MARINE BOUNDARY-LAYER STRUCTURE AND FRACTIONAL CLOUDINESS

Radiosonde data collected during the First International Satellite Clo ud Climatology Project (ISCCP) Regional Experiment (FIRE), 1987, the A tlantic Stratocumulus Transition Experiment (ASTEX), 1992, and the Tro pical Instability and Waves Experiment (TIWE), 1990, were used to deve lop four composite thermodynamic soundings of the marine boundary laye r for cloud conditions ranging from solid stratocumulus to trade cumul us and sea surface temperatures ranging from 16 degrees to 27 degrees C. Average cloud fraction and cloud base height for the composite soun dings were estimated using laser ceilometers located at each of the fo ur sites. The average fractional cloudiness varied from 0.83 off the c oast of California at San Nicolas Island (33.4 degrees N, 119.6 degree s W) during FIRE to 0.26 over the central equatorial Pacific during TI WE (0 degrees N, 140.0 degrees W). During ASTEX, two sites in the vici nity of the Azores generally experienced cloud conditions characterize d by cumulus clouds penetrating into a stratocumulus layer. At the mor e northerly site, the island of Santa Maria (37.0 degrees N, 25.2 degr ees W), the average fractional cloudiness was 0.67 compared with 0.40, 1000 km downstream at the German R/V Valdivia (28.0 degrees N, 24.0 d egrees W). The two composite soundings from ASTEX and the composite so unding from TIWE indicate decoupled boundary layer structures. These t hree soundings have a cloud layer that is conditionally unstable and s how a systematic increase in relative humidity with increasing fractio nal cloudiness. The effect of cloud top entrainment on fractional clou diness was evaluated. For the four composite soundings the fractional cloudiness decreases as the slope of the normalized w-theta mixing lin e increases (greater instability), but this decrease is 4 times less t han that from previous studies. Fractional cloudiness diagnosed from p arameterizations using cloud layer relative humidity compares well wit h the observed mean cloudiness.