At late stages of winter monsoon pressure pattern, shallow cloud streets spread over the Japan Sea. The instrumented aircraft flew along the mean wind in the mixed layer. Shallow cumulus clouds were loosely organized as cloud streets. Their orientation was deviated from the mean wind direction in the mixed layer to the right by 40 degree. During the air mass traveled over the Japan Sea for 300 km distance, the depth of the convectively mixed layer increased from 1.5 to 1.9 km. The mixed layer was also heated by 2.5 K and moistened by 0.3 gkg-1 during the travel. Liquid water path in the clouds was much less than adiabatic condensation amount although it gradually increased toward downwind. Ice crystal concentrations increased with traveling distance, corresponding to the decrease in cloud top temperatures, and the depletion of cloud water by ice crystals compensated the increase of condensation water in the mixed layer.
On the basis of heat and moisture flux measurements, development of the mixed layer and cloud formation in it are discussed quantitatively.