  Recent research has community have shown that tropical convection and
rainfall is sensitive to mid-tropospheric humidity. Therefore it has been
suggested to improve the representation of moist convection by making cumulus
parameterizations more sensitive to mid-tropospheric moisture. Climate models
show considerable rainfall biases in coastal tropical areas, where approx. 33 %
of the overall rainfall received is associated with coastal land-sea
interaction. Building on an algorithm to objectively identify rainfall that is
associated with land-sea interaction we investigate whether the relationship
between rainfall in coastal regions and atmospheric humidity differs from that
over the open ocean or over inland areas. We combine 3-hourly satellite
estimates of rainfall with humidity estimates from reanalyses and investigate
if coastal rainfall reveals the well known relationship between area-averaged
precipitation and column integrated moisture. We find that rainfall that is
associated with coastal land-sea effects occurs under much drier
mid-tropospheric conditions than that over the ocean and doesn't exhibit a
pronounced critical value of humidity. In addition, the dependence of the
amount of rainfall on mid-tropospheric moisture is significantly weaker when
the rainfall is coastally influenced. Our findings indicate that the
representation of convection in weather and climate models must cater for the
special character near coasts rather than rely on a common set of rules across
the globe.
