A Satellite Proving Ground

The ER-2 is a high-altitude aircraft that flies at 20 km altitude. From the lower stratosphere, the ER-2 is above 99% of the atmosphere. This unique vantage point makes the ER-2 an ideal platform for testing instruments destined for space, and calibrating / validating new space-based sensors.

The NASA ER-2 aircraft observe the Earth-atmosphere system at a ~20 km nominal cruising altitude. The ER-2 have observed severe weather, atmospheric chemistry, ocean dynamics, and even the stars above. The ER-2 are particular valuable as validation platforms for new satellites including TRMM and GPM.

A common precipitation measurement ER-2 payload configuration includes a high-altitude doppler radar (EDOP), an Advanced Microwave Precipitation Radiometer (AMPR), and a Lightning Instrument Package (LIP). The EDOP measures vertical profiles of rainfall below the aircraft, and uses two beams (one forward-pointing, and one nadir-pointing) to measure the doppler velocity and reflectivity of the precipitation in the overflown storm. AMPR, meanwhile, is a total power passive microwave radiometer operating in 4 channels: 10.7 GHz, 19.35 GHz, 37.1 GHz, and 85.5 GHz. AMPR provides high-resolution passive microwave imagery compared to satellite instruments, but does not report separate horizontal and vertical polarization measurements.

The ER-2 LIP consists of electric field mills positioned along the fuselage of the aircraft and a Gerdien atmospheric conductivity probe. The number of field mills has increased over the years in order to compute the vector components of the atmospheric electric field during each overflight of electrified weather. Electric field timeseries can be used to compute the conduction currents (Wilson currents) that supply the Direct-Current branch of the Global Electric Circuit (GEC). Transient changes in the electric field data can also be used to identify lightning discharges.

I used the AMPR and LIP data to construct a retrieval algorithm that estimates the Wilson current generated by an overflown storm based on its extent and intensity in the passive microwave data.