uwe.roland.gross

Soundings, Weather Balloons, and Vapor Pressure Deficit

From Watts Up With That?

By Charles Blaisdell, PhD ChE                                                

Abstract

Yes, it is about hot air, hot lower humidity air from any parcel of land that has lower annual evaporation of water with time (many years).  Due to lack of cooling from evaporation this type of parcel has a higher temperature and lower specific humidity, SH, than in its virgin state and can produce a plume of cloud retarding higher “vapor pressure deficit”, VPD, air.  Urban Heat Islands, UHI’s, forest to cropland, forest fires, and mining are good examples of this type of parcel.  The size of this plume is an amplification factor in cloud retardation.  The high VPD air mixes with passing air in the lower cloud zone (cumulus clouds altitude) and retards cloud formation somewhere.

Data from weather balloons “Soundings” suggest that a plume of higher VPD air is created over cloud free UHI’s and can be 1 to 4 time greater than the area of the UHI, agreeing with models.  The plume is created by the lower density of the hot-lower specific humidity, SH, air (lower ET, EvapoTranspiration) rising from the UHI and forcing turbulence (mixing) with the much low SH air in the upper atmosphere.  On cloudy days the cooler higher SH air has higher density and specific humidity and does not rise as fast or not at all.  The cloud free soundings data also suggest that the cloud retarding VPD is retained as the air rises from this parcel.

The plume amplifies the size of high VPD air from special parcels and can be a factor in climate change if the size, water evaporation, or albedo of the special parcel changes.  Urban areas are getting bigger, forested land is decreasing, and mining has increased.  The amount of change in all the earth’s special parcels is not known but could be significant.  Plume size will amplify the special parcel effect on cloud cover (or reflectivity) in climate change.

Introduction

              Scientist have long known that cloud cover, CC, (fraction) of the earth is a key part of seasonal and yearly climate change (1). WUWT’s Willis Eschenbach (2)  has proposed a theory on how increasing cloud fraction cools the earth (or decreasing cloud fraction heats the earth).  This author is in full agreement with Willis theory,  and has proposed a theory on what can causes cloud cover to change.  The Cloud Reduction Global Warming, CRGW, theory is: The sum total of the earth’s special parcels of land had deceased water evaporation over time (UHI’s, deforestation, mining, etc.) which results in higher VPD air rising in a plume to retard cloud cover or thinner clouds. Less clouds, more sun and higher temperature and more evaporation of water which can be seen as higher global specific humidity.  CRGW theory is most applicable in the 1970 to today period of time.   The subject of this paper is the plume part of this theory.

Figure 1. A visual of a plume is provided by Ann Cosgrove & Max Berkelhammer (3)

An important variable in the CRGW model is the size of the plume of hot-dry air that rises from local land change, see Figure 1 for visual. The plume size increases the area of the earth that high VPD air can retard (or thin) cloud formation. For example, if a UHI has an area of X and produces a plume of twice the size of the UHI then the area of the earth that is influence by that UHI is 2X.   Ann Cosgrove & Max Berkelhammer (2021) (3) modeled a plume over Chiago as being 2-4 time the size of the UHI it came from.  Yifan Fan et al. (2017)  (4) also modeled with about the same results.   This plume is warmer and dryer than surrounding air, giving this plume a higher VPD (potential less clouds). 

VPD and cloud cover (fraction)

              VPD, vapor pressure deficit, is defined as the difference between the saturated vapor pressure, Psw, and the actual vapor pressure, Pw, (VPD = Psw – Pw).  VPD is a logical relationship between atmospheric temperature and moisture, (specific humidity, SH) that may predict the probability of cloud formation.  As VPD approaches 0 the atmosphere becomes saturated clouds are likely to form.  (Although, super saturation can happen (no clouds) or particles in air can cause clouds before 0).  On a single point basis VPD is very nonlinear: clouds at 0, no clouds > 0.  The global average VPD has been increasing since 1970 indicating that there are less 0 VPD (less clouds) in the average than > 0 VPDs.  See Blaisdell (2024) (10)  for more information on VPD and clouds.

Sounding and Plume size

            To better understand the rising air over UHIs weather ballons “soundings” were analyzed for some hint of plume size.   Weather ballon soundings are released around the world twice a day at 12pm and 12am Greenwich time (Zulu time).  For plume size, the data needs to be when the sun is shining and no clouds.  The site picked was a group of cities called the Quad Cities Ia.-Il.  (Davenport Ia. Bettendorf Ia, Moline Il. Rock Island Il). The area has grown to include other cities including the airport at Coal Valley for land-based weather data).   The ballon is released in the middle of the Quad Cities (Davenport, Ia) not far from the airport at 6:00 am and 6:00 pm local time.  The 6:00 pm time works for the summer but not the winter (no sun light at 6:00 pm in the winter).  Sounding for the month of July 2022 from University of Wyoming College of Engineering (5) and daily weather data from Weather Underground (6) was sorted for cloud free days in order to get a representative sample of days with a high probably of a plume.

