Representation of dry tropical layers and their origins in ERA‐40 data

The focus of this paper is the occurrence of dry intrusions in the tropical atmosphere and their origin. Dry layers are observed to have a wide range of thicknesses, so as guidance on the scales of interest the radiative impact of dry intrusions is investigated. Typically such intrusions would contribute an increase in OLR of 3 Wm −2 per 100 hPa thickness in a clear‐sky scenario and less if clouds are present. The velocity fields from ERA‐40 are used to calculate the back trajectories of air masses from radiosonde profiles. The humidity observed by the sondes is compared with humidity profiles reconstructed by taking ERA‐40 specific humidity from the origin of the trajectories, reducing the value only if saturation occurs along the trajectory. The agreement found provides a validation of the trajectory technique used, which is then used to infer the origin of dry air masses during November and December 1992 at Kapingamarangi, in the tropical West Pacific. A variety of behaviors is found, with many dry events clearly associated with air masses descending from the extratropics. Four particular dry events are discussed.