High-Resolution Tomography of Upper Tropospheric and Lower Stratospheric Water Vapor with Spaceborne Active Microwave Limb Sounding

Resolution of many outstanding science questions in the Upper
Troposphere and Lower Stratosphere (UT/LS) demands atmospheric
composition observations with the continual near-global coverage
attained by past and current passive limb sounders but with greater
spatial resolution than currently achievable from space. We describe a
new spaceborne active microwave occultation sounder system under
development, making two-dimensional tomographic atmospheric composition
observations with unprecedented spatial resolution (~500m vertical, 10km
along track). The measurement approach employs multiple small (e.g.,
6U-“CubeSat”-class) transmitters orbiting in the same plane and flight
direction as a separate receiver instrument. The transmitters emit
continuous distinct tones, and the receiver observes all transmitters
simultaneously and continuously, in an occultation viewing geometry.
The vertical resolution of the measurements is set, to first order, by
the along-orbit spacing of the transmitters, with the horizontal
resolution set by signal to noise and radiative transfer considerations.
We review the underlying receiver and transmitter technologies being
developed, along with our plan to test the measurement system in a
balloon-to-balloon configuration. Specific science targets for such a
measurement system include the contribution of overshooting convection
to the budget of stratospheric water vapor.