Meteorologists plot sounding data on a strange graph call a “Skewed T log P” (the x axis (temperature) is skewed right at 45⁰ and the y axis (pressure) is a log scale (this method of plotting is probably used to keep all the data on a single piece of paper).  Added to the graph are iso lines of mixing ratio (specific humidity), dew point, and more.  Meteorologists are mainly interested in weather change and have many terms (and abbreviations) for these graphs.  Calm clear sky data boors them; but for climate change, clear sky data has some insight into the UHI’s rising air that creates an invisible plume.  See the web site (7) for a good summary of Skew T Log P diagrams.

Figure 2 Skewed T Log P diagram from (7).

The only part of Figure 2 data of interest to understanding plumes from UHIs. Is the part below cumulus cloud forming area about  600-800 mb (4000m – 2000m).  At the surface radiation is reflected as short wave radiation or absorbed and reflected as long wave radiation (heats the land and air and evaporates water) or is used by plants and transpires water.  The air from these processes can rise, stay put, or sink depending on its density vs the surrounding air. Hot air rises cold air sinks. Water added to air decreases its density at the same temperature, but the process of evaporating water causes the air temp to decrease making it denser.  A buoyancy calculation is needed to determine which way the air is going.  Rising air will mix with the very dry (and cold) air from the upper atmosphere.  The initial speed (if rising) of this air (in the 0 to about 3000m) should be related to plume size.  The specific humidity SH, profile (Figure 3-a) shows this dilution from low SH air from the upper atmosphere (above 4000m).  The slope and height of the SH profile in the lower atmosphere is an indication of ground moisture availability (Denissen (2021) (8)), the higher to more vertical the slope suggest lower soil moisture.  Likewise, the shorter the rise (cooler air does not rise as fast as hot air), suggest higher ground moisture.

 Figure 3b shows VPD, Pws-Pw, and Figure 3c the rising air velocity data from one sounding.  Above the initial rise (about 1600m in this example) of hot air the passing weather fronts mix with this ground air and form a plume of higher VPD (or T-Td) air that mixes with the total atmosphere’s air and may reduce the cloud fraction some place in the atmosphere.  Each one of these UHIs is a very very small contribution to the total global increase in VPD but the sum total of all UHIs (and other similar phenomena like deforestation and mining) over years may be significant.

From the sounding temperature data, the buoyancy can then be calculated.  From the buoyancy the speed of the rising air can be estimated, assuming the air rises to an average of 3000m.   The data for 38 days in July-August of 2022 was screened for cloud free daylight (higher probability of a plume with more than 3 sounding data points in the stable region).  Table 1 is the surviving 12 days.

The velocity of rising air is calculated from the buoyancy equation (see (9) for derivation):

 B = (Ti – Ts)/Ts * 9.8

Where:

B = buoyancy in m/sec^2 or N/kg

Ti = laps rate temp of sounding data point = Tii – 9.8 * (Hi – Hii)/1000 in K

Tii = dry laps rate of initial sounding temperature in K

Hii = initial altitude in meters

Hi = altitude of data point from sounding in meters

Ts = Temperature of data point from sounding in K

Rising air velocity from buoyance:

V = incremental distance traveled/time to travel that distance, d/t in m/sec

d = sounding incremental distance between data points

t = (d * 2 / B)^(1/2)  in sec

The initial average velocity (see Table 1) from clear sky soundings is not directly related to plume size but is a very simple estimate:

Plume size, P = D / V * Hr

Where :

V = average velocity m/sec

D = distance from ground to cloud level (assume 3000m)

Hr = time during the day this velocity is maintained.  (assume 8 hr)

The conclusion for Tabe 1: there is a lot of variability in the buoyant velocity (and thus the plume size).  The plume size factor, Pf, is calculated by:

Pf =  3000 m to cloud height / v(m/sec) * 3600 sec/hr * 8 hr/day

Pf = How much more area is added to the area of the special parcel.

Table 1 is by no means a rigorous calculation of plume size but it does compare well with models (4) (3).  It also implies that the plume size is variable at each site and probably different averages at other locations.  Locations with less evaporation are expected to have hotter air rising thus higher velocities and larger plumes; like wise, locations with high evaporation should have smaller or no plumes. 

Table 1.  Clear sky data from one month in July 2022.

The actual plume may be smaller than this due to the turbulent mixing in the 2000m 4000m range indicated by the sounding rapid decrease in SH in this area.  The clear sky sounding data does indicate that the high cloud killing VPD (> 0) is maintained in the 2000m to 4000m range.

Plume size is important multiplying factor in the CRGW model.  A literature search indicates the study of UHI or other land change plume size is very much understudied leaving opportunity for other researchers.  No physical measurement of plume size could be found.

Discussion

Balloon soundings showed there are clear sky plumes that agree with the models and the cloud killing high VPD air generated from low ET surface air reaches cloud level.  Plume size will remain a wide range (1-4x) variable in the CRGW model.   More work is needed on the global variability and size of the plumes.  Could satellites help?  Satellites can see large smoke plumes from forest fires.  This short study of soundings data gave me a high respect for meteorology and is as far as I go in Meteorology!

Bibliography

  1. “Clouds and relative humidity in climate models; or what really regulates cloud cover?”  by Walcek, C. (1996)  web link Clouds and relative humidity in climate models; or what really regulates cloud cover? (Technical Report) | OSTI.GOV

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