Inherent Properties of Clouds in the PBL Derived from Multi-angle
Spectro-Polarimetric Imaging at the “Edge of Space:” New Capabilities
of JPL’s AirMSPI Sensor on NASA’s Airborne ER-2 Platform
Abstract
Commonly-occurring stratification and synoptic tendencies lead to liquid
clouds and warm precipitation processes in the PBL over large portions
of the globe. The climate is so sensitive to these low-level clouds that
they are identified in IPCC reports as major uncertainty sources for
climate prediction; their representation in GCMs thus needs improvement.
PBL clouds have therefore been scrutinized in numerous field campaigns
over both ocean and land. The main method for measuring clouds in field
campaigns is in-situ airborne probing and, though these data are
invaluable, it is widely recognized that spatial and temporal sampling
is innately poor. We then turn to remote sensing as a way of drastically
improving spatial sampling since it delivers cloud properties over more
than a line-of-flight through 3+1D space. The obvious tradeoff is,
however, generally complicated connections between remotely-measured
radiances and inherent cloud properties of real interest to cloud
process modelers. Active remote sensing from below or above the clouds
improves vastly over in-situ sampling, but its outcome remains confined
to a “ribbon” of vertical profiles ordered in time (from below) or
space (from above). Passive imaging has the complimentary problem of
delivering a potentially wide horizontal swath of cloud properties, but
integrated along the vertical. At least that is the conventional wisdom
when it comes to the solar spectrum, where observed radiances from
clouds are dominated by multiple scattering. Based on recent results
from AirMSPI imaging at 20 km altitude, we challenge the perceived
limitation of passive shortwave radiometry to deliver only
column-integrated properties. We demonstrate that multi-pixel
exploitation of multi-angle spectro-polarimteric imaging at solar
wavelengths can be used to extract not only maps of microphysical
properties but also 3D cloud structure for both PBL-topped stratiform
layers and vertically-developed 3D clouds in convective regimes. A key
realization is: airborne and space-based sensors offer radically
different spatial and angular sampling opportunities with unique
advantages in both cases. We look forward to future PBL-specific
missions in space for their global reach. At the same time, there is a
clear case for deploying high-altitude imagers in all future campaigns.