Tropical Rainfall

Tropical rainfall often comes from the release of static instability by thunderstorms—what is called 'convective rainfall' (Chapter 9). This is true especially over land and over archipelagos

Table 7. 1 Pasquill's classification of the stability of the surface atmosphere, according to the degree of insolation and wind speed

Pasquill class

Condition of the air

Lapse rate (K/km)

Standard deviation* of wind direction (degrees)

A

Very unstable

-17

25

B

Unstable

-15

20

C

Slightly unstable

-13

15

D

Neutral

-lot

10

E

Stable

+5

5

F

Very stable

+25

2.5

* A measure of the scatter, discussed in Chapter 10 t That is, the dry a adiabatic lapse rate

* A measure of the scatter, discussed in Chapter 10 t That is, the dry a adiabatic lapse rate

Table 7.2 The circumstances of various degrees of surface-air stability

Rs of 290-580 W/m2

Slight Sunt, Rs of 145-290 W/m2

Cloudy, day or night

Clear night

0-2

A

A-B

B

-

-

2-3

A-B

B

C

E

F

3-4

B

B-C

C

D

E

4-6

C

C-D

D

D

D

6 +

C

D

D

D

D

* A clear sky, with the Sun higher than 60° above the horizon; Rs is the incoming solar radiation in W/m2 t A clear sky, with the Sun only 15-35° above the horizon

* A clear sky, with the Sun higher than 60° above the horizon; Rs is the incoming solar radiation in W/m2 t A clear sky, with the Sun only 15-35° above the horizon like Indonesia, where rainfall is more common during the afternoon because of destabilisation of the lower atmosphere by surface heating.

The role of atmospheric instability in causing rainfall over tropical oceans remains uncertain. One theory suggests that rainfall there happens chiefly in the early morning, because of particularly deep convection at that time when the water surface is warmer than the adjacent air. The convection is promoted by cooling of the upper air by longwave radiation lost to space during the night, making the troposphere unstable.

Figure 7.7 Typical conditions associated with a thermal. The dimensions are only indicative; the vertical dimension is exaggerated.

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