Recent studies on tropical cumulus convection have been focusing on the various modes of cumulus clouds: not only shallow trade wind cumulus and deep cumulonimbus, but also cumulus congestus. It has been known that congestus clouds exhibit a significant part of tropical convection over the western Pacific warm pool region. This study investigates the relationship between the vertical development of tropical cumulus convection and environmental temperature/moisture profiles through the analyses of observational data obtained in a region of the tropical western Pacific and a series of numerical experiments with a cloud-resolving model in order to identify the factors responsible for determining the heights of tropical cumulus clouds, that is, the mode of tropical convection: cumulus; congestus; and cumulonimbus.

The data used in this study were obtained by a Japanese research vessel called 'Mirai' during the four cruises in a western tropical Pacific region 1999-2001 (the time period of the each cruise was about two to four weeks). The observational periods are divided into three phases depending on the amount of rain observed: no rain (DRY1); a little rain (DRY2); and a lot of rain (RAINY). In the DRY1 period the most prominent cumulus mode is shallow cumulus, while in the other two periods three modes of cumulus clouds constitute the population of tropical cumulus clouds. The sounding data analysis reveals that there is a close correlation between the shallow and middle-topped cumulus clouds and the existence of dry layers at middle to upper levels, while a stable layer at around the 500-hPa level (about the 0-degree-C level) can be often found in all the observation period.

Cloud-resolving simulations with the Advanced Regional Prediction System (ARPS) are performed in a two-dimensional idealized framework starting from a resting atmosphere initialized by the Mirai sounding data. A series of sensitivity experiments are conducted by gradually increasing the height of moist layers from lower levels to upper levels and by setting stable layers at the 5-km level. With the dry condition throughout the troposphere only shallow clouds are simulated. With moist layers within the 3-km height most of the convection is still composed of shallow cumulus and its vertical development is strongly limited. As moist layers becomes above the 3-km level the development of cumulus convection that has a congestus or cumulonimbus character is simulated; and further increasing the depth of the moist layer results in prevalent deep convection and the existence of the three mode of cumulus convection. A mid-level stable layer that has an averaged feature does not prevent the vertical extent of convection; only a strongly stable layer that is apart from one standard deviation can inhibit deep convection. The numerical experiments showed that the height of tropical convection is sensitively controlled more by mid-level moisture profiles than by mid-level stable layer.

Moisture profiles at middle to upper levels play an important role in the vertical development of tropical convective clouds, and mid-level moist layer is favorable for the development of cumulus congestus and cumulonimbus clouds. Entrainment processes around cloud tops is a vital mechanism for the vertical development of convective clouds, and thus dry-air entrainment seems to be unfavorable for that development.