An Airborne Microwave Radiometer for Improved Thermodynamic Observations of the Atmosphere

Most of the existing airborne microwave radiometer instruments are platform specific, thus they operate only on the aircraft they were designed for. They have a limited view, usually just under the aircraft, and limited observational capabilities, either temperature or humidity profile, based on operation in a preselected range of a few microwave absorption lines or atmospheric windows. Any additions or modifications of such an instrument is prohibitively expensive and thus nearly impossible.

Passive microwave radiometers are one of the most valuable observational assets for meteorological research, weather forecasting, and climate observations from space, ground, or airborne platforms. Radiometer observations provide temperature and humidity profiles, cloud liquid water content, total moisture content of the atmosphere, and some information about surface properties. These meteorological variables can be observed under all weather conditions because of the nature of microwave radiometer observational frequencies, between approximately 1 and 1,000 GHz, and corresponding wavelengths of 30 cm and 0.3 mm. For example, a ground-based radiometer requires correction for cosmic background radiation (2.73 K) when it is pointed skyward. Thus, a ground-based radiometer provides information about the whole atmospheric column and can even “see” all the way to the Big Bang (13.8 billion years ago). Similarly, an airborne instrument can provide a complete profile below and/or above the flight line from any altitude.

A paper by Zuidema et al. “Recommendations for Improving U.S. NSF-Supported Airborne Microwave Radiometry”, BAMS, December 2016 recommend “Radiometers emphasizing a compact design (that) should fit into a standard cloud probe canister. Irrespective of size, radiometers should possess

• multifrequency humidity and liquid water sens­ing (20–30, 90, 183 GHz) capability;
• modularity of frequency components;
• the ability to be deployed upon multiple aircraft;
• views above and below the aircraft …”.

Boulder Environmental Sciences and Technology (BEST) is developing a small airborne scanning radiometer, the Profiling Airborne Microwave Radiometer (PAMR), for airborne observations. PAMR is built to operate independently of the aircraft, requiring only power, with all other supporting measurements, such as ambient pressure, temperature, humidity, position (latitude, longitude, altitude) and attitude (pitch, roll, yaw), magnetic orientation, and others are internal to PAMR. Internal storage can accommodate data from up to three 8-hour flights, with an option to communicate with PAMR during the flight via an Ethernet connection. PAMR has and estimated mass of ~4.5 kg and power consumption under 50 Watts, with a length of 60 cm and a diameter of 10 cm. PAMR’s small size, low mass, and low power consumption enables deployment on various type of aircraft, including medium size UAVs such as the ArcticShark, owned by the Atmospheric Radiation Measurement program of the Department of Energy.

PAMR scans in a plane perpendicular to the flight line (cross track scan mode), and thus it samples the whole atmosphere through which the airplane if flying, both above and below the aircraft. This 360° scanning mode during the aircraft’s ascent or decent allows observations of cloud parameters vertical distribution with vertical resolution that depends only on the aircraft’s rate of ascent (descent) and the available time. PAMR’s modular concept allows not only easy and fast field replacement of the modules, but also allows development of various additional sensors that can be added to it at any future time.

We will present the PAMR design and its current configuration and measurement capabilities. We will also describe its proposed integration into the ArcticShark.

PAMR has the potential to significantly improve observations of clouds, boundary layer thermodynamics, other atmospheric phenomena, as well as providing some information about surface conditions.