An Automated System for Measuring Microphysical and Radiative Cloud Characteristics from a Tethered Balloon

Abstract

OAK-B135 The rate of climate change in polar regions is now felt to be a harbinger of possible global warming. Long-lived, relatively thin stratus clouds play a predominant role in transmitting solar radiation and trapping long wave radiation emitted from open water and melt ponds. In situ measurements of microphysical and radiative properties of Arctic and Antarctic stratus clouds are needed to validate retrievals from remote measurements and simulations using numerical models. While research aircraft can collect comprehensive microphysical and radiative data in clouds, the duration of these aircraft is relatively short (up to about 12 hours). During the course of the Phase II research, a tethered balloon system was developed that supports miniaturized meteorological, microphysical and radiation sensors that can collect data in stratus clouds for days at a time. The tethered balloon system uses a 43 cubic meter balloon to loft a 17 kg sensor package to altitudes u p to 2 km. Power is supplied to the instrument package via two copper conductors in the custom tether. Meteorological, microphysical and radiation data are recorded by the sensor package. Meteorological measurements include pressure, temperature, humidity, wind speed and wind direction. Radiation measurements are made using a 4-pi radiometer that measures actinic flux at 500 and 800 nm. Position is recorded using a GPS receiver. Microphysical data are obtained using a miniaturized version of an airborne cloud particle imager (CPI). The miniaturized CPI measures the size distribution of water drops and ice crystals from 9 microns to 1.4 mm. Data are recorded onboard the sensor package and also telemetered via a 802.11b wireless communications link. Command signals can also be sent to the computer in the sensor package via the wireless link. In the event of a broken tether, a GMRS radio link to the balloon package is used to heat a wire that burns 15 cm opening in the top of the balloon. The balloon and sensor package slowly descends to the ground and a radio tracking beacon is activated to locate the balloon and sensor package. The tethered balloon system was deployed in upslope clouds at the Smokey Hills Bombing Range in western Kansas and at Ft. Carson Air Force Base near Colorado Springs, Colorado. Both of these areas are FAA Restricted Airspace up to FL180 (18,000 ft MSL) so that the tethered balloon could be flown to its maximum height without violating FAA regulations. Because the feasibility field programs took place at the very end of the research period covered by this DOE grant, a detailed analysis of the results are beyond the scope of this report. However, examples of water drops and ice crystals recorded by the CPI demonstrated the feasibility of the balloon and sensor package. Based on our initial analysis of results from the feasibility field deployments, we have determined that the tethered balloon system is capable of making long -term measurements of meteorological, microphysical and radiation properties of polar stratus clouds up to a height of about 2 km. However, further field trials should be conducted before deploying the system in a full-up field campaign.