Fig. 1. Airborne Cloud-Radiation Observing System (ACROS) for aircraft experiments in JACCS.
The major scientific objectives of the aircraft-based experiments in JACCS are (1) to expand our knowledge of the relationship between cloud structures and radiative properties, (2) to improve our understanding on the structure and evolution processes of clouds, and (3) to assess and improve the currently used remote-sensing techniques for retrieving cloud-microphysical parameters. Intensive airborne observations of clouds and radiation from multiple aircraft have been planned for midlatitude low-level and mid-level clouds in the western north Pacific region. In the JACCS program during FY1993-1995, we have developed an Airborne Cloud and Radiation Observing System (ACROS) by employing two instrumented aircraft for simultaneous measurements of clouds and radiation (Shiobara et al. 1994, 1995). Synchronized formation flights by multiple aircraft are our essential strategy for simultaneous observation of clouds and radiation.
In ACROS, for remote-sensing and radiation measurements flying over clouds, Cessna 404 Titan aircraft (C404) was equipped with a microwave radiometer, an FTIR radiometer, and a multichannel cloud pyranometer (MCP) system developed by Asano et al.(1995a), as well as conventional flux radiometers. For in-situ measurements of cloud microphysical and thermodynamical properties, Beechcraft B200 Super King Air aircraft (B200) was equipped with a video-microscope system for sizing cloud droplets (AVIOM) developed by Tanaka et al.(1989), the PMS 1D- and 2D-probes, a wind probe, the King LWC probe, the Gerber's microphysics probe PVM-100A (Gerber et al. 1994), Lyman-alpha and dew-point hygrometers, etc. These platforms are chartered from a commercial air service company (Nakanihon Air Service Co.). By FY1994, the ACROS as schematically shown in Fig 1 has been almost completed installing these instruments (Shiobara et al. 1996).
In FY1995, the ACROS performance has been tested through the field experiment for stratocumulus clouds over the Sea of Japan during 10-25 January 1996. The results have proved good performance of ACROS as demonstrated in the following examples. The stratocumulus cloud layer observed on 20 January 1996 was caused by an outbreak of winter monsoon over the Sea of Japan. The altitude and temperature of the cloud top were 2000 m and -7 C, and those of the cloud base were 1100 m and -3 C, respectively. Figure 2 shows the flight altitudes of B200. C404 flew at a constant altitude over the cloud layer during the synchronized flight with B200.
The stratocumulus layer was mostly super-cooled water cloud and partially glaciated (mixed-phased) sometimes showing a weak subsun phenomena caused by ice crystals. However, water droplets were rather predominant with frequent glory phenomena. Figure 3 and Figure 4 show vertical profiles of the liquid water content (LWC) and the effective radius of cloud droplets measured by PVM on board B200 flying in the stratocumulus layer. LWC had maximum at the middle level of the cloud layer. However, the maximum value was 0.3 g/m3 for the descending flight and 0.6 g/m3 for the ascending flight (Fig.3). In contrast with the profiles of LWC, the effective radius of cloud droplets increased with height during both descending and ascending flights, that is, being around 4um at the cloud base and around 8um near the cloud top (Fig.4). The cloud particle concentration measured by FSSP decreased from 500/cm3 just above the cloud base to 200/cm3 near the cloud top.
We have compared the cloud microphysical properties measured with different probes. Figure 5 shows a consistency between the effective particle radii measured with FSSP and PVM probes. The liquid water contents measured by the PMS King wot-wire probe and the PVM probe were in good agreement for the range of 0 - 0.6g/m3 (Fig.6). Spectral solar reflectance measurements with the MCP system on board C404 enable us to estimate effective radius of cloud droplets (Asano et al. 1995b). Figure 7 compares the effective radius estimated from MCP measurements with those from in-situ measurements by PVM. The MCP-retrieved effective radii were slightly larger than the in-situ measured radii.
A dual-frequency microwave radiometer (MWR; Radiometrics WVR1100) was on board C404 to measure the cloud liquid-water-path (LWP) and water vapor amount from nadir-looking observations above cloud layers. Figure 8 shows the temporal/spatial variation of the LWP measured by MWR, and compares them with those retrieved from the MCP measurements. The procedure to estimate LWP from MCP measurements is described in Asano et al.(1995b). The remote-sensing of LWP from MWR and MCP were consistent with each other .
Acknowledgments. We thank Nakanihon Air Service Co., Prede Co., Finetec Co., Science Engineering Associates, and Gerber Scientific, Inc. for their help to the aircraft experiment.
Fig. 1. Airborne Cloud-Radiation Observing System (ACROS) for aircraft experiments in JACCS.
Revised 1 October 1998
(Previous revision; 1 August 1996)
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