Anvil Clouds of Tropical Mesoscale Convective Systems in Monsoon Regions

Cetrone, J., University of Washington

Cloud Distributions/Characterizations

Cloud Life Cycle

Cetrone J and RA Houze. 2009. "Anvil clouds of tropical mesoscale convective systems in monsoon regions." Quarterly Journal of the Royal Meteorological Society, 135(639), 10.1002/qj.389.


CFAD showing the frequency distribution of CloudSat CPR reflectivity as a function of height of thick (>6 km) MCS anvil over (a) West Africa, (b) the Maritime Continent and (c) the Bay of Bengal. The contours show bin counts divided by total counts. The contour interval is 0.001, and contour values range from 0.001 to 0.018. Bin dimensions are 5 dBZ by 250 m.


CFAD showing the frequency distribution of CloudSat CPR reflectivity as a function of height of thick (>6 km) MCS anvil over (a) West Africa, (b) the Maritime Continent and (c) the Bay of Bengal. The contours show bin counts divided by total counts. The contour interval is 0.001, and contour values range from 0.001 to 0.018. Bin dimensions are 5 dBZ by 250 m.

In the tropics, upper-level clouds containing ice and mixtures of ice and liquid water strongly affect the transfer of radiation through the atmosphere. A large proportion of these clouds are associated with deep convection, which generates and lifts hydrometeors to upper levels. Systems of deep convection that are larger than ~100 km in dimension are called mesoscale convective systems (MCSs), which often develop large stratiform precipitation regions, and they account for a large fraction of tropical precipitation, anvil cloud and cirrus. In addition to producing much of the precipitation and latent heating in the tropics, they also affect the profile of radiative heating. This study constructs a joint climatology of both precipitation structure and anvil characteristics of tropical MCSs to help lay the groundwork for global calculations of the effect of MCSs on radiative heating in the tropics.

MCSs are analyzed over three tropical locations: the Maritime Continent (Indonesia/Malaysia/North Australia), the Bay of Bengal, and West Africa. These regions are heavily impacted by monsoonal precipitation falling from large MCSs. West African MCSs are associated with strong positive buoyancy through much of the troposphere and are frequently composed of intense squall lines with high rain rates and dense anvil clouds. The Maritime Continent consists of islands, peninsulas, and intervening oceans and is often a mix of continental and oceanic air masses. Maritime Continent MCSs tend to have large stratiform rain areas and extensive anvil clouds and cirrus. MCSs over the Bay of Bengal exhibit characteristics of oceanic convection with large stratiform rain areas and extensive anvil clouds.

The precipitation structures of MCSs were analyzed using data from the Tropical Rainfall Measuring Mission’s (TRMM) Precipitation Radar, and the anvil cloud structures were investigated using the CloudSat Cloud Profiling Radar. We verified the CloudSat against the ARM vertically pointing cloud radars in Niamey and Darwin and found a high degree of consistency. The precipitation structures were partitioned into convective and stratiform profiles. The anvil clouds were subdivided by thickness: thin (0 to 2 km), medium (2 to 6 km), and thick (greater than 6 km). The data are displayed using Contoured Frequency by Altitude Diagrams (CFADs).

In general, the anvils generated by MCSs in West Africa, the Maritime Continent, and the Bay of Bengal have similar statistics. However, these statistics are dominated by the large numbers of measurements of the extensive thin portions of MCS anvils. The thin anvil clouds are generally located at about the 12- km level and are nearly identical in structure on radar. The portions of anvil clouds that are of medium depth also have very similar structures in all venues.

Most of the variation in anvil structure among MCSs is evident in the thick portions of anvils. The CFAD of West Africa thick anvils (Figure 1) shows a broad, flat histogram of reflectivity and a maximum of reflectivity in the lower reaches of the anvils. In contrast, the CFAD of the Bay of Bengal thick anvils has a sharply peaked distribution of reflectivity at all altitudes with modal values that increase monotonically downward. The reflectivity histogram of the Maritime Continent thick anvils is intermediate between that of the West Africa and Bay of Bengal anvils, consistent this region’s mix of land and ocean influences. This observed difference between the statistics of the continental and oceanic anvils implies that future parameterizations and representations of anvil clouds in climate and numerical prediction models will have to be flexible enough to account for different anvil physics in the two regions.

The difference between the statistics of the continental and oceanic anvils appears to be related to some combination of two factors. (1) The West African anvils tend to be closely tied to the convective regions of MCSs, while the oceanic anvils are more likely to be extending outward from large stratiform precipitation areas of MCSs. (2) The West African MCSs result from greater buoyancy; consequently, the precipitating cores are deeper and have heavier rain, and stronger convective updrafts almost certainly produce copious graupel. Because graupel has higher fall velocities than smaller, less rimed ice particles, it is therefore likely to fall out of or be detrained from the updraft cell before being lofted to cloud top.