Reevaluate Hadley Circulation

Reevaluate Hadley Circulation Wei Huang Universal Weather & Aviation

Hadley circulation can be described as that strong heating at the equatorial surface, convection occurs as strong flow near the surface towards the equator. The heated air rises at equatorial area and forms upward flow to the higher latitude at upper level, and then sinks at higher latitude. The arising air at the equatorial area forms the Inter Tropical Convergent Zone (ITCZ). The ITCZ should be closer to equator and strong in Spring and Fall as the sun is near equator in these two seasons.

Since ITCZ is resulted from strong surface heating, and it is in the form of strong convective cloud, and it produces lots of rains, we can see there are two issues here: * Cloud in the equatorial will block the solar radiation reach the ground. * The precipitation will cool down the sea surface temperature. The two of them together will cause the ITCZ to oscillate, as: 1. The sun heats the sea surface, and cause SST increase, which cause evaporation and then eventually to form cloud. 2. The cloud blocks solar radiation, and rainfall cool down SST, which reduces the evaporation and causes the cloud decreasing. 3. Solar radiation become strong, and heats the surface to evaporate, and forms cloud. 4. Back to 1. And we did not see strong ITCZ at equator in Spring and Fall.

Here, we want to introduce a new concept, in which the sun heat the equatorial region, and generates outward flow in the surface; the air flow converge at higher latitude, produce precipitation there, and flow back towards the equator in the upper tropopause, and then descending in the equatorial area. There are few advantages of such a circulation: The equatorial area has sink motion, which keeps it clear, so the solar radiation can reach the surface all the time. So the evaporation can be continuous, as there is no cloud to block the sun. The precipitation happened in the higher latitude and cools the surface there, but that does not have negative impacts on the generation of evaporation and the surface flow. The latent heat released at higher latitude forms a warm high in the upper tropopause, which produces equator-ward flow.

The above conclusion can be derived by combine motion equation and state equation. Then we can see that the motion is controlled by the combination of temperature (gradient) field, and air density (gradient). As there are two terms at the right hand side of the new motion equation, we will define two motion systems, can name the gradient term as thermal term, and the density gradient term dynamic term, where if in a system that the (absolute of) thermal term is great than the (absolute of) dynamic term, we can name such system as thermal system, otherwise, we name it dynamic system.

Now let us see what a thermal system look like: a. A warm centered anti-cyclone, or warm high; b. A cold centered cyclone, or cold low. And dynamic system will be: a. A warm centered cyclone, or warm low; b. A cold centered anti-cyclone, or cold high.

We can also image that the thermal system is easy to play a leading role, as it is resulted from temperature field, where temperature field can be changed as result from the net effects of solar and earth radiation, and certainly from other heating effects such as latent heating release. The dynamic system is kind of a response to some other forces, which such forces have created density field changes.

Back to the Hadley circulation, where it is said ITCZ is the leading ascending motion, and the subtropical is the responsive descending motion. Which is certainly contradictory to what we have defined the roles of thermal and dynamic systems.

Some people may argue that the subtropical high warm center was maintained by the heating effects of the subsiding. We will prove that it is, at least, not always the case, from the adiabatic equation where most meteorology text books used.

But there are still issues with this new proposal, which is based on this approach, there should be a high at equator at Spring and Fall equinoxes, but observations did not see this. How to explain this, please refer to my other talk: Diurnal and its Impacts on Trade winds and ITCZ.