Recent studies on tropical convection have been focusing on the various type 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 tropical cumulus convection and environmental temperature and moisture profiles by use of observational data over the tropical western Pacific and performs a series of numerical experiments with a cloud-resolving model to examine the sensitivity of tropical convection to temperature and moisture profiles. The data used in this study were obtained by a Japanese research vessel called 'Mirai' in the western tropical Pacific region during the four cruises in 1999-2001 (the time period of the each cruise was about two to four weeks). The observational periods are divided into three types depending on the amount of rain observed: no rain, a little rain, and rain. In the no-rain period the most prominent cumulus mode is shallow cumulus, while in the other rainy periods three modes of cumulus clouds constitute the tropical convection. The sounding data analysis reveals that there is a close correlation between the shallow and mid-level cumulus clouds and the existence of dry layers at middle to upper levels, while a stable layer at around the 500-hPa level (0 degree C level) can be often found in all the observation period. The mid-level moisture variability is significantly large during the no-rain and a little-rain periods.

The cloud-resolving simulations with the Advanced Regional Prediction System (ARPS) employ two-dimensional idealized conditions in a resting atmosphere initialized by the Mirai observations. A series of sensitivity experiments are performed by gradually increasing the height of humid layers from low levels to upper levels and setting stable layers at the 5-km level. With the dry condition throughout the troposphere only shallow clouds can be seen. With the humid layers within the 3-km height most of the convection is still composed of shallow cumulus. As the humid layers reach above that level congestus-like and cumulonimbus-like convection appear, and further increasing the height of humid layers results in prevalent deep convection. An averaged stable layer at middle levels do not prevent the vertical extent of convection; only a strong stable layer can inhibit deep convection. These numerical experiments demonstrate the height of tropical convection is sensitively controlled by mid-level moisture profiles.