عنوان مقاله [English]
Water vapor is considered to be one of more significant atmospheric constituents as it contributes in precipitation and is also a critical element of the climate system. This climate element extended from earth surface to upper level of troposphere that potentially precipitable. Accordingly it called perceptible water. Therefore The total atmospheric water vapor contained in a vertical column of unit cross-sectional area extending between any two specified levels, commonly expressed in terms of the height to which that water substance would stand if completely condensed and collected in a vessel of the same unit cross section. The total perceptible water is that contained in a column of unit cross section extending all of the way from the earth's surface to the “top” of the atmosphere.
The water vapor component of air requires monitoring because of its importance to weather and climate and the essential role it plays in the operation of the global water cycle. This is the reason that atmospheric perceptible water determines the potential value of precipitation as well as an index for forecast precipitation for a certain time of year. So it is an important variable in climatic study and climate change survey.
Precise calculation of perceptible water needs to know about the atmospheric water vapor amount. However the amount of water vapor is determined by many factors including atmospheric density, temperature, cloudiness, wind direction and velocity, humidity in top of geographical situation. All of these factors determine the capacity of humidity, distances from the water bodies as well as leading the water vapor to any location.
In this paper it has attempted to calculate perceptible water in atmospheric column from the earth's surface to the top of troposphere. This purposeful effort has been done to achieve the long term trends of precipitable water over Iran atmosphere. In order to accede this purpose the NCEP/NCAR data has been applied.
Data and Methods
Consider a column of liquid water with cross-sectional area, and certain height. This height is the perceptible water. One way to determining the mass of water vapor (in grams) in a vertical column one square centimeter in cross-sectional area that extends from Earth's surface to the upper reaches of the atmosphere. Mathematically, if x(p) is the specific humidity at the pressure level, p, then the perceptible water vapor, PW, contained in a layer bounded by pressures p1 and p2 is given by
Where g refers to gravity acceleration.
For the purposes of our study, the pressure and specific humidity of NCEP/NCAR which exist for each 6 hours has been used. This data characterized by 2.5 longitude*2.5 latitude degree resolution.
In order to find out what we had decided to find, many software including Grads, Surfer and Matlab have been used. So at first general characters including mean and coefficient of variation of percipitable water has been mapped. Then the trends maps have been represented. In addition we have used sequential Man-Kendal method to diagnosing trends during the time sequence.
To find the cause and the result of this trends, humidity advection and long term change in temperature as well as precipitation over Iran has calculated.
The mean of precipitable water in Iran is about 14.3 mm. According to coefficient of variation, the most stationary amount of the mean is at 18:00 o’clock meanwhile the most non stationary take place at 06:00 o’clock. Central Iran which located in dry region as well as far from water bodies, it contains much more precipitable water in compare to Zagros mountain chain. The most precipitable water as well as lowest coefficient of variation is happened in Caspian Sea coast as well as Persian Gulf coasts. In general mountains area has experienced more variation in compare with law areas.
In regard for trend analyses, spatial distribution of daily trends has been mapped in fig 1. As it is clear almost all over Iran (except in Bandar Abass and Chabahar) decreasing significant trend by -0.000043 to -0.000067 mm/day especially in south as well as southwest of the country has been occurred.
Fig 1: Spatial distribution of precipitable water over Iran (Mm/day)
In order to survey temporal variation and base on Man-Kendal test has been used. The result is displayed in fig 2. As it is clear since the first year under investigation, trend had decreasing behavior, while it beams significant in 1987.
Fig 2: Trend changes in precipatable water over Iran Using Man-Kendal Sequential test
These trends caused by decreasing in water vapor advection also result in increasing in temperature in addition to decreasing of precipitation over Iran.
Surveying precipitable water over Iran has shown that this climatic parameter is dramatically depending on local as well as global factors. Accordingly the most spatial variations take place in 00:00 and 06:00 o’clock.
Trends analyses have shown decreasing significant trend by -0.000043 to -0.000067 mm/day in the country.