Precipitation

Question 1

What are the three main types of precipitation?

Answer 1

1) Convective precipitation:
Heated air near the ground expands and absorbs more water moisture. The warm moisture-laden air moves up and gets condensed due to lower temperature, thus producing precipitation. Convective precipitation spans from light showers to thunderstorms with extremely high intensity.

2) Orographic precipitation:
The uplifting of air is caused by natural barriers such as mountain ranges.

3) Cyclonic precipitation:
The uneven heating of the earth’s surface by the sun results in high and low pressure regions, and air masses move from high pressure regions to low pressure regions. If warm air replaces colder air, the front is called a warm front. If cold air displaces warm air, its front is called a cold front.

Question 2

What effects precipitation?

Answer 2

Precipitation is derived from atmospheric water. Atmospheric moisture is a necessary but not sufficient condition for precipitation. Other factors such as wind, temperature, atmospheric pressure and local landscape can influence precipitation.

Question 3

How is it measured?

Answer 3

Precipitation events are recorded by gauges at specific locations. Point precipitation data are used collectively to estimate areal variability of rain and snow. Rainfall data are usually represented as mm/hour, mm/day, etc.

Question 4

Why may a rain gauge give bad readings?

Answer 4

This change could be due to gauge re-siting, growing trees, etc.

Question 5

What are the methods of finding areal precipitation?

Answer 5

1) Arithmetic Mean
- This is a simple method and can be used when the gauges are uniformly distributed.
- method is course, should only be used for monthly and annual rainfalls, & for many uniformly spaced rain gauges

2) Thiessen Polygon Method
- area of influence given by bisecting lines joining the closest raingauages
- does not take into account altitude so can not be used for mountains

3) Isohyetal Method
- rainfall contours (isohydrets) are drawn between rain gauges over a contour base
- arithmetic mean formula
- method subjective requires skill, however exposure and topographic effects can be taken into account

4) Geostatistics
The conventional methods cannot estimate the uncertainty with the result. Geostatistical methods can be used to compute best estimates as well as error bands that describe the potential magnitude of the estimation error. The uncertainty information is useful for decision making (e.g., to add extra rain gauges if the uncertainties are large at certain points). Kriging is a typical method in this category.

Question 6

Precipitation is typically measured using weather radar and rain gauges. Explain the benefits and drawbacks of each method

Answer 6

Raingauges

• measure actual rainfall that falls on the the earths surface
• remote sensing are calibrated against these
• point accurate do not show variability over catchement
• Rain gauge errors can occur if the rain is blocked, the raingauage is overshadowed by growing trees/bushes, mechanical failure

Weather Radar

• provides spatial variability of the rainfall field
• give indirect estimate as it falls in the air, calculated from the echo reflected from raindrops
• precipitation may have evaporated before it hits the ground

Precipitation may be overestimated:

1) reflectivity
- melting snow has a higher reflectivity than rain drops

2) Clutter
- birds, insects also provide reflectivity measurements which are hard to distinguish from rainfall

3) radar beam may overshot rainfall landing leading to underestimate