08 Tropical meteorology


Focus on tropical meteorology concepts… extrapolation from mid-latitude

experience not necessarily direct…

Fundamental differences between extratropical and low-latitude weather and climate.

Lack of strong temperature contrasts and daily temperature changes

No ice or snow except at high altitudes

Synoptic changes small compared with diurnal changes

Convective storms much more common at low latitudes

Small atmospheric variations can produce large rainfall responses

Local effects tend to be larger in tropics over land areas

Most developing countries lie in the tropics or subtropics – this is simply true because most countries worldwide, by the Earth’s geography, are in this region and, with relatively few countries lying primarily in higher latitudes, especially outside of Europe. Thus it is not realistic to expect that all experiences of met services in Europe, Japan or the US will be adaptable to tropical developing countries. But why exactly isn’t this the case and why is it very important?

With the exception of some tropical countries with high altitude terrain (Andes in South America or small parts of South Asia and east Africa) there is little snow or ice at any time of the year. Thus measurements systems do not need to account for freezing conditions. Then, the synoptic changes from one day to the next can be quite small – at least in temperature, humidity and surface pressure. The diurnal changes tend to be larger than the synoptic changes. This is evident by comparing surface pressures from meteorological buoys at higher and lower latitudes (Fig. xx). The changes associated with synoptic-scale weather systems are dominant at the middle-latitude buoys, while the diurnal and semidiurnal changes dominate in near-equatorial locations.

Moist convective storms are much more common in the tropics than in higher latitudes, though they can also occur there. Since widespread stratiform rain events are less common in the tropics, the measurement of rainfall tends to be more challenging, since it is more likely to be associated with convective-scale storms. (FIG) The higher amounts of atmospheric water vapor in the tropical atmosphere means that more rainfall is usually produced by locally-generated breezes (such as the sea breeze) than at higher latitudes. The weaker synoptic scale forcing and stronger forcing by circulations driven by the diurnal cycle of solar heating leads to a more challenging situation with regards to forecasting rainfall, since it tends to be much more locally controlled. Departures for the “average” climatological conditions are then important, since these control the strength of the local circulations. This leads to difficult observational challenges. (need Fig’s?)

A quick assessment of the adoption of inappropriate higher-latitude procedures can be carried out when visiting any forecast office in the tropics. If surface pressure analyses are being produced then it is an indication that forecasters have been trained at higher latitudes or have adopted techniques commonly used for producing analyses for the middle latitudes. More will be said about why this is not particularly effective later in section (xxx).