By Charles Blaisdell, PhD ChE
Abstract
Yes, it’s about warm air, lower humidity air from any parcel of land that has lower annual water evaporation over time (several years). Because of the lack of cooling of the evaporation of this type of parcel has a higher temperature and a lower specific humidity, SH, than in the virgin state and can produce a plume of clouds retarding more “vapor pressure deficit”, VPD, online. Urban Heat Islands, UHI’s, forest to cropland, forest fires, and mining are good examples of this parcel type. The size of this plume is an amplifying factor in cloud retardation. High VPD air mixes with passing air in the lower cloud zone (cumulus cloud height) and prevents cloud formation everywhere.
Data from the “Soundings” weather balloon show that higher VPD air is created over the cloud-free UHI and can be 1 to 4 times larger than the UHI area, agreeing with models. The plume is created by the lower density of the heat-lower specific humidity, SH, in the air (lower ET, EvapoTranspiration) rising from the UHI and forcing turbulence (mixing) with SH air much less in the upper atmosphere. On cloudy days the cooler SH air has a higher density and specific humidity and does not rise as quickly or at all. The cloud-free noise data also show that the cloud-decreasing VPD is retained as air rises from this parcel.
Plumes increase the size of high VPD air from a specific parcel and can be a factor in climate change if the size, water evaporation, or albedo of a specific parcel changes. Urban areas are expanding, forest land is shrinking, and mining is increasing. The amount of change in all of Earth’s special parcels is unknown but may be significant. The size of the plume will increase the effect of parcel-specific cloud cover (or reflectivity) on climate change.
Introduction
Scientists have long known that the Earth’s cloud cover, CC, (fraction) is an important part of seasonal and annual climate change. (1). WUWT’s Willis Eschenbach (2) has proposed a theory of how increasing cloud fraction cools the earth (or decreasing cloud fraction heats the earth). This author agrees with Willis’s theory, and has proposed a theory about what might cause cloud changes. Cloud Reduction Theory Global Warming, CRGW, is: The sum of specific areas of the earth that have lost water evaporation over time (UHI, deforestation, mining, etc.) cover or thin clouds. Less clouds, more sun and higher temperatures and more water evaporation can be seen as higher global specific humidity. The CRGW theory is most applicable to the period 1970 to the present. The subject of this paper is part of this theory.
Figure 1. Visualization of the plume courtesy of 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 changes, see Figure 1 for a visual. The size of the plume increases the area of the Earth where high VPD air can prevent cloud (or thin) formation. For example, if a UHI has an area of X and produces a plume twice the size of the UHI, then the area of the Earth affected by that UHI is 2X. Ann Cosgrove & Max Berkelhammer (2021) (3) modeled the plume in Chiago as 2-4 times the size of the original UHI. Yifan Fan et al. (2017) (4) were also modeled with similar results. This plume is warmer and drier than the surrounding air, giving it a higher VPD (low potential cloud potential).
VPD and cloud cover (fraction)
VPD, vapor pressure deficit, is defined as the difference between the saturation vapor pressure, Psw, and the actual vapor pressure, Pw, (VPD = Psw – Pw). VPD is a logical relationship between atmospheric temperature and humidity, (specific humidity, SH) that predicts the likelihood of cloud formation. When VPD approaches 0 the atmosphere becomes full of clouds likely to form. (Although super saturation can occur (no clouds) or particles in the air can cause clouds before 0). On the basis of one point VPD is very nonlinear: clouds at 0, no clouds > 0. The global average of VPD has increased since 1970 showing that there are less 0 VPD (less clouds) on average than > 0 VPDs. See Blaisdell (2024) (10) for more information on VPD and clouds.
Plume volume and size
To better understand the air rising through UHIs weather balloon “soundings” are analyzed for some indication of the size of the plume. Weather balloons are launched around the world twice a day at 12:00 PM and 12:00 PM Greenwich time (Zulu time). For plume measurements, the data must be when the sun is shining and there are no clouds. The selected site is a group of cities called the Quad Cities Ia.-Il. (Davenport Ia. Bettendorf Ia, Moline Ill. Rock Island Ill). The area has expanded to include other cities including an airport in Coal Valley for land-based weather data). The balloons were launched in the middle of the Quad Cities (Davenport, Ia) not far from the airport at 6:00 and 18:00 local time. The time of 6:00 pm works for summer but not winter (there is no sunlight at 6:00 pm in winter). Sound for the month of July 2022 from the University of Wyoming College of Engineering (5) and daily weather data from Weather Underground (6) were sorted for cloud-free days in order to obtain a representative sample of days with a high number.
