Convective systems

Question 1

Transport mechanisms by convective clouds with large vertical extent.

Answer 1

vertical transport from surface to tropopause
heat transport to higher latitudes
affect radiation fluxes

Question 2

Severe weather caused by convective clouds

Answer 2

Because of transport of latent heat. Flooding, hail and lightning

Question 3

Convective clouds cause mixing of air

Answer 3

including aerosols, moisture and pollutants and other gases.

Question 4

General types of hydrometeor

Answer 4

cloud-particles (aerosols) <0.1 mm suspended in air by updraft
precipitation >0.1 mm fall out of clouds that are deep enough for them to be created

Question 5

condensation nucleus (CCN)

Answer 5

an aerosol needed to form cloud droplets. Are abundant everywhere (100-1000 /cm^3). Can chemically reduce saturation pressure (ie. reduce supersaturation needed to start growing)

Question 6

formation of cloud droplets

Answer 6

Super saturation is needed for droplets to form on their own therefore there is condensation on CCN initially. limited by local supersaturation of vapour. Condensation is fast on small particles but slow on larger.

Question 7

Curved vs. flat surfaces

Answer 7

saturation vapour pressure is higher over curved surfaces so water evaporates easierly from droplets.

Question 8

Köhler-curve

Answer 8

Curve that shows the critical radius where a CCN gets activated at a certain supersaturation. Droplets that have been activated grow into size classes of cloud droplets. CNN get activated at about 100 nm

Question 9

approximate radius, range of concentration and typical concentration of small (aitken) CCN

Answer 9

<0.2 micrometers, 1000-10000 /cm^3, 1000 /cm^3

Question 10

approximate radius, range of concentration and typical concentration of large CCN

Answer 10

0.2-1.0 micrometers, 1-1000 /cm^3, 100 /cm^3

Question 11

approximate radius, range of concentration and typical concentration of giant CCN

Answer 11

> 1 micrometer, <1-10 /cm^3, 1 /cm^3

Question 12

approximate radius, range of concentration and typical concentration of fog and cloud droplets

Answer 12

> 10 micrometers, 10-1000 /cm^3, 300 /cm^3

Question 13

Two ways precipitation can be formed

Answer 13

Warm-rain process and ice-crystal process

Question 14

Warm-rain process

Answer 14

only creates liquid precipitation. Requires warm cloud base (>10 degrees) and no minus degrees in the cloud. Common in tropical and mid-latitudes. Activated CCN grow into rain drops. Rain drops fall through the cloud and grow through Coalescence. Too many aerosols will decrease the probability of rain (competition for water vapour). A certain updraft is needed to keep the big droplets suspended.

Question 15

Coalescence

Answer 15

Larger drops fall through a cloud and collide with smaller droplets that will stick to the drop. The collection efficiency depends on drop size and terminal velocity. This is low for small drops but increase at about 20 micrometers. Decrease for larger drops due to turbulence around the drop. Clouds need to be deep enough for larger droplets to be formed. Turbulence increases the collection efficiency since this bring more small droplets to the big drop to collide with.

Question 16

Ice-crystal process

Answer 16

Requires sub-zero cloud temperature
Works even if droplets are too small for coalescence
All sorts of precipitation
Mid-latitudes and high latitudes or high reaching clouds.
Most relevant in Sweden

Question 17

Ice nucleus (IN)

Answer 17

aerosols that are insoluble and solid. Much rarer than soluble CCN
Without IN ice would only form at -40 degrees
Can nucleate ice in mixed-phase clouds

Question 18

Wegener-Bergeron. Findeisen-process

Answer 18

If RH falls below 100% after activation ice droplets grow on the expense of water droplets since the saturation is lower over ice than over water. Difference largest at -15 degrees.

Question 19

Formation of ice-crystals

Answer 19

Might grow from evaporation if ascent is weak.
When they are large enough they start to fall
In clouds with high liquid water content they can collide with supercooled droplets. This is growing by accretion and form graupel.
When they collide with each other thay can splinter and form tiny secondary ice particles
In clouds with low liquid water content they can stick together and form snow flakes (aggregation).

Question 20

Different forms of crystals and aggregates

Answer 20

depend on temperature and vapor availability.

Question 21

Hail, snow, graupel or rain.

Answer 21

Hail: Take turns in the cloud and grow through accreation. Fall during thunderstorms
Graupel: Similar to hail but fall during snowstorms and is softer and smaller
Snow: below freezing all the way to the ground
sleet: warm in between but deep freezing layer at ground
Freezing rain: sub-zero ground but warm above
Rain: warm all the way to the ground

Question 22

Classification of precipitation systems

Answer 22

Convective precipitation and Stratiform precipitation. Can co-exist in the same system

Question 23

Convective precipitation

Answer 23

Narrow and short lived
More intense because if strong updrafts.
Rarer since strong ascent is rare
Turbulent and nonhydrostatic

Question 24

Stratiform precipitation

Answer 24

widespread and long-lived
Weak because of lower ascent
Hydrostatic and Laminar

Question 25

Scales of weather studied in this course

Answer 25

Convective scale <20km - convective clouds
Mesoscale <2000 km - cloud systems, clusters of convective cells
Synoptic scale >2000 km - Low and high pressure system, continental scale

Question 26

Mechanical turbulence day vs. night

Answer 26

Day: Unstable conditions, strong flow, thermals, mechanical and thermal turbulance => Large eddies, strong turbulance
Night: Stable conditions, weak flow, mechanical turbulence => small eddies, low turbulence

Question 27

Losses and production of turbulent kinetic energy at surface vs. upper planetary boundary layer (PBL)

Answer 27

Surface: Gain from wind shear and bouyancy. Loss from transport and dissipation
Upper PBL: Gain from shear at inversion layer and advection from below. Loss from negative bouyancy and entainment

Question 28

Planetary boundary layer (PBL)

Answer 28

• Lower layer of the atmosphere which is influenced by the surface
• up to 1000-2000 m above surface
• Higher in summer when unstable and lower in winter when stable
• Wind speed increases with height
• Structure depends on surface parameters
• Strong vertical mixing

Question 29

Layers of PBL during fair weather

Answer 29

Day: High mixed layer (unstable) then entainment zone (stable). Above free atmosphere
Night: Low stable boundary layer, residual layer (may be unstabe), Capping inversion (stable). Above free atmosphere

Question 30

Horizontal and time scale of Conditional instability, symmetric instability and baroclinic instability

Answer 30

Conditional instability: <10 km and 8 min
Symmetric instability: <200 km ans 3 h
Baroclinic intability: ~ 2000 km and 3 days

Question 31

Necessary factors for thunderstorms

Answer 31

• High CAPE and some CIN to overcome
• Moisture
• Warmth in lower troposphere and coolness aloft (high CAPE)
• Most convective clouds have a warm low base (low LCL and LFC)

Question 32

What is needed for a parcel to have a LCL?

Answer 32

It needs to have high moisture and start close to surface

Question 33

Triggers for convective clouds

Answer 33

• Boundaries between air masses (fronts and frontal lifting, sea-breeze fronts, dry lines, gust fronts from other thunder storms)
• Coastal fronts (convergence due to friction)
• Mountains
• Areas of surface heating
• Areas of turbulent transport (evaporation)
• Atmospheric waves (gravity waves)