Cumulus Clouds. What goes on inside a cumulus cloud?
-
Upload
bethanie-oconnor -
Category
Documents
-
view
246 -
download
0
Transcript of Cumulus Clouds. What goes on inside a cumulus cloud?
Cumulus Clouds
What goes on inside a cumulus cloud?
Conceptual Model
• Series of convective plumes rising to form individual turrets comprising cloud
• Each rising pulse a toroidal circulation
• Successive toroids give rise to mean upward current called updraft
• Sustained downward current between toroids, if existing, would be downdraft
Liquid Water Content
What causes liquid water content to be below adiabatic LWC?
• Lateral entrainment– Neutral mixing– Dynamic entrainment
• Cloud Top Entrainment
Bubble and JetModels of Convection
<= mixing lateral entrainment
Dynamic Lateral Entrainment
Dynamic Entrainment
Effects of Dynamic Lateral Entrainment
Effects of Dynamic Lateral Entrainment
Cloud Top entrainment
Deep Cumulus
• Must consider impact of precipitation on cumulus circulation
• Must consider pressure effects because of cloud depth– Thermodynamic pressure, ie hydrostatic pressure
– Dynamic pressure due to inertia of air motions
• Friction layer small compared to cloud and we generally ignore friction
Vertical Acceleration(using Pressure)
31 1 2 2 2 1
'
3
'
( ) ( )
1
i
o
u kf u f u
t x
p
x
g
Inertia
Pressure
Buoyancy
Vertical Acceleration(Using Total Pressure)
31 1 2 2 2 1
3
( ) ( )
1i
u kf u f u
t x
p
x
g
Inertia
Pressure
Buoyancy
Vertical Acceleration(using Exner function)
31 1 2 2 2 1
'
3
'
( ) ( )i
o
vv
o
u kf u f u
t x
x
g
Inertia
Pressure
Buoyancy
Traditional Buoyancy
'
1 0.61 1vvv l i o
o o
gg q q q
| | | |
Warm/Cold air rises/sinks
Vapor less dense than dry air
Liquid water loading
Ice water loading
Anelastic Approximation• Neglect frequencies higher than those
associated with meteorological phenomena such as sound wave frequencies
• Similar to incompressible assumption, but for a compressible system
0 (anelastic approximation)
or
0
i
i
i i
i
i
u
x
u ux tt x
Continuity Equation
t
j
j
j jj
j j j
ud
dt x
u uu
x x x
Multiply momentum equation (momentum form) by density:
3
3
3
1( )
1
hence,
ji i ii i
j
ijk j j k ii i
ij ijk j k i
j i
i jiijk j k i
i j
uu u uu u
t t t t x
k pf u g
x x
u pu f u g
x x
u uu pf u g
t x x
Multiply momentum equation (vorticity form) by density:
3
1( )
hence,
ji i ii i
j
ijk j j k ii i
jii
j
uu u uu u
t t t t x
k pf u g
x x
uuu
t x
3
3
( )
( )
ijk j j k ii i
ijk j j k ii i
k pf u g
x x
k pf u g
x x
Decomposition of Pressure into Dynamic and Buoyancy Pressure
u
v
w w
u pDynamic
t x
v pDynamic
t y
w pDynamic Buoyancy
t z
Dynamics (or inertia) Terms
2 2 3 3 3 21
1 1 3 3 3 12
1 1 2 2 2 13
( ) ( )
( ) ( )
( ) ( )
u
v
w
kDynamic f u f u
x
kDynamic f u f u
x
kDynamic f u f u
x
Buoyancy Terms
1
where
w
d v l i
m l i
m d v
ll
m
ii
m
Buoyancy g
g
g q q
q
q
Take divergence of density multiplied by three momentum equations and then result set to zero and
solve for pressure:
2 u v u wDynamic Dynamic Dynamic Buoyancyp
x y z z
or
2
2
''
u v ud
wb
o o d b
Dynamic Dynamic Dynamicp
x y z
Buoyancyp
z
p p p p p p
Where pressure is divided into dynamic and buoyancy pressure contributions
Buoyancy vs. Dynamic Pressure
• Dynamic pressure, , is zero if flow is at rest.• Buoyancy pressure, , is hydrostatic pressure for flow
at rest.• Dynamic pressure results from inertia such as:
– Rotation (cyclostrophic pressure)– Straight line accelerations – Coordinate system accelerations (coriolis)
• Buoyancy pressure results from:– Moisture anomalies– Thermal anomalies– Condensate (precipitation drag)
dpbp
Real Buoyancy Acceleration
• True buoyancy acceleration is :
• Where we see the acceleration is caused by thermal, moisture or precipitation drag anomalies
'1true buoyancy acceleration = b
o
p
z
Dynamic Pressure Acceleratrion
• True dynamic pressure gradient acceleration is :
• Where we see the acceleration is caused by inertial effects of rotation, straight line movement and coordinate system movement
1dynamic pressure acceleration = d
o
p
z
Conditional Instability of the First Kind
• Occurs when a parcel is statically unstable when saturated but stable when dry
• Results in the formation of moist convective thermal plumes, ie cumulus clouds
• Instability favors horizontal scales ~ vertical scale of overturning, i.e. meso-gamma scale for deep convection
Three Stages of a Deep Convective Thermal
• Simplest Case:– Conditionally unstable for deep convection– No environmental wind– Dry middle layers– Moist unstable boundary layer
Stage 1 : Cumulus Stage
• Updraft only• Cloud droplets only (no precipitation)• Level of Non-divergence (LND) near top of moist Planetary Boundary
Layer (PBL) • Cloud positively buoyant throughout:
• Environment neutrally buoyant• Low pressure under updraft• High pressure throughout cloud
'' 1
>0 or more precisely, 0bvv
p
z
Stage 2 : Mature Stage
• Updraft and downdraft• Precipitation and cloud droplets throughout cloud• Level of Non-divergence (LND) at middle levels • Cloud positively buoyant at middle levels,
negatively buoyant in lower part• Cold air dome (density current) at surface• Environment neutrally buoyant but warming• Low pressure at middle levels• High pressure at surface and top of cloud
Stage 3 : Dissipating Stage
• Downdraft only• Precipitation only throughout cloud• Level of Non-divergence (LND) at upper levels • Cloud negatively buoyant throughout• Environment positively buoyant• Low pressure at middle levels and above in cloud• High pressure at surface• Low pressure at surface of environment
Reasons for Breakdown
• Water loading of updraft from precipitation drag
• Cooling due to dynamic entrainment of mid level dry air
Introduce Environmental Wind Shear to Prevent Breakdown
• Assume:– two-dimensions, i.e. infinitely long convective
line– Straight-line shear with height, I.e. wind speed
change with without direction change
Three-Dimensional Effect of Wind Shear
• As before but now assume convective plume is initially circular rather than infinitely long
• Also start by assuming a straight line shear profile again
• Assume westerly shear and veering winds in lowest 6 km
View from South
View from East
Helicity
j jH u
Convective Richardson Number
212 i
CAPER
u
CAPE
( ) ( )
( )
z zCAPE g dz
a
Wind Shear
6
06
0
km
i
i km
u dz
u
dz