EVOLUTION OF THE SUBTROPICAL MARINE BOUNDARY-LAYER - COMPARISON OF SOUNDINGS OVER THE EASTERN PACIFIC FROM FIRE AND HARP

The mean time rates of change of temperature, total water mixing ratio and ozone along airflow trajectories in the lower troposphere over th e eastern Pacific are inferred by comparing aircraft soundings from th e First ISCCP Regional Experiment (FIRE) and the Hawaiian Rainband Pro ject (HaRP). Through the use of the estimated mean fluxes of temperatu re and total water mixing ratio, it is found that the tendency for str atus layers to grow or dissipate is very sensitive to the assumed turb ulence structure below the capping inversion. A mixed-layer model that assumes a well-mixed boundary layer up to the capping inversion predi cts a solid cloud layer extending all the way to Hawaii, whereas a mod el that allows decoupling predicts rapid dissipation of the stratus la yer. It is concluded that stratus dissipation here is due to the slowd own of turbulent mixing throughout the layer below the capping inversi on, not the drying out of a well-mixed layer; hence, the mixed-layer m odel cannot be expected to predict realistic cloud dissipation. The di fferences in ozone concentration observed in the boundary layer during HaRP and FIRE suggest a chemical loss of ozone of 3-8 ppb day-1, corr esponding to a lifetime of 3-9 days. This implies that ozone cannot be treated as a conserved tracer when dealing with ozone budgets over pe riods of days. The ozone sink is probably of photochemical origin, and it requires further investigation.