Meteorology plots sound data in a strange graph called “Skewed T log P” (the x-axis (temperature) is tilted to the right at 45⁰ and the y-axis (pressure) is a log scale (this plotting method is probably used to keep all the data in one piece of paper) Sky data that boos people; but for climate change, the sky data has some insight into the UHI plume. See the website (7) for a good summary of the Skew T Log P diagram.
Figure 2 Skewed T Log P diagram from (7).
The only part of the data in Figure 2 is of interest to understand the plumes of UHIs. The part below the cumulus clouds forms an area around 600-800 mb (4000m – 2000m). On the surface of the radiation is reflected as short-wave radiation or absorbed and reflected as long-wave radiation (heat of land and air and water evaporation) or used by plants and water transpires. Air from the process can rise, stay put, or sink depending on the density and surrounding air. Hot air rises cold air sinks. Water added to the air decreases its density at the same temperature, but the process of water evaporation causes the temperature of the air to decrease making it denser. A buoyancy calculation is required to determine the direction of the wind. The rising air will mix with the dry (and cold) air from the upper atmosphere. The initial speed (if rising) of this air (at 0 to about 3000m) should be related to the size of the plume. SH specific humidity, profile (Figure 3-a) shows this dilution of SH air is less than the upper atmosphere (above 4000m). The slope and height of the SH profile in the lower atmosphere is an indication of the availability of soil moisture (Denissen (2021) (8)), the higher the more vertical the slope indicates the lower soil moisture. In addition, shorter ones rise (cooler air does not rise as quickly as hot air), indicating higher soil moisture.
Figure 3b shows the VPD, Pws-Pw, and Figure 3c airspeed data increases from one sound. Above the initial rise (about 1600m in this example) of hot air, the passing weather front mixes with this ground air and forms higher VPD (or T-Td) air that mixes with the total air of the atmosphere and can reduce the cloud fraction. some place in the atmosphere. Each of these UHIs is a very small contribution to the total increase in global VPD but the total sum of all UHIs (and similar phenomena like deforestation and mining) over the years can be significant.
From the sound temperature data, the buoyancy can then be calculated. From the buoyancy of the air speed increase can be estimated, assuming the air rises to an average of 3000m. Data for 38 days in July-August 2022 are filtered for cloudiness (higher probability of plume with more than 3 sounding data points in stable area). Table 1 is 12 days of survival.
The rising air speed 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 = sound data point rotation rate temperature = Tii – 9.8 * (Hi – Hii)/1000 in K
Tii = dry lap level of the initial sound temperature in K
Hii = initial height in meters
Hi = the height of the data point of the sound in meters
Ts = Temperature of the data point of the sound in K
Air speed rising from buoyancy:
V = additional distance traveled / time to travel that distance, d / t in m / sec
d = noise additive distance between data points
t = (d * 2 / B)^(1/2) in sec
The initial average speed (see Table 1) of the clear sky noise is not directly related to the size of the plume but is a simple approximation:
Plume size, P = D / V * Hr
where:
V = average speed m/s
D = distance from ground to cloud level (assume 3000m)
Hr = time during the day this speed is maintained. (assuming 8 hours)
Conclusion for Tabe 1: there is a lot of variation in buoyant velocity (and thus plume size). The plume size factor, Pf, is calculated by:
Pf = 3000 m to cloud height / v(m/second) * 3600 seconds/hour * 8 hours/day
Pf = How much more area is added to the special parcel area.
Table 1 is not meant to be a strict plume size calculation but can be compared to model (4) (3). This also suggests that the plume size varies from site to site and may be different on average at other locations. Locations with less evaporation are expected to have warmer air rise at higher speeds and larger plumes; like wise, locations with high evaporation should be small or no plumes.
Table 1. Clear sky data from one month in July 2022.
The actual plume may be smaller than this due to turbulent mixing in the 2000m 4000m range indicated by the rapid decrease in SH in this region. Clear sky sounding data show that high cloud-killing VPDs (> 0) are maintained at 2000m to 4000m.
Plume size is an important multiplying factor in the CRGW model. The literature search revealed studies of UHI or other land change plume measurements that were not studied and provided opportunities for other researchers. No physical measurements of feather size could be found.
talk
Balloon soundings show clear skies agreeing with the model and high VPD air quenching clouds resulting from low ET surface air reaching cloud level. Plume size will remain a wide range (1-4x) variable in the CRGW model. More work is needed on global variation and plum size. Can satellites help? Satellites can see huge plumes of smoke from forest fires. Studying this short data soundings gave me a high respect for meteorology and as far as I go in Meteorology!
Bibliography
- “Clouds and relative humidity in climate models; or what really governs cloud cover?” by Walcek, C. (1996) Clouds and relative humidity in climate models? (Technical Report) |
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