Precipitation Analysis

Question 1

Types of precipitation

Answer 1

Depends on the mechanism of lifting

-Orographic (due to topography)
-Convective (due to convection)
-Cyclonic (due to cyclones)

Question 2

Lifting due to topography

Answer 2

1. Warm ocean - evaporation
2. Prevailing winds
3. Ascent up mountain - rain
4. Descent down mountain - lee-side downdraft - rain shadow

Question 3

Congestive precipitation

Answer 3

1. The air near ground gets heated and mixing
2. The air mass starts ascending
   - if the temp lapse rate of the air mass is smaller than that of one of the surrounding air, the vertical motion continues (INSTABILITY)
3. Condensation to precipitation

Question 4

Dry adiabatic lapse rate

Answer 4

Lifting a dry air mass without heat exchange. Temperature decreases due to expansion

Question 5

Moist adiabatic lapse rate

Answer 5

Lifting a moist air mass without heat exchange. Temperature decreases due to expansion but not as fast as that of the dry air mass

Question 6

Ambient temperature lapse rate

Answer 6

The surrounding air temperature also decreases with height. This is measured by air balloons

Question 7

Cyclone genesis

Answer 7

-air flow due to pressure gradient (high to low)
-deflection by Coriolis force
-resulting in a rotating motion
-twisting temp distribution

Question 8

Rain gauges (TBR) - factors that affect accuracy

Answer 8

• location
• strong winds
• field animal activities
• human activities
• unreliable in heavy rainfall
• ‘point’ measurement

Question 9

Weather radars

Answer 9

-uses reflected EM energy
-related to rainfall intensity (radar equation)
-can reveal spatial coverage by rotational scanning
-composes images with multiple radars

Question 10

Cons of weather radars

Answer 10

-not as accurate
-complex algorithm to calibrate before use
-large errors
-expensive to build/maintain

Question 11

Mass curves

Answer 11

-used due to rainfall being affected by non meteorological factors -> consistency needs to be checked
-plots of cumulative values of time dependant observations

Question 12

Measurements

Answer 12

• Depth (R) : mm
• Intensity ( I) : mm/h
• Duration (D) : mins, hours or days
• Areal Extent (A) : km2, miles2
  -Frequency (n) : occurrences out of N years
• Return period (T): one in T years

Question 13

Intensity

Answer 13

Measured as depth/time
-higher intensity causes less water infiltrated and therefore more surface runoff
-higher intensity can cause more damage

Question 14

Areal extent

Answer 14

-connective precipitation tends to be localised and sporadic whereas cyclonic precipitation often covers large areas with smooth distribution
-> discontinuation
-> should be a reduction factor when use point rainfall