of biomass burning aerosols. Using
satellite measurements from the Clouds and the Earth's Radiant Energy System
(CERES) instrument from the Tropical Rainfall Measuring Mission (TRMM)
platform, this proposed effort will also test and validate the algorithms
during the upcoming Large-Scale Biosphere-Atmosphere Experiment in Amazonia
(LBA) experiment over South America. The algorithm and products developed
through this research proposal will be directly applicable to the CERES Surface
Radiation Budget (SARB) efforts.
This proposed effort also brings together an interdisciplinary team
that provides expertise in different areas. 1) Sundar A. Christopher
(satellite retrievals and flux calculations), 2) Brent Holben (Sunphotometer
measurements and DRF issues), 3) Thomas Eck (pyranometer measurements and
radiative transfer calculations). Both Brent Holben and Thomas Eck have agreed
to collaborate in this research effort.
