By Richard Willoughby
(The author appreciates the availability NASA’s Earth Observations satellite data sets used in this analysis.)
Now a four part series that analyses the role of atmospheric water in regulating Earth’s thermal balance.
Part 4: The Atmospheric Gear Change
Total Precipitable Water & Level of Free Convection
In Part 2 of this analysis it was briefly mentioned that the atmosphere cannot form a Level of Free Convection (LFC) unless the Total Precipitable Water (TPW) exceeds 30mm. The listed conditions below all create an LFC at 500m:
- Surface temperature 298K, relative humidity 52% and TPW 3.1cm
- Surface temperature 293K, relative humidity 71% and TPW 3.1cm
- Surface temperature 288K, saturated and TPW 3.2cm
This demonstrates that an LFC can form under widely varying surface temperature and relative humidity but TPW remains consistent close to 3.1cm for an LFC 500m above surface level.
Part 2 also quantified the rate of condensation at 7.3mm/day if all OLR exists via the atmospheric column. This condition certainly holds once the TPW reaches 3.1cm due to the high long-wave absorption of water vapour, water condensate and ice. With the LFC at 500m and TPW of 3.1cm, there is 2.3cm (23mm) of water above the LFC. It would therefore take 75 hours for the column to develop full CAPE. It is likely that divergence or other disturbance disrupts the full development such that any cloudburst is weak.
It is observed that cloudburst cycles become more frequent once the TPW reaches 4.5cm. Typical surface conditions for occasional cloudburst are 296K and 80% humidity. This results in the LFC at 2000m with 10mm of water vapour above the LFC and 7mm of water above the level of freezing if the relative humidity is constant. The full CAPE can be recharged in 33 hours and the cirrus cloud persistency is nominally 70% of the full CAPE development phase.
Atmosphere in Overdrive
An interesting observation results when combining an understanding of deep convection as discussed in Part 2 and above with the actual ToA outgoing EMR data analysed in Part 3. Here in Figure 17, the twelve monthly regression lines for the EMR versus TPW are replotted on a single chart.
Figure 17: Regression lines for twelve monthly plots of ToA outgoing EMR flux versus TPW
With reference to Figure 17, there is a vertical line shown at 4.5cm, termed “Threshold” that gives the least squares error to the points of intersection. This is the TPW where the atmosphere goes into overdrive and deep convection sets in. Above the Threshold, the atmospheric water provides cooling principally by regular cloudbursts catapulting water vapour above the LFC to form highly reflective cumulus cloud and then persistent cirrus cloud while the CAPE is recharging. Once deep convection sets in, the increased reflection of ToA insulation trumps the reduction in OLR so ToA radiating power increases.
Below the Threshold, the water vapour acts as a warming agent by reducing the radiating temperature of the atmospheric column thereby reducing ToA OLR without producing high level reflective cloud.
When the surface temperature is cooling, the slope of the regression lines or Atmospheric Water Cooling Coefficient (AWCC), introduced in Part 3, is negative. It is evident that there is a reduction in the radiating power of the atmosphere above the Threshold compared with the months where the surface was warming and the AWCC is positive.
Atmospheric water imbues the atmosphere with the ability to change gear in response to changes in the surface temperature. The ordinary gear and overdrive conditions are distinct around the Threshold TPW of 4.5cm. In ordinary gear, the atmospheric water is a warming agent and in overdrive it is a cooling agent. It is certainly not causing a “Greenhouse Effect” that is solely warming the planet. Atmospheric water is able to stabilise the surface temperature by allowing more surface insolation and reducing OLR power when the ocean surface is cool and restricting surface insolation more than reduction in OLR with reflective cloud when the surface is warm. Deep convection provides a precise regulating temperature of 30C annual average over open ocean warm pools.
Current climate models parameterise clouds and atmospheric water is treated as a “Greenhouse Gas” when it exists in the atmosphere as gas, liquid and solid. The solid phase is a key factor in the formation of reflective clouds. These phases are all responsive to surface temperature at the base of the atmospheric column and surface pressure to a much lesser degree in the observed range. The atmospheric gear change around 45mm TPW is not a simple process that can be emulated with a few cloud parameters.
Date Links for the Referenced Data
Note that the data was sourced for various time intervals, usually monthly, from these locations.
Richard Willoughby is a retired electrical engineer having thirty years experience in the Australian mining and mineral processing industry with roles in large scale operations, corporate R&D and mine development. A further ten years was spent in the global insurance industry as an engineering risk consultant where he developed an enduring interest in natural catastrophes and changing climate.