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 (15ЊS-15ЊN, 130ЊE-180Њ) 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-today 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 Ϫ1. 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 largescale 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.

The diurnal cycle of rainfall over New Guinea in convection-permitting WRF simulations

“…These high rainfall rates are concentrated along a relatively linear stretch of coastline where there is no enhanced localised convergence from colliding land breezes. Interestingly, the location of these rainfall rates coincides well with the offshore maximum in diurnal brightness temperature difference reported by Liberti et al (2001) (their Fig. 4), who documented the life cycle of convective cloudiness for New Guinea using infrared satellite data.…”

The Diurnal Cycle of Convection over the Northern South China Sea

“…• Whereas other studies have shown a similar propagation signal in satellite data (e.g., Yang and Slingo 2001;Liberti et al 2001;Mapes et al 2003b), the propagation signal over the northern SCS was shown to exist similarly in both satellite and radar data, with both phase and propagation speed in agreement. Propagation was also implied by the increase in the stratiform fraction of convective systems moving southward away from the coast, suggesting maturing MCSs as they move southward.…”

“…This is defined as a "propagation signal," or a systematic variation of the phase of maximum convective activity with a coherent spatial structure. Other recent studies have provided observational evidence of propagation signals in the Bay of Bengal (Yang and Slingo 2001;Webster et al 2002;Zuidema 2003) and off the northern coast of New Guinea (Liberti et al 2001;Zhou and Wang 2006). In the latter study, Zhou and Wang found that gravity waves generated by deep convective heating over the mountains of New Guinea help to initiate rainbands in the coastal regions offshore.…”

Diurnal Cycle of Cloud System Migration over Sumatera Island

“…The land-water interface drives convection initiation and thereby affects the spatial distribution of convection (Zuidema 2003). Cloud systems propagate northward from the northern coast of New Guinea; these propagating cloud systems arise from low-level convergence between the large-scale flow and a land breeze (Liberti et al 2001). Mapes et al (2003a, b) and Warner et al (2003) described observational and numerical studies of westward rainfall peak migration to offshore regions in the night time over northwestern South America.…”