Diurnal and its Impacts on Trade Wind and ITCZ

Diurnal and its Impacts on Trade Wind and ITCZ

Wei Huang

It is known from Hadley circulation that the trade wind is the steady and persistent winds which blow on the equatorial side of the subtropical high pressure in both hemispheres. Where the trade winds from each hemisphere approach each other, the rising air creates instability and results in a line of cumulonimbus clouds, which is known as Inter Tropical Convergence Zone (ITCZ). ITCZ and trade winds are closed related as this ITCZ is considered as the ascending part of Hadley circulation, where trade winds are the result of ITCZ near the surface.

There are few issues to believe that the ITCZ is the ascending part: first, it should follow the move of the sun, it may not necessary just beneath the sun's zenith point, as the earth surface is not in uniform, and the surface had variety in heat capacity, but it should be close the heating zone, and following the heating zone (with some reasonable delay); secondly, the strength of ITCZ varies, as it block the solar radiation, when it is strong, which again should make the trade winds fluctuation, which will be contradictory to steady trade winds observed.

There are nowhere people have linked the solar diurnal changes to the trade winds and/or ITCZ. In this talk, we want to link the solar diurnal to trade winds and ITCZ.

Following the discussion in the talk of Rethink Hadley Circulation, we can derive that solar diurnal circles play big role in the thermal system, as the thermal term changes the wind fields. As the solar diurnal forms a thermal tide traveling from east to west around the earth forever, this thermal tide generates an easterly wind with it, and it is also a function of latitude. The distribution of the easterly will cause negative vertical relative vorticity in the northern hemisphere, and positive vertical relative vertical vorticity in southern hemisphere, when the sun is at equinox, which in general, to create anti-cyclone in both hemisphere, and cause relative low pressure in the equatorial region, since there is no cyclone genesis, that is why the ITCZ is weak in the Spring and Fall season, which in Hadley circulation theory, the ITCZ should be located near equator and should be stronger in Spring and Fall.

We will discuss how the easterly produce very different effects in the Summer and Winter in our presentation.

We will present in theory how diurnal impacts trade winds and ITCZ, and then show some results from Model for Prediction Across Scales (MPAS), which is a collaborative project for developing atmosphere, ocean and other earth-system simulation components for use in climate, regional climate and weather studies.