The FY1997 experiment has been carried out for maritime stratiform clouds over the sea north of the Amami islands in a few hundreds-kilometer wide area centered at (29N, 129E) during 23 January through 2 February 1998. We have observed several cases of stratocumulus clouds caused by outbreaks of cold airmasses and extended stratiform clouds associated with fronts. On 2 February, we encountered stratocumulus cloud layers which were laying adjacently to each other with different thicknesses. The cloud layers consisted of pure water clouds with cloud-top temperature of 2`4 at 2.0km altitude for the thicker layer. The measured microphysical properties are shown in Fig. 1. For the cloud layers, we have carried out synchronized stacked-flights of two instrumented aircraft to simultaneously measure clouds and radiation (Fig. 2). Fig. 3 demonstrates the coincidence of the flight tracks by C404 and B200 aircraft. The maximum time lag between the two aircraft on the synchronized flight legs was less than one minute. Thus, it is assured that the two aircraft observed the same part of the cloud layers. The observational data were analyzed to study solar radiation budget by cloud layers and to validate remote-sensing techniques developed in JACCS.
The cloud-physical properties such as visible optical thickness, effective-particle-radius, liquid-water-content (LWC) and liquid-water-path (LWP) have been retrieved from the spectral solar reflectance measured by the MCP system on board C404 (Asano et al. 1995a, 1995b), and they were compared with the in-situ measured values (Fig. 4, Fig. 5, Fig. 6, Fig. 7). The MCP-retrieved effective radius and LWC were in good agreement with those measured by the PVM and KING hot-wire probes on board B200 (Fig. 5 and Fig. 6). Further, the retrieval algorithm of LWP from the upwelling microwave radiation measured by the downlooking microwave-radiometer installed on C404 has been developed (Takayama et al. 1998) and applied to the aircraft observational data. The MCP-derived LWP and microwave-radiometer measured LWP are reasonably agreed with each other (Fig. 7).
The solar radiation budget due to the stratocumulus cloud layers observed on 2 February 1998 has been precisely analyzed (Fig. 8, Fig. 9,Fig. 10). The visible solar flux was estimated by taking differences between the total and near-IR solar fluxes measured by the total-band and near-IR-band pyranometers. The visible net fluxes measured above and below the cloud layer were almost the same within the measurement accuracy; this means no appreciable extra absorption of the visible solar radiation by the cloud layer. On the other hand, there were reasonable amounts of absorption in the near-IR region; the absorption agrees quite well with that calculated theoretically by employing the measured cloud microphysical properties. The present result is the first direct observational evidence which clearly denies the existence of the so-called anomalous solar absorption due to water clouds (Stephens and Tsay, 1990).
The result will be further discussed in a coming paper submitted to JGR.
Fig. 1. Vertical distributions of liquid-water-content (A), cloud-particle concentration (B), and effectiv-particle-radius (C) for the stratocumulus cloud observed on 2 February 1998.
Fig. 2. Time series of the flight heights (RIGHT scale) and the downward solar irradiances (LEFT scale) measured on C404 and B200 aircraft, respectively, for stratocumulus cloud layer observed on 2 February 1998. The C404 and B200 aircraft made stacked flights in the time intervals denoted by S-1, S-2 and S-3.
Fig. 3. Horizontal flight tracks of C404 and B200 aircraft along the legs S-1 (A) and S-2 (B).
Fig. 4. Time series distributions of the visible optical thickness (LEFT scale) and effective-particle-radius (RIGHT scale) estimated from spectral reflectance measurements by MCP along the synchronized flight legs S-1 (A), S-2 (B) and S-3 (C).
Fig. 5. Comparison of the MCP retrieved effective-radius and in-situ measured effective-radius by the Gerber probe on board B200 along the north-south leg S-2b.
Fig. 6. Comparison of the MCP retrieved liquid-water-content (LWC) and in-situ measured LWC by the King hot-wire-probe(KLWC) and Gerber probe(PVM), respectively.
Fig. 7. Time series comparison of the MCP retrieved liquid-water-path (LWP) with the LWP measured by the microwave radiometer (MWR) on board C404 for stratocumulus cloud layer observed on 2 February 1998.
Fig. 8. Distribution of the visible (VIS) and near-infrared (NIR) net solar fluxes measured on C404 and B200, respectively, above and below the cloud layer in time series (A) and in longitudinal position (B) along the S-1a leg shown in Fig. 3.
Fig. 9. Same as Fig.8, but for the S-1b leg. For the BOTTOM panel, a geometrical correction was applied for latitudinal position of B200.
Fig. 10. Distributions of cloud albedo, transmittance and absorptance for the near-IR (A), (C) and visible (B), (D) solar riation along the flight legs S-1a and S-1b, respectively, for the stratocumulus cloud observed on 2 February 1998.
Released 1 October 1998
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