A Combined Passive-Active, Multi-Sensor Approach to Earth’s Planetary Boundary Layer (PBL) Thermodynamic Sounding

Characterizing the complex three-dimensional (3D) thermodynamic structure of the Planetary Boundary Layer (PBL) from a global perspective is an unmet grand challenge. Current Program of Record (POR) space-borne passive sounders (infrared, microwave) were not designed with a specific PBL focus. Consequently, and despite their relative successes in retrieving atmospheric thermodynamic information on a global scale, current operational retrieval methods have limitations that preclude them from profiling PBL temperature and water vapor with the resolution and quality (~1K/km, 10% accuracy for temperature and water vapor, respectively) required by key PBL science questions and applications outlined in the NASA Decadal Survey Incubation (DSI) PBL Study Team Report (Teixeira et al., 2021).

The fundamental problem with all current space-borne passive sensors and correlative operational retrieval algorithms is an ill-conditioning one: the scientific needs for high PBL thermodynamic vertical resolution (i.e., <1 km) are not met by the degrees of freedom present in the passive space-based sounding measurement. The thermal emission of the PBL constituents is inherently absorbed by the overlaying atmospheric layers. Thus, existing passive sounders reach their highest sensitivity to temperature and water vapor in the mid-tropospheric region, leaving the PBL largely opaque.

Spaceborne backscatter lidar (BSL) measurements on the contrary can observe the structure of the PBL with high vertical (<100 meters) and spatial (~300 meters) resolution, although with limited spatial coverage (~nadir only). Passive sensors can provide information about cloud concentrations over large areas, but BSLs can help distinguish differences in height, phase, and layer type (e.g., smoke, dust, pollution, water droplet, ice crystal) and estimate PBL depth from the vertical variance of the backscattered signal associated with aerosol and shallow clouds (Palm et al., 2021; McGrath-Spangler and Denning, 2013). In this vein, spaceborne BSL can be combined with passive sounding geometries in a data fusion complementary approach to improve both PBL sounding vertical resolution and spatial coverage.

This presentation provides an overview of our passive-active data fusion sounding approach to the Earth’s Planetary Boundary Layer from space. This work is a NASA ESTO 2021 Decadal Survey Incubation PBL funded research project. This presentation revolves around two main foci. First, we describe ongoing simulation sensitivity experiments on the use of a novel hyperspectral microwave sensor (the Hyperspectral Microwave Photonic Instrument, HyMPI - a 2021 NASA ESTO Incubation Instrument Program funded project). We discuss the improved PBL thermodynamic information content harnessed in these hyperspectral microwave measurements, provide first of a kind comparisons with correlative hyperspectral infrared measurements and study the optimal combination of the two hyperspectral measurements that can help unlock the traditional sources of opacity (mainly clouds) and uncertainty (trace gas interference) affecting the conventional infrared measurements from the program of record, in a complementary, data-fusion approach. Second, a data-fusion approach of MW and IR hyperspectral passive measurements with a novel, space-based backscatter lidar concept is be presented.

This work is intended to quantify the gain in PBL information content obtained by the use of this passive-active data fusion approach and is intended to inform future pathways to filling technology and uncertainty gaps in the next generation of NASA and NOAA sensors. We also present an overview of the West-coast Hyperspectral Microwave Sensor Intensive Experiment (WHyMSIE), a joint 2021 - 2024 NASA-NOAA funded field campaign which will demonstrate first of a kind airborne hyperspectral passive + active measurements of the Earth’s PBL, under multiple weather and surface regimes.