TROPOSPHERIC TEMPERATURE GRADIENTS AND CONVECTION ARE VITAL FOR RAINFALL

[iapennell]

Dear Readers

Underpinning depressions (particularly the tropical variety), tall cumulonimbus clouds bringing heavy showers and thunderstorms and even widespread heavy rainfall or snowfalls occurring over hills in high latitudes have one vital ingredient for their occurrence: Convection. 

Atmospheric convection lies at the root of strongly rising air (or even gently- rising air originating over an ocean surface or moist land) that brings about cloud- formation and rainfall. If the surface of the land were much colder, the oceans frozen and/ or the upper troposphere a good deal warmer convection could not occur- anywhere, even in areas where the atmosphere rises because of convergence into the region (and rising air in the hot, steamy tropics and in the vicinity of deep mid-latitude depressions largely depends upon convection and the release of latent heat in the rising cooling air to fuel up-draughts). The situation with an atmosphere without convection means that the areas of rising air would invariably cool faster than the surrounding air- and become denser in the process: Thus the convection currents would be still-born (and probably before the moisture contained in the rising air- parcels cool to the point where the moisture condenses out).

The Earth's Hydrologic Cycle would be much weaker, and it would be largely restricted to the oceans- because rainfall over land requires moist air to be pulled inland by areas of vigorously- rising air associated with depressions (and, of which, convection, condensation and the release of latent heat as moisture condenses in rising air  is a major part). If the oceans were frozen, very little evaporation could happen in any case but- without mixing with air above it, the atmosphere in a fairly shallow layer over the surface (no more than 500 metres with strong winds, below 50 metres with light winds) would soon reach saturation during periods of nocturnal cooling leading to widespread dense fog (or freezing fog) out of which fine water droplets or ice-crystals would fall back to the surface. This process would balance the net evaporation rate from a frozen/ cold ocean surface. If convection cannot occur and rising air gets colder than the air around it (and no warmer than air up to 500 metres above it), that shallow layer of moist air is not going to go very far and even mixing with the air above is greatly limited by the air not been warmer than that above it- in the same way that today any rising moist air in the upper troposphere mixes very little with air higher up in the Stratosphere because of the very temperature profile of the atmosphere there.

It follows, therefore that the only moist layers of air would be shallow layers of air over the oceans if there could be no atmospheric convection on Earth: These shallow layers of air over the oceans would be at or very close to saturation all the time- with the result being that the world's oceans would be largely shrouded in fog- particularly at higher latitudes and over parts of the oceans in low latitudes that had come through a night of radiative cooling. With so much of the watery 70% of planet Earth shrouded in sunlight- reflecting fog the planetary albedo would be well above 40% resulting in sharply lower global temperatures (with the oceans freezing at higher latitudes).  Over the continents the lack of moisture and rainfall would quickly turn vast areas into desert: And bright deserts reflect more heat than dark vegetated land-surfaces and the very dry atmosphere above would assist in bringing about greater radiative heat loss- all of which would help keep the Earth even colder.

Continued below.    

                    

[iapennell]

Continued.

Whilst a World without convection is out of the question, mainly because the surface and lower atmosphere have a net heat surplus of about 50 Watts per square metre whilst the upper atmosphere loses heat at a rate of 50 Watts per square metre  (averaged globally throughout the year), one cannot underplay the importance of deep atmosphere convection in bringing clouds and rainfall across the World.  Even in higher latitudes in winter, the deep depressions that bring rain and snow depend (to a considerable degree) on convection to remain healthy. Oceans at higher latitudes cool only gradually with the season whilst prevailing westerly winds keep the western continental margins furnished with surface air that is just above freezing- point. Aloft, the air is extremely cold (-60C at 10,000 metres above the surface). The air temperature drop with height is thus just over 6C per 1,000 metres' elevation and moist air rising in a depression- and condensing it's moisture content as it goes- cools at a rate of almost exactly 6C per 1,000 metres- as the moisture condensed and frozen out gives up a lot of latent heat in the process of freezing out.  Thus the air stays marginally warmer than the air around it as it rises, causing the air to rise faster because being a little warmer it is lighter.  So the air rises faster and provides energy and impetus to the depression.  If our Winter depression then moves into Russia and entrains air that is below -10C at sea- level, then with the air at 10,000 metres still at -60C the lapse rate drops to 5C per 1,000 metres. Rising air in the depression still cools at 6C per 1,000 metres- and in rising higher becomes colder (and relatively denser) than the atmosphere around it. Thus convection stops, the colder air in the low elevations of the depression can no longer rise effectively- and our Winter depression quickly fills. This is why depressions penetrating the Central Arctic in February quickly run out of steam and fill and why deep depressions are normal in the Southern Ocean around Antarctica but you rarely, if ever, get a deep depression over the South Pole in any season.

Even in the deep tropics, areas of cooler sea- surface temperatures or warmer than normal conditions in the high atmosphere can sharply weaken convection and the rain- bearing thunder-storms that depend on it. The Intertropical Convergence Zone (ITCZ), where hot steamy air in low latitudes rises and fuels spectacular thunderstorms, is largely driven not by converging Trade Winds from the Northern and Southern Hemisphere but by the vigorous up-draughts of a myriad of thunderstorms which pump large amounts of latent heat into the upper atmosphere causing atmospheric divergence aloft and a fall in surface- pressure below. It is the powerful convection currents of thousands of thunderstorms that fuels the ITCZ and that, in turn draws in the Trade Winds below- sending Westerly Atmospheric Angular Momentum (AAM) through the frictional interaction of the easterly Trade Winds with the underlying surface. And that Westerly AAM, under current climatic conditions also helps to fuel depressions in higher latitudes with the attendant surface Westerlies acting as a sink for Westerly AAM.

Without the cloudy thunder-storm convection of a myriad thunderstorms along the ITCZ the entire house of cards of the global weather- machine would collapse. If the surface of the lands and oceans near the Equator were 5C colder than nowadays, but the upper-air is the same temperature almost all of the convection driving the ITCZ would collapse because the temperature drop with height would be such that rising parcels of air would become cooler than the surrounding air before much moisture could condense out. Relatively dry air at temperatures typical of the low atmosphere in the tropics cools at a rate of 10C per 1,000 metres, so convection would quickly run out of steam before condensation (and latent heat release) of warm moisture- carrying air reduces the effective lapse rate to 5C per 1,000 metres (which is what fuels deep thunder-storm convection).  So an Equatorial zone just a few degrees cooler than today at the surface would scarcely maintain an ITCZ, the Trade Winds converging from north and south of the Equator would be very weak and, without the transfer of Westerly AAM into the global atmospheric circulation there would be very little Westerly AAM transferred to higher latitudes- for which read, means no Circumpolar Vortex, no extra-tropical depressions and no Westerlies in higher latitudes. And that would be all for the want of vigorous thundery convection in the deep tropics!

The effectiveness in cooler- than- usual surface conditions in killing convection (and thus rainfall) in the tropics can be seen from low rainfall amounts on the coast of Ecuador and Peru- where the cool Humboldt current effectively kills convection and leads to semi desert conditions just inland.