Land Effect on the Diurnal Cycle of Clouds over the TOGA COARE Area, as Observed from GMS IR Data

Abstract

The diurnal cycle of clouds over the western equatorial Pacific region (15°S?15°N, 130°E?180°) is studied analyzing hourly GMS-4 infrared brightness temperature images during the intensive observation period (Nov 1992?Feb 1993) of TOGA COARE. Although the area studied is essentially (93%) oceanic, differences of diurnal behavior of the clouds are noticed over different ocean subareas, depending both on the general circulation conditions and on the vicinity of landmasses. This study focuses on the effects of New Guinea and other major islands on the diurnal cycle within the surrounding ocean areas, as for example, the TOGA COARE Intensive Flux Array. The major observable feature of the influence of land is the presence of a diurnal, rather than semidiurnal, average cycle of cloudiness with a high day-to-day repetitivity. The signal is observed up to 600 km off the coast of New Guinea and it is characterized by a variable phase propagating at an average speed of about 15 m s?1. For smaller islands, the effect extends over a distance approximately comparable to their size. The genesis of the propagating cloud systems is assumed as due to the low-level convergence between the large-scale flow and a possible land breeze. This conceptual model has been previously proposed to explain a similar signal observed offshore of Borneo. Within this framework, the influence of the large-scale circulation on the intensity and spatial organization of the propagating cloud systems is discussed. The diurnal signal vanishes when the expected convergence is weaker or when overshadowed by large-scale disturbances crossing over the considered area. In the first 3 months of the period such disturbances are nearly always cloud clusters accompanying the active phase of the Madden?Julian oscillation. Finally it is shown that the small islands in the TOGA COARE domain can corrupt the ?oceanic? signal by as much as 10% of the diurnal cycle.