Now that we understand the cells and the rising motion and divergence at the upper level, convergence at the lower level or vice versa, let's take it a bit further to explain rainfall distribution and the most prominent feature that occupies the whole world in the lower latitudes like the jet streams at higher latitudes and that is the inter-tropical conversion zone. So everything we have done so far assumed symmetry and trade winds were converging into the low and the hot region on the equator, but now we'll add a few more complications. This is something we already saw, the lapse rate. Maybe we didn't use exactly the same terminology, but we looked at this kind of idea before. It's warm near the surface, gets colder as you go above.
So you have so-called dry adiabatic rate at which temperature of the rising air drops at about 10 degrees centigrade per kilometer. So as we said, as air rises, pressure falls, so the air expands and cools. So how fast the temperature drops is the lapse rate. But once you hit the so-called lifting rate, condensation level, then remember the Clausius Clapeyron where we said we can increase the vapor pressure either by adding more water vapor or by cooling the temperature. You should stare at that curve as many times as needed to make sure you understand this concept.
So there was temperature versus vapor pressure and you had the exponential curve. And if you take a water parcel that's away from the saturation water vapor pressure curve, You can move towards the curve by reducing the temperature or by increasing vapor pressure, right? So here we are lifting air parcel from near the surface where it's not saturated, but it has water vapor in it.
As it rises, it expands and cools, so you are cooling the temperature and moving it towards saturation curve. So here is the lifting condensation level where the temperatures get cold enough, something like 2 degrees centigrade at, three kilometers where water within the droplet or the parcel begins to condense. And remember, we define the parcel as something that is moving so rapidly that it's not mixing with the surrounding environmental air.
What happens when the water vapor condenses? The condensational heat is released, so the cooling of the air parcel slows down. If it's dry, then it'll continue to cool as it rises, but then you have to understand where the buoyancy comes from, because if it has to rise, then it has to be lighter than the air that's already there. So if it got very warm, it's getting lighter and that's why it's rising and it'll continue to rise till it's lighter than the surrounding air, right?
That's buoyancy. So you hit lifting condensation level and then water vapor begins to modify the rate at which the temperature drops with height. So there you get wet.
adiabatic lapse rate, so temperature of rising air drops at 5 degrees centigrade per kilometer as opposed to 10 degrees centigrade per kilometer. So the wet adiabat and the dry adiabats are different. Adiabat is still the assumption that the parcel is rising without mixing with the surrounding environmental air.
Okay, take those and now put together our picture of the Hadley cell, Ferrel cell, polar cell into this map. which is showing now the winds, so we still have the northeast and southeast trade winds, we still have the subtropical highs which are changing seasonally, so this is in January and this is in July, this is the northern hemisphere winter, so the sun is to the south, and we have the subtropical highs that are still there, so on the ocean, where remember the Hadley cell is rising, where there is... warm air rising and convergence of trade winds so that is the ITCZ except now instead of the zonal mean meridional picture we have opened it up and we are looking at what we call before the zonal asymmetries so the distribution in the longitudinal direction which is the zonal direction, latitudinal direction which is the meridional direction it's not uniform in the longitudinal direction that's what we call asymmetry.
If you are mathematical, then take any quantity and derive and take a derivative with respect to x, where x is the derivative of y. This is the longitudinal direction. That gradient is not zero, then you have asymmetries in the longitudinal direction.
So there is a seasonal waxing and waning of the subtropical high and the subpolar low. Why? Because as the Sun moves north-south, the trade winds get stronger and weaker, and the convergence region moves approximately with the Sun.
If the Sun is to the south, you can see that the convergence zone here is way to the south of the equator, and there is reasons why it remains to the north of the equator in these two places here, for example. On land, it's digressing quite further because the heat content of the land, the rate at which land warms and cools, as the Sun is overhead or not, is much more rapid than the ocean. You expect the ITCC to be largely in the southern hemisphere with the sun, which we see here in a simplistic way.
There are more details which we are not getting into. When the convergence zone moves south, now the rising air and the sinking air in the Hadley cell also change. So if the convergence has moved south, the rising and sinking is stronger to the south than to the north.
So go back to the Hadley cell and think about, what is the difference between the two? moving that convergence zone. So you can see that the subtropical high is stronger in the southern hemisphere in the winter season when the sun is overhead and the ITCZ has been pulled south because the heating and the convergence is pulled south.
