We diagnose formation processes of thin moist layers using the advection equation of water vapor. Because a thin moist layer is a local maximum of water vapor in vertical, we evaluate D(∂2q/∂z2)/Dt, where q denotes water vapor mixing ratio. The formation processes of thin moist layers by advection can be classified into four types. Two of them are essential: intrusion (∂2u/∂z2•∂q/∂x) and linear-shear (2∂u/∂z•∂2q/∂x∂z), where u denotes a horizontal wind component. In this study, we use relative humidity instead of water vapor mixing ratio.
We performed numerical experiments using a non-hydrostatic regional model PSU/NCAR MM5. We apply the above method to the case study of an observed thin moist layer in September 1999 to the south of the ITCZ in the equatorial eastern Pacific region. The intrusion of the moist air mass from the convective region to the dry region south of the ITCZ is properly captured by the diagnosis. We also performed downscaling experiments for 2005-06 for the same region. Two source terms show different vertical structures in probability distribution functions. The intrusion term has two local maxima at 5 and 15 km, whereas the linear-shear term has three local maxima at 5, 10, and 15 km.