Abstract
The infrared window region (780-1250 cm-1, 12.8 to 8.0 µm) is of great
importance to Earth’s climate due to its high transparency and thermal
energy. We present here a new investigation of the transparency of this
spectral region based on observations by interferometers of downwelling
surface radiance at two DOE Atmospheric Radiation Measurement program
sites. We focus on the dominant source of absorption in this region, the
water vapor continuum, and derive updated values of spectral absorption
coefficients for both the self and foreign continua. Our results show
that the self continuum is too strong in the previous version of
Mlawer-Tobin_Clough-Kneizys-Davies (MT_CKD) water vapor continuum
model, a result that is consistent with other recent analyses, while the
foreign continuum is too weak in MT_CKD. In general, the weaker self
continuum derived in this study results in an overall increase in
atmospheric transparency in the window, although in atmospheres with low
amounts of water vapor the transparency may slightly decrease due to the
increase in foreign continuum absorption. These continuum changes lead
to a significant decrease in downwelling longwave flux at the surface
for moist atmospheres and a modest increase in outgoing longwave
radiation. The increased fraction of surface-leaving radiation that
escapes to space leads to a notable increase (~5-10%)
in climate feedback, implying that climate simulations that use the new
infrared window continuum will show somewhat less warming than before.
This study also points out the possibly important role that aerosol
absorption may play in the longwave radiative budget.