So the subtropical highs in the northern hemisphere have shrunk and become weaker except over land where Himalayan... glaciers, Siberian snow, Eurasian snow and so on make for high reflectivity in the winter, very cold temperatures and very high pressures, so you have a Siberian high there. Whereas in the boreal summer months when the Sun moves to the north, northern land is going to get warmer, northern ocean is going to get warmer, trade winds are going to now converge into the ITCZ which has followed the Sun approximately and it's to the north, you can see this is gone.
Way north over here, again there are details, this is the monsoon that is convergence zone over land, but you can look at the ocean. So these large changes in the ITCZ with the sun are important also to change the strength of the anticyclone, the cyclone in the seasons and so on. So the southern ocean on the other hand is very different, it is a channel. So the winds are going around and around, we will see the consequence on the ocean later on. So you can see that the polar front here and here are very different, because the land distribution here and the ocean land contrasts make for very different distribution of pressure centers, as opposed to the southern hemisphere, where there is almost uniform high pressure here, which has got seasonal waxing and waning as well.
But the subpolar low is not so obvious as it is over here. So the semi-permanent pressure systems as they are called are the subpolar lows and the subtropical highs in both hemispheres and then the ITCZ is moving north and south. The asymmetry of the ITCZ used to be always blamed on the fact that there is more land in the northern hemisphere.
land warms faster, so ITCZ can be in the warm hemisphere, but it turns out that it's a bit more complicated because the ocean is also moving energy as we will see later. So the ocean moves energy from the colder hemisphere into the northern hemisphere because of the ocean circulation that we will study later on in this chapter. And to balance the energy, atmosphere moves the energy back south across the equator and the southern hemisphere. and that makes the ITCZ the energy equator, so it moves to the northern hemisphere to balance the ocean heat transport.
If you don't understand it, don't worry about it, just remember that there is an asymmetry in the ITCZ that is related to energy transport as well. So you can look at the specific situation over here. You can see that the winds here are like northeast trade winds during the boreal winter months because the land is colder, ocean is warmer and lower pressure, higher pressure, so winds are going from higher pressure to lower pressure southward, so Coriolis is doing its thing and creating northeast.
trade wind like winds called northeast monsoon over India. But during the summer, land is warming lower pressure than the ocean. So high pressure, low pressure in a simplistic sense.
So winds again want to go from high pressure to low pressure. Coriolis is again tilting to the left in the southern hemisphere. They crossed the equator, so the winds are again being tilted to the right in the northern hemisphere.
So northward winds with the... pressure gradient are being moved to become southwesterly, so they're going from the southwest towards the northeast. So these are southwesterly monsoons in the summer, northeasterly monsoons in the winter, which is seen much more clearly here.
Winter monsoon, high pressure, wind direction is from the land onto the ocean. Some regions here get rain during the boreal fall winter months. During the summer there is low pressure, high pressure over the ocean, so wind is coming from high pressure to low pressure with Coriolis and then there are additional factors which are swinging them around. So you have south westerly winds and south west monsoon over India during the boreal summer months. So you can see there are different kinds of clouds involved in the whole process as well.
I won't go into the details. Cumulonimbus are the tall, deep convection clouds in the tropics. There are also cumulus clouds, which are not rain clouds. There are stratocumulus clouds, which are not as loaded with water droplets as the cumulus clouds. There are stratus clouds, low clouds, stratocumulus clouds, nimbostratus, altostratus, and so on.
These matter in terms of... how much solar radiation they allow in and how much outgoing long wave they trap. So that affects the budget and global warming as well.
Okay, so this is the distribution of rain you get. So here is the ITCZ with Amazon rain and the ITCZ or the Atlantic Ocean rain over Central Africa, the monsoon. So this is annual mean. So you can see that the monsoon, summer monsoon, winter monsoon, winter ITCZ, summer ITCZ have all been averaged here. So you get this kind of a split structure.
Maybe I'm wrong, maybe this is for, yeah, this is annual mean precipitation. And the northern regions where we had the convergence between the polar cell and the, so the ferrel cell where we said, There is a polar front that creates rain. This is the rain here. So this is dynamic ascent, tall, deep, convective clouds, whereas this is more of a convergence, rising motion and rain due to those kind of processes that are different than the warming and the rising air here.
This is forced ascent or dynamic ascent and this is kind of... forced ascent. Don't worry about the terminology.
So I'll come back and show a nice animation of the seasonal cycle of rainfall, which tells us a lot about how ITCZ and rainfall distribution works. There are a lot of issues involved in terms of scale selections. There are large areas where evaporation happens.
That evaporation is being supplied into the convergence zones, and the rainfall is distributed in this thin, narrow ITCZ. that is circum global so it's not very easy to explain those dynamic scale selections with just physics you need a lot of mathematics and other dynamics to explain those so we will deal with that when we come back okay