Kevin Kilty

In a recent guest blogger essay a side debate broke out between Nick Stokes and I over whether or not it is possible for thermal convection from a surface, such as Albuquerque on August 3, 1993, which was my boundary layer example, to reach high above, and produce substantive cooling of the surface.

The debate started thusly from Nick:

“The atmosphere is a heat engine/refrigerator, and it works like this. When the lapse rate is less than the DALR (about 9.8 K/km), when air is forced to rise by turbulence, it expands and cools adiabatically as pressure reduces. The cooling rate is basically the DALR. With this greater than the lapse rate, the air becomes cooler and denser than the neighboring air, and so work is done pushing it higher. That comes from the KE of the wind. Work is done to transfer “cold” upwards – ie heat downwards, against the temperature gradient. It is a heat pump….”

Continued thusly:

“If the lapse rate is 7 K/km, and DALR is 10 K/km, then air that leaves the surface 3K warmer than surrounds [sic] will lose that excess at height 1000m. Then it works against gravity to go higher, so slows down.

If the lapse rate is 9 K/km, it can rise to 3 km. It can happen, but such thermals are not easy to find.”

And ended here with my final reply:

“ I would agree with you if all there was was a parcel with a tiny bit of K.E. in an environment lapse rate of 7-10 C/km; but that’s not all there is, and I am sayin’ no more until I have thought it all out…”

Last night I went looking for data that I hoped to find which would make my point. I found it in several forms in several different places.

First, rather than reiterate all the examples I gave in the original essay, I will reference the paper by Renno and Williams from which the RPV data came, and point specifically to Section 6, the discussion, which is jam-packed with such, and which ends with this…[1]

“The observation that the air participating in natural moist convection originates in the surface layer is not surprising. More than half of the solar radiation energy absorbed at the surface is transferred to the atmosphere as both sensible and latent. This energy is vertically transported by the convective motions, and then radiated to space.”

I would only add that the same is true for dry convection as well.

In addition, a typical spring/summer fair weather day here as one that begins largely clear, by 10 am or so fair weather cumuli appear, these may grow to a steady state density covering a great deal of the sky by 4 pm some days. After midday the cumulus may attain vertical development but by 8 pm the cumuli become pancake shaped and may vanish completely. 

I would also ask readers to examine the video in a comment by David Dibbell found here. What this fascinating Band 16 timelapse of the western hemisphere shows is, repeatedly one day after another is this. The hottest portion of the day (brightest yellow) begins in the east and propagates across Argentina/Chile, then Mexico and finally across the U.S. Southwest. In between, in what is the Amazon basin, and in time order with all the other activity in the video, is the flaring up of thunderstorms with tops near the tropopause. Action follows the Sun. Surface heating organizes it all.

Now, regarding the RPV gathered data at Albuquerque on August 3, 1993, the story is told in a couple of atmospheric soundings, which I fetched from the archive at University of Wyoming. A sounding made at 6am local time and a second one made at 6 pm local time are shown in Figure 2.

Figure 2. Soundings at 6 am and 6 pm. Albuquerque. August 3, 1993

Overnight, the ground surface and air with a kilometer of the surface cooled, became mechanically stable. At 6 am (Figure 2A) the potential temperature (theta) is lowest near the ground surface, but the Sun begins heating the surface to a superadiabatic layer. This is soon unstable and begins to convect – at first in a shallow layer near the surface then step by step to increasingly higher levels. Radiation from the warming surface also contributes heat to this layer.

The atmosphere is not static. It is dynamic and it is bootstrapping its way into a deeper and deeper layer of instability. By 6 pm (Figure 2B) the entire atmosphere from ground to 4,200m (2,600 above surface) has uniform potential temperature and any parcel heated to as little as 1K higher theta will always be adequately buoyant to reach a high elevation. Recall that surface measurements at 1:00 pm showed a pool of superadiabatic air at the surface 6K or more warmer than rising parcels. Overnight something like Figure 2A re-establishes itself through radiation cooling, perhaps some subsidence and adiabatic warming, and contact with the ground surface.

MONEX

In the summer of 1979 a field experiment (MONEX) designed to study the South Asian monsoon had discovered this same scenario taking place in the atmosphere above the Empty Quarter of the Arabian peninsula.[2] Peter Webster devotes Chapter 12 in his textbook to desert climates and makes use of the MONEX results.[3]

Conclusion

Just to head-off one objection early, I admit that mesoscale conditions may contribute to lesser or more development of this boundary layer convection. I have, in fact, observed surface turbulence and even a hydraulic jump in the atmosphere downstream from wave clouds over the Front Range. But not in the warm season. Perhaps boundary layer convection draws occasionally on some pre-existing potential or kinetic energy. Yet, where did this potential energy or kinetic energy come from? My view is it is a fraction of remaining available work from heat transfer by many other, earlier regional weather events, some of which makes its way into the general circulation.

References:

1-Renno and Williams, Quasi-lagrangian measurements in convective boundary layer plumes and their implications for the calculation of CAPE, 1995, Mon. Weather Rev., September, 2733.

2-Eric A. Smith, 1986, The Structure of the Arabian Heat Low. Part II: Bulk Tropospheric Heat Budget and Implications, Mon. Weather Rev., v.114, p. 1084. Note: I can’t seem to find Part I of this publication, but Part II contains more than adequate data and commentary.

3-Peter Webster, Dynamics of the Tropical Atmosphere and Oceans, Wiley Blackwell, 2020.