Lec 4: Precipitation II

Question 1

Define return period and probability of occurrence

Answer 1

return period (T): the time for an event to return (ie the time between two occurrences of an event
T=100/P

Probability: probability that an event occurs in a given time
P = 100/T

Question 2

How can you derive P and T from a sample of data?

Answer 2

You need a sample of events ranked by magnitude.
n = rank of a specific event (in terms of magnitude)
y = number of events in the sample
You can use the Weibull or Hazen plotting positions.

The probability (p) that a storm of a given magnitude and duration occurs (or is exceeded) in any one year can be calculated as (Weibull method):

P = n/(y+1)
T = 1/P = (y+1)/n

Question 3

What are the variables on probability paper?

Answer 3

x-axis: T
y-axis: Magnitude of precipitation (in mm)

Question 4

How do you figure out the probability of an event of a given magnitude occurring during a given number of years?

Answer 4

P(T, n) = 1 - (1 - 1/T)^n

1 minus the probability of it not occurring in each of the n years

Question 5

What is precipitation driven by?

Answer 5

climate, specifically large scale circulation patterns

Question 6

What is the range of annual precipitation globally?

Answer 6

Global long-term annual averages range from 2
to 12 000 mm

Question 7

What can hold more water vapor, cold air or warm air?

Answer 7

warm air can hold more water vapor

Question 8

In terms of the water balance equation, what is the difference between rain and snow?

Answer 8

rain is a flux
snow is a store
snowmelt is a flux

Question 9

Will there be more or less snow in the
future due to climate change?

Answer 9

It could increase the amount of snow.
As temperature increases, humidity increases (it generally doesn’t snow much at very low temperatures)

Question 10

Compare 4 types of snow and their ages/densities

Answer 10

ranked from youngest to oldest:
- snow (youngest, least dense)
- neve (max 1 year, 500 kg/m3)
- firn (more than 1 year, 500 kg/m3)
- ice (many years, 850 kg/m3)

Question 11

Where does snow mostly occur?

Answer 11

in the northern hemisphere

Question 12

What are some of the roles of snow? (2)

Answer 12

• modifies energy and moisture fluxes (evaporation)
• represents a dynamic store in the hydrological system

Question 13

What is the albedo of new snow?
How about in forests?

Answer 13

0.8-0.9

Albedo of snow within forests is low (0.2), thus change of forest cover can have strong effects on large scale albedo

Question 14

How does snow affect energy exchange between surface and atmosphere?

Answer 14

Snow reduces energy exchange between
surface and atmosphere

Question 15

How does snow affect air temperature?

Answer 15

albedo

Positive feedback: the more snow, the more solar radiation is reflected, and thus the colder the air (but ground stays warm)

Question 16

How does climate change affect snow? (not just pertaining to its quantity)

Answer 16

• changes timing of snowmelt
• changes ratio of liquid-to-solid precipitation
• overall increase in precipitation (and thus snow) in a warmer atmosphere

Question 17

How does snow resemble and differ from soil?

Answer 17

► Snowpack acts like soil (or a sponge): it has grains of
different sizes and pockets of air that can fill with liquid
water (e.g., during rain event, or when melting)
► But unlike soil, it can change its structure and properties
quite quickly (snow metamorphosis)

Question 18

What are typical measurements of snow?

Answer 18

• snowfall (depth of snowfall from one snowfall event)
  errors due to snow drift and settling
  measured with a ruler?
• snow depth (depth of snow cover that accumulated on the ground)
  measure with a ruler, radar, ultrasound etc
  influenced by location of measurement
• snow water equivalent (of snow cover)
• timing and duration of snow cover on the ground

Question 19

What is solid precipitation?
What errors are associated with it, and what are they due to?

Answer 19

Water equivalent of snowfall
50% undercatch due to wind
measured with a standard precipitation gauge

Question 20

What is the density of fresh vs old snow cover?

Answer 20

Density of fresh snow ranges from 10 to 150 kg/m3, but mostly 50-120 kg/m3

Older snow cover has an average density of ~ 300kg/m

All this is highly heterogeneous in space
and time

Question 21

What is snow water equivalent (SWE)?
How is it measured?

Answer 21

SWE is defined as the DEPTH (in m) of water if the snow cover is completely melted

gravimetric method which involves
taking a vertical core through the snowpack and weighing or melting it; or by using “snow pillows” (which measure the change in pressure of an air cushion)

Question 22

What do you need to know to figure out the SWE of snow cover?

Answer 22

The mass of snow, and the cross-section area of the snow core

SWE = 0.001(mass of snow/area)

Question 23

What local characteristics do snow cover extent and depth depend on?

Answer 23

• topography (snow line, slopes, aspect)
• shading
• vegetation

Question 24

Compare the energy balance of snowcover in the middle of the winter and at the beginning of spring

Answer 24

middle of winter
- sensible heat flux into snow
- net radiation flux is out of snow

melting snow
- heat flux from rainfall, sensible heat flux and latent heat flux into snow
- net radiation flux into snow

Question 25

How are estimates of snow melt calculated?

Answer 25

temperature index snowmelt models

degree-day method
M = K(Ta-Tb)

M = mass of melt for the day
K = degree-day factor
ta = average air temperature
tb = base temperature, usually 0C

Question 26

Why does rain make snow melt?

Answer 26

condensation of water (latent heat transfer)
- rain = high air humidity
- water vapor condenses at cold surface of snow
- as it condenses, it releases heat
- condensed water infiltrates snowpack and distributes energy
- once the water cools to 0C, it freezes, releasing more latent heat

Sensible heat introduced by the rainwater itself is typically much smaller as the rain is close to freezing temperatures

Question 27

Why does snow melt quickly in warm, sunny conditions?

Answer 27

Advection
- once some snow disappears, the ground heats up as it has a lower albedo
- the spots of warm, wet ground deliver heat and humidity to the remaining patches of snow
- condensation speeds up the ripening of the remaining snow
- this effect stops once the bare ground dries up