Wade_Review_of_Kumjian_Ryzhkovx

swedishstreakMécanique

22 févr. 2014 (il y a 7 années et 5 mois)

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Size Sorting in Bulk & Bin Models


Onset of
precip



development of particles large
enough to sediment relative to cloud droplets &
ice crystals.


Larger particles tend to fall faster.


Differential Sedimentation (D.S.)


Atmospheric flows (e.g. updrafts) can prolong
D.S. due to the removal of small drops upward &
exhausted through the anvil region.

Size Sorting


In rainfall,
Zdr

tends to increase w/ increasing
Zh

because heavier rain tends to have large conc. of
bigger drops.


Leads to an increase in both
Zdr

and
Zh
.


Thus, high
Zdr

values alone are not sufficient to
identify size sorting.


**
T
his true for interpretation of nearly all
polarimetric

signatures**


Review of common size
-
sorting on
polarimetric

variables w/ the use of simplistic bin models.

Size Sorting in Bulk and Bin Models


Fall speed increases w/ increasing drop diameter


From
Brandes

et al. (2002):




Matches historical observations and recent
observations from
Thurai

&
Bringi

(2005).


Power law from Atlas &
Ulbrich

(1977)
substantially overestimates fall speed of large
drops (>6mm), while bulk microphysical
schemes slightly underestimate
Vt

for D>6mm.

Bin
vs

Bulk Microphysics


Bulk Schemes


Particle size distributions are
assumed to have a shape describe by an analytic
function (e.g. three parameter gamma distr.)




Previous studies (Table 1) have demonstrated
that microphysics schemes with only one
prognostic moment are unable to capture D.S.


Bulk models can better match Bin models for
D.S. after an extended amount of time.

Bin
vs

Bulk Microphysics


Spectral or Bin Microphysics


Each particle size is
assigned to a “bin” and each bin is assigned its own fall
speed.


Thus Bin models are able to explicitly capture D.S. much
better than bulk schemes, particularly during the initial /
early time periods.


None of the studies in Table 1 has investigated the
maintained size sorting possible by updrafts or vertical
wind shear.


Investigations of
supercell

storms have revealed repetitive
polarimetric

radar signatures (
Zdr

Arc,
Zdr

Column) that
are seemingly characteristic of such storms (wind shear,
air flows, buoyancy, life cycle)

Bin
vs

Bulk Microphysics


Spectral or Bin Microphysics


Each particle size is
assigned to a “bin” and each bin is assigned its own fall
speed.


Thus Bin models are able to explicitly capture D.S. much
better than bulk schemes, particularly during the initial /
early time periods.


None of the studies in Table 1 has investigated the
maintained size sorting possible by updrafts or vertical
wind shear.


Investigations of
supercell

storms have revealed repetitive
polarimetric

radar signatures (
Zdr

Arc,
Zdr

Column) that
are seemingly characteristic of such storms (wind shear,
air flows, buoyancy, life cycle)

Size Sorting Models


Models for a particular size soring mechanism
were developed, applied to both bin and bulk
models, & resulting DSDs were converted to S
-
band
polarimetric

radar variables.


Raindrops assumed to be pure water at a T=20
°

C, w/ mean canting angle of 0
°
.


Models applied to Pure Sedimentation and
Vertical Wind Shear.

Pure Sedimentation


A distribution of raindrops is prescribed at the top of the
domain & drop begin falling at Ti.


Fresh drops are continuously replenished at the top of
the domain (“cloud base”) at each time step & domain is
3km tall.


Simulate sedimentation of drops in bulk


moment
-
weight fall speeds are calculated based on prognostic
moments (0
th
, 3
rd
, 6
th
) & every drop falls @ same speed.

Pure Sedimentation

Sedimentation of q:

Sedimentation of
Ntot
:

Sedimentation of Z:

Pure Sedimentation

No drops reaching
the ground

Excessive size sorting

Overestimate in
Kdp

due to an
overprediction

of smaller &
medium sized
drops

Vertical Wind Shear


Provides nonzero storm
-
relative flow, allowing
raindrops to be
advected

away from directly
beneath the cloud.


Smaller drops encounter storm
-
relative flow for
longer periods of time & are thus transported
farther downstream.


Size Sorting


Explains enhancement of
Zdr

along the leading edge of MCSs (
Ulbrich

&
Atlats

2007, Morris et al. 2009,
Kumjian

&
Ryzhkov

2009,
Teshiba

et al. 2009)


Vertical Wind Shear


2
-
D

model

similar

to

previous

1
-
D

except

a

vertical

wind

profile

is

introduced
.


Storm
-
relative

winds

increase

linearly

toward

the

ground

from

0

m/s

@

cloud

base

(
3
km

AGL)

to

20

m/s

at

the

surface
.


1

km

wide

precipitating

“cloud”

is

placed

at

the

top

left

of

the

model

domain

w/

a

Gaussian

Dist
.

For

the

precip

intensity

pattern
.


Raindrop

motion

is

determined

purely

by

advection

and

sedimentation,

governed

by

Eqn

(
10
)
.

Wind Shear


Bin Model Results

No drops reaching
the ground

Excessive size sorting

Z
H

Z
DR

K
DP

Rho
H
V

Wind Shear


Single Moment Bulk

Z
H

Z
DR

K
DP

Rho
H
V

Wind Shear


Two
-
Moment Bulk

No drops reaching
the ground

Excessive size sorting

Z
H

Z
DR

K
DP

Rho
H
V

Excessive size sorting

Wind Shear


Two
-
Moment Bulk

Z
H

Z
DR

K
DP

Rho
H
V

Overestimate in
Kdp

due to an
overprediction

of smaller &
medium sized
drops

Wind Shear
Difference
Fields

Z
H

Z
DR

Rho
H
V


1M


Zdr

underestimated nearly
everywhere, especially
near the ground
where size sorting
most pronounced.


2M
-

Overpredictions

in
Zdr

nearly
everywhere.


3M


Minimal
differences and
generally only slight
underestimates.

Polarimetric

Observations


Large

values

of

Zdr

along

Zh

gradient

in

the

leading

edge

of

MCSs
.


Zdr

enhancements

found

at

the

base

of

developing

convective

cores
.


Zdr

Arc

signatures

in

supecell

storms

due

to

size

sorting

by

vertical

wind

shear
.


Zdr

values

in

excess

of

4

dB

which

are

present

outside

the

30

dbZ

Zh

contour
.


Strong

wind

shear

in

supercell

environments


Light

to

moderately

precipitating

storms

in

an

environment

with

vertical

wind

shear

=

highest

Zdr

at

the

leading

edge

of

a

rain

shaft

along

a

gradient

in

Zh
.


Biological

scatterers

differentiate

between

precip

and

biological

scatterers
.

Polarimetric

Observations

Summary


Size

sorting

in

bin

models

have

significant

impact

on

polarimetric

radar

variables


Differential

sedimentation

can

be

maintained

by

updrafts

and

vertical

wind

shear
.


Single

moment

bulk

parameterizations

are

incapable

of

simulating

size

sorting,

and

thus

the

impact

of

D
.
S
.

on

dual
-
pol

variables
.


Significant

overestimates

in

Zdr
,

Zh
,

&

Kdp


Double

moment

gets

somewhat

closer

to

bin,

but

often

results

in

over
-
sorting

or

excessive
-
sorting
.


Three

moment

fairly

close

to

replicating

the

bin

models

for

D
.
S
.

and

the

impact

on

dual
-
pol

variables
.


Size

sorting

can

thus

have

a

tremendous

impact

on

the

assimilation

of

dual
-
pol

data

into

numerical

models
.