CE322 Basic Hydrology

Jorge A. Ramirez

Computation of Precipitable Water - Example

Compute the precipitable water for a saturated column of atmosphere extending 10 km up from the ground surface. Assume the surface pressure is 101300 Pa, the surface temperature is 30 C and the ambient lapse rate is 6.5 K/km.

Precipitable water:

The solution is presented in tabular form. The columns of the table are computed as indicated below.

Column 1: Elevation in km

.

Column 2: Linear vertical temperature distribution.

Column 3: Vertical pressure distribution assuming hydrostatic conditions and linear temperature distribution:

Column 4: Saturation vapor pressure:

Column 5: Moist air density

Column 6: Specific humidity

Column 7: Average specific humidity

(1)

Elevation

(2)

Temperature

(3)

Pressure

(4)

Vapor Pressure

(5)

Density

(6)

Specific Humidity

(7)

Average Specific Humidity

(8)

Dp/g

(9)

-qv Dp/g

(km)

C

K

Pa

Pa

kg/m3

qv

   

kg/m2

0.00

30.00

303.00

101300.00

4310.66

1.15

0.03

     
             

0.02

-2126.62

44.05

2.00

17.00

290.00

80437.86

1934.12

0.96

0.01

     
             

0.01

-1757.16

20.10

4.00

4.00

277.00

63200.08

804.92

0.79

0.01

     
             

0.01

-1438.96

8.50

6.00

-9.00

264.00

49083.90

307.27

0.65

0.00

     
             

0.00

-1166.85

3.30

8.00

-22.00

251.00

37637.08

106.16

0.52

0.00

     
             

0.00

-935.99

1.16

10.00

-35.00

238.00

28454.98

32.66

0.42

0.00

     
                   
                 

W=77.10

W is in units of mass (M) per unit area (kg/m2) which can be expressed also in units of volume (V) of liquid water per unit area.

W = 77.10 kg/m2 = 77.10 mm