Convective and stratiform rainfall in the tropics

B. Geerts


Stratiform clouds are, by definition, stably stratified, and stratiform precipitation results from such clouds. Hydrometeors (snow or rain) in stratiform clouds grow primarily by descent through a widespread updraft whose magnitude is less than 1 m/s. The growth occurs primarily by continued condensation/deposition (i.e. the diffusion of water vapour onto droplets or ice crystals, respectively). Purely stratiform rain results from mid-latitude frontal systems, convergence into lows, or upslope flow, all situations in which the lower troposphere is stably stratified.

Convective clouds, on the other hand, result from buoyant ascent, whose magnitude (by definition at least 1 m/s) is 1 to 2 orders of magnitude larger than the widespread, forced ascent that produces stratiform rain. Convective rainfall, then, results from convective clouds. Hydrometeors are lifted in the updraft, where they grow primarily by accretion (i.e. collection by raindrops or riming by graupel). Localised convection can occur in areas of widespread, slow ascent as well (near fronts, lows, and in upslope flow), because uplift destabilizes the atmosphere. Therefore convective ‘cells’ are often found embedded in an area of stratiform precipitation. Recently it has been shown (1) that stratiform rainfall also occurs within mesoscale convective systems (MCSs), not just in their decaying stage, but more importantly, also in their mature stage, and the presence of a stratiform region is essential for the longevity of some MCSs. In mature MCSs, stratiform rainfall contributes 10-50% of the storm total. For orographic rainfall the boundary between the two types is less distinct, because even in a stable environment the updrafts may be strong (>1m/s), depending on the wind and the topography.

Raindrop growth in a stratiform cloud is slow, so its rain consists of small drops. Convective rainfall is heavier and the drops are larger. Therefore maps of radar reflectivity (which is a measure of rainfall intensity) can be used to diagnostically separate areas of convective and stratiform precipitation (2). Convective rainfall is also characterized by sharper spatial and temporal intensity gradients. The algorithm proposed by (2) combines both characteristics (high reflectivity and high reflectivity gradient) to define areas of convective rainfall.

A distinction between the two types of precipitation is quite useful because in the tropics (and in mid-latitudes in the warm season), the latent heat release (by condensation/deposition) peaks at a higher level in the troposphere in areas of stratiform rainfall. General circulation models are very sensitive to the profile of latent heating, especially in the tropics.

Tropical rainfall may appear to be essentially convective in nature, but experiments over the eastern tropical Atlantic, northern Australia, and the western equatorial Pacific have shown that almost all convection occurs in association with stratiform rain (3). The younger parts of the cumulonimbus clouds are 100% convective. Later, when convection decays, clouds become stratiform and co-exist with the embedded convective columns of rapid updraft. Stratiform rainfall generally occurs more frequently in the tropics, yet convective rainfall accounts for most (~70%) of the cumulative rainfall, because its intensity is so much higher.



  1. Houze R.A. Jr. 1993. Cloud dynamics. Academic Press, 573pp.
  2. Steiner, M. and R.A. Houze Jr., 1997. Sensitivity of estimated monthly convective rain fraction to the choice of Z-R relation. J Appl. Meteor., 36, 452-462.
  3. Houze, R.A. 1997. Stratiform precipitation in regions of convection. Bull. Amer. Meteor. Soc. 78, 2179-95.