The real tropospheric atmosphere is neither absolutely dry nor completely saturated. It is in general moist but not saturated. Here the generalized potential temperature (GPT) was introduced to describe this humid feature of real moist atmosphere. GPT's conservation property in moist adiabatic process was discussed and proved. Comparisons of GPT in moist atmosphere with the equivalent potential temperature (EPT) in saturated moist atmosphere were made by analyzing three torrential rain cases occurring over Jianghuai Valleys in 2003, the north China in 2004, and with the typhoon Fung-Wong in 2008, respectively. Results showed that the relative humidity is not up to 100% even in torrential rain systems, the saturated condition for EPT is not always held, and thus GPT can describe the moisture concentration and moisture gradient better than EPT. The GPT's definition includes the process that the air changes from dry to moist, then up to saturated. Therefore, potential temperature (PT) and EPT can be considered as its two special status. Similar as PT and EPT, GPT can be used to study atmospheric dynamic and thermodynamic processes more generally because of its conservation property in moist adiabatic process.
Potential temperature (abbreviated as
PT and denoted by
Other than the absolutely dry or
completely saturated air, the moist atmosphere is the real object of atmospheric
research. Because the phase changes and microphysical processes are hard to
observe and estimate, progress in these fields is very slow. Xie [
Gao et al. [
The expression of GPT for the
nonuniformly saturated air, introduced by Wang and Luo [
On the basis of the molecular
statistics, if the specific humidity of the air is
The real atmosphere is neither absolutely dry nor completely saturated, therefore it is rational to introduce the condensation probability function and further study the dynamic and thermodynamic processes of the moist air.
In view of the small changes of a closed system, the first law of thermodynamics can be expressed by
Equation (
Based on the property of nonuniform saturation, the latent heat release can be expressed as
Furthermore, employing the
appropriate form of
Taking (
From the curve change of
Moreover, ignoring the individual
change of
From (
Recently Gao et al. [
The Meiyu is a typical weather
phenomenon along the Yangtze River and Huaihe River basins
during the early summer season. The comparison of the distribution for EPT and
GPT will be made within these Meiyu frontal rainstorms. In late June and middle
July, 2003, strong rainstorm and flood disaster occurred in Yangtze
River and Huaihe River basins. The onset of Meiyu was on 21st June and the outset on 22nd July. The precipitation mainly occurred
during two periods, one from 21st to 28th June when the
rainfall was almost concentrated in the Yangtze River basin with a center at the middle
Yangtze River, and the other lasting from 29th June to 11th July when the extensive rainbelt was located at Huaihe River basin.
Detailed surface precipitation observations can be found in Zhou et al. [
Figure
Distributions of the averaged (a) EPT and (b) GPT at 850 hPa from 29th June to 11th July in 2003 (unit in K; isoline interval of EPT and GPT: 2; isoline interval of RH: 10%). The blank zone is the Tibetan Plateau.
Seen from the meridional
cross-section (Figure
Meridional cross-sections of the averaged (a) EPT and (b) GPT (solid line) and RH (dashed line) along 115°E from 29th June to 11th July in 2003 (unit in K; the intervals: 2, 3, resp.; the interval of RH: 10%).
The above cases show the contrast
among EPT for saturated moist air and GPT for moist atmosphere during the Meiyu
period. China
is also often experiencing high-frequent and wide-range torrential rains, so it
is necessary to make a comparison with other forms of rainstorms. The heavy
rainfall occurring in North China in
August 2004 and Typhoon Fung-wong in 2008 is taken as examples. From 11th to 13th August 2004, a heavy rain occurred in North
China. Several past studies have simulated and analyzed this event [
An obvious wet trough with the
southwest to northeast orientation can also be discerned in Figure
Distributions of (a) EPT and (b) GPT (solid line) and RH (dashed line) at 700 hPa on 12UTC August 2004 (unit in K; the intervals: 4, 4, resp.; the interval of RH: 10%).
From the meridional cross
sections, similar to Figure
Meridional cross-sections of (a) EPT and (b) GPT (solid line) and RH (dashed line) along 117°E at 12UTC 12th August 2004 (unit in K; the intervals: 2, 4, resp.; the interval of RH: 10%).
The above analysis is on a heavy rainfall event occurring in North China. In the following, the features of Typhoon Fung-wong with plenty of water vapor are discussed in detail. As the first strong typhoon landed in China in 2008, Fung-wong is special for its large size, large-range influence, and high water vapor content. Particularly it had an imbalanced shape with clouds primarily seen in the south of the center during its early stage. The thick clouds were always located at its southeastern quadrant, accompanied by plenty of water vapor supply, inducing torrential rain with high intensity, large scopes, and long-time influence.
At 14UTC on July
25th, 2008, formed over the east ocean of Philippines, Fung-wong moved westward.
Subsequently, it evolved into a severe tropical storm at 08UTC
on 26th and into a typhoon at 20UTC on 27th. After landing
over the Hualian area of Taiwan
at 0630UTC 26th, Fung-wong continued to move northwestward. At 22UTC
28th, it landed at the town of Donghan in Fuqing city of Fujian
Province again with the central pressure of 975 hPa and the central maximum wind
speed of nearly 33 m s-1 (12-grade), and then moved northwestward but decreased gradually. In the
afternoon of 30th, it weakened into a tropical storm depression in
the northwest of Jiangxi
Province with its center shifting
southwestward slowly and passing through the southeast of Anhui Province. Fung-wong caused heavy
precipitation and flooding in the southeastern coastal region of southern China as well as Jiangxi Province. From the isobaric chart at 850 hPa
at 00UTC 28th July, the wind field of Fung-wong (the center located around
23°N, 121°E) had an asymmetrical structure (Figure
Distributions of (a) wind field (solid: velocity, arrows: wind vector, unit in m s-1), (b) PT (solid line) and RH (dashed line), (c) EPT, and (d) GPT at 850 hPa at 00UTC 28th July in 2008 (unit in K; the intervals: 2, 2, 4, resp.; the interval of RH: 10%).
The meridional cross-section (Figure
Distribution of (a) EPT and (b) GPT (solid line) and RH (dashed line) along 121°E at 00UTC 28th July 2008 (unit in K; the intervals: 2, 4, resp.; the interval of RH: 10%).
From the heavy Meiyu front rainfall of Yangtze River and Huaihe River basin to North China rainstorm and to the typhoon Fung-wong, the comparisons of the EPT and GPT distributions indicate that the GPT is able to reflect the effects of moisture and water vapor gradient better than EPT.
A further exploration of the
properties and application of the GPT introduced by Gao et al. [
This paper is supported by the State Key Development Program for Basic Research of China (Grant no. 2009CB421505), the Meteorological Special Project of The Ministry of Sciences and Technology of the People's Republic of China (Grant no. GYHY200706020), and the Project of the Natural Science Foundation of China (Grant nos. 40775031 and 40620120437).