New Radar Technologies for the Aerosol/Cloud/Ecosystems Mission

By Wanda Archy, posted on November 16th, 2012 in Atmospheric Chemistry, Earth Observation, Featured Article

NASA’s future Aerosol/Cloud/Ecosystems (ACE) Mission will address fundamental science questions concerning the role of aerosols on cloud development and ecosystems. Recommended by the National Research Council’s (NRC) Earth Science Decadal Survey, ACE targets a broad class of hydrometeor types. The mission’s instruments will measure cloud droplets, ice crystals, rain and snow, which are subject to change in the presence of aerosols. These changes also impact the Earth’s ecosystems and the ocean’s storage of carbon dioxide. The ACE measurements aim to improve the understanding of aerosol, cloud and ecosystem interactions. Also, ACE measurements will have spin-off benefits, including aiding in air-quality forecasting.

Aerosol pollution in Northern India and Bangladesh. Source: Wiki Commons.

ACE seeks to study cloud and aerosol height, organic material in surface ocean layers, and aerosol and cloud type properties. Overall, the mission will provide improved climate modeling and better predictability of climate change variability, measure ocean productivity and health, and create air-quality models and forecasts. Results of the mission will help decrease uncertainty in climate predictions. ACE will focus primarily on two phenomena: aerosol-cloud interactions and carbon uptake in oceans.
The ACE mission calls for the use of two radars, ACERad, which represent a significant advancement compared to earlier atmospheric radar technologies. The radars will operate at 94 gigahertz in W-band and 35 gigahertz in Ka-band, and unlike previous missions, have Doppler and polarimetric capabilities. The dual-frequency Doppler measurements will provide valuable information about how particle sizes change with height. Dual-polarization will provide information about the water particle state and shape.
The Ka-band radar will scan to provide images for a more comprehensive picture of precipitating systems. The use of these two radars will create stronger vertical profiles for measurement of cloud droplet size, glacier height, and cloud height.

A three-dimensional, detailed structure of Hurricane Sandy measured by the TRMM Precipitation Radar (PR) on Oct. 28, 2012 at 1725 UTC (1:25 PM EDT). Source: NASA.

Dr. David Starr, chief of the Mesoscale Atmospheric Processes Laboratory at NASA’s Goddard Space Flight Center (GSFC), explains that cloud systems are extremely difficult to understand and cloud physics continues to be an important focus of the mission. The sensors that will be used in ACE will look at precipitation in cloud systems, using active and passive sensors for a more complete picture. Paying attention to how variables evolve with height also is important, since some variables may enter in a lower section, but ultimately have a more of an effect at a higher section. Aerosols’ reflection of solar radiation also interferes with ocean color — depending on the uptake of carbon. To eliminate any confounding variables, the ACE mission will study aerosols simultaneously with ocean color. “There are many tradeoffs between the ACE mission and its radar specification,” said Dr. Gerald Heymsfield, a cloud radar expert and research meteorologist at NASA’s GSFC. “One of the biggest (tradeoffs) being the imaging capability of the radar and its sensitivity. A significant challenge is to obtain high-sensitivity measurements while scanning.”
Imaging capability, for example over convective storm cells, provides context for cloud generation, and improved understanding between precipitation, aerosol and cloud formation. As a trade between measurement sensitivity and imaging capability, the ACE mission calls for the use of a nadir-only pointing W-band radar and a scanning Ka-band radar. Such dual-frequency and dual-mode operation presents challenges in implementing on a single spacecraft. A primary challenge relates to the large antenna size of about 3 meters by 5 meters required for each radar to achieve the sensitivity and spatial resolution to meet the ACE science objectives. The antenna architecture to achieve fixed-beam pointing differs from that needed for scanning, thus, complicating the shared use of the large antenna aperture by the radars.
“It is very challenging to have two frequencies on the radar that will fit in the spacecraft and still have adequate performance,” Heymsfield said.

Candidate antenna architecture for ACE radar. Source: NASA.

Since ACE is not scheduled to launch until 2022, there is time to develop technologies to address the challenges of implementing the ACE radar. NASA’s Earth Science Technology Office (ESTO) has made a number of investments in active microwave technologies during the past several years to address the challenges of ACE. These investments include: an Instrument Incubator Program (IIP) project in 2007 to study “A Multi-parameter Atmospheric Profiling Radar for ACE (ACERAD);” an Advanced Component Technology (ACT) project in 2008 for a large, high-precision deployable reflector for Ka-band and W-band Earth Remote Sensing; and a 2010 ACT project for advanced W-Band Gallium Nitride Monolithic Microwave Integrated Circuits (MMICs) for cloud Doppler radar supporting ACE. A 2011 IIP project aims to develop an antenna technology to support dual-frequency, dual-mode operation from a shared aperture. The principle investigator is Earthzine’s Editor-in-Chief, Paul Racette. Developed in partnership between the NASA Goddard Space Flight Center and Northrop Grumman Electronic Systems, the approach calls for the use of a novel dual-band reflector/reflect array using a W-band and a Ka-band Active Electronically Scanned Array (AESA) feed module. The reflector is a parabolic cylinder that permits scanning at Ka-band using an AESA line feed. A reflector is formed with a frequency selective surface that is reflective at Ka-band and focuses the W-band beam to a point. This approach offers cost advantages over other dual-reflector architectures. The architecture allows wide-swatch scanning of more than 100 kilometers at Ka-band and effectively uses the full antenna aperture to maximize the sensitivity at W-band while using W-band radar technologies also used for CloudSat and EarthCare.

GPM’s DPR. Source: NASA.

The ACE radar will extend the technological capability of predecessor atmospheric radars including The Tropical Rainfall Measuring Mission, Global Precipitation Measurement, CloudSat and EarthCare. The Tropical Rainfall Measuring Mission (TRMM) was launched in 1997 as a cooperative effort between NASA and the Japan Aerospace Exploration Agency (JAXA) and uses a 13.8 gigahertz in Ku-band Precipitation Radar (PR) for rainfall measurements. The PR was the first radar dedicated to map three-dimensional images of storm structures from space. Scheduled to launch in February 2014 is the Global Precipitation Measurement (GPM) mission. Like TRMM, GPM also is a joint venture between JAXA and NASA. The satellites used in this mission will provide detailed measurements of rain and snow every three hours. GPM’s Dual-Frequency Precipitation Radar (DPR) will be used in this mission and is scheduled to launch in February 2014. The DPR is composed of two radars that will provide 3-D measurements of physical characteristics in droplets. (See also “‘Revolutionary’ Space Project to Improve Weather and Climate Forecasting”)
NASA’s CloudSat, flying in tandem with the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO), studies the vertical structure of clouds and the role of clouds and aerosols on weather, climate and air quality. In 2006, CALIPSO and CloudSat joined other satellites in the A-Train formation to study the Earth. EarthCare, a global observation mission of clouds, aerosols, and radiation by the European Space Agency (ESA), will use two active instruments including a lidar and W-band radar, and two passive instruments, including a multispectral imager and broadband radiometer, for data collection. The EarthCare mission launch is scheduled for 2014. CloudSat and EarthCare, unlike the future ACE mission, do not have scanning capability and instead focus on high-sensitivity measurements at nadir. The ACE mission plans on using radar technologies that have both capabilities, making it an innovative new radar system.

source:  http://www.earthzine.org