Download W(cm/s)

Vertical motion in the stratiform
precipitation region observed with
Equatorial Atmospheric Radar (EAR)
NISHI Noriyuki (Kyoto Univ.)
with:
M.K. Yamamoto, T. Shimomai, S. Mori,
A. Hamada, S. Fukao
SOWER 2006/07/19
• Purpose – observing the fine structure in various
upper and middle troposphere clouds – Cirriform
clouds, Altostratus, Nimbostratus…
• In this study, we present the fine structure in the
nimbostratus with larger.
Group of
cumulonimbus
•The understanding of
the nimbostratus (anvil)
is important also for the
generation of cirriform
clouds.
Cumulo
nimbus
Nimbo
stratus
Cirrus
Mesoscale system
convective part
and
stratosphere part
Vertical motion in the
stratiform region
• Average 10-20cm/s
– Few exceptions
W in stratiform precipitation region by sonde,
weather radar and VHF radar etc. (m/s).
Houze(1989,QJRMS)
Line shape system
LAYER ASCENT
W in stratiform region
• Few study on fine W
structure
– Cifelli and Rutledge(1994,
JAS): Two cases in
monsoon and break periods
in N. Australia
Line-type system in break monsoon
Z
W(contour 50cm/s)
Rain
Fine W structure in nimbostratus
• Observation of W with 10cm/s and 10-min scale in the
upper troposphere -- difficult
• Comparison of instruments
– radiosonde → Low time resolution
– Weather radar → Problem in boundary condition when integrating
horizontal divergence vertically
– Lidar etc. → Need hypothesis
– VHF radar → Possible,but…
• VHF radar(e.g. Equatorial Atmospheric Radar: EAR)
– In upper troposphere – Even EAR, difficult – weak Bragg
scattering and weak partial reflection
– Special vertical mode (only vertical beam) – alternately operated
with normal standard mode (5 beams)
– Intensified quality control of Doppler spectral data
Method and data
• EAR special obs. with vertical wind mode (Nov2003)
– Resolution resolution 3min、150m(Z)
– Accuracy 2.6cm/s
• Weather radar (λ= 3cm) Not directed vertical direction
– Average at circle with 29.5 ˚ elevation angle (dBZ) and the ratio
of above-noise-level data (％)
• Other datasets
– GOES9 Tbb – Calculating estimated cloud top height with
temperature in NCEP reanalysis Version 2. [In most cases,
lower values are calculated than actual top]
– Optical raingauge (averaged in 10 minutes)
Typical case(6 Nov 2003)
• Almost isolated
cloud cluster passes
over EAR from SE
to NW
Movement of cluster (GOES-9 Tbb: K)
Conv./Strat. By Weather radar
Convective 1200 UTC 06Nob2003
Stratiform 1647 UTC 06Nob2003
Gentle upward
motion in the later
stage of stratiform
region
• Stratiform
precipitation
region –
identified by the
presence of
bright band
– “When Bright
Band is present,
stratiform.
– The reverse is
not always true
(Steiner et al.,
1995, JAM)
W(cm/s)
Z (dBZ)
14
3
Stratiform
• Downward motion
and strong upward
motion during
earlier stage of
stratiform period
• During later stage,
only weak upward
motion (0-40cm/s)
– two to three
hours, 3-5km
thickness in height
14
3
Gentle upward motion in the later stage
of stratiform region
• Fluctuation component is
small
12-min average
– Particularly in the vertical
direction
• Confirmation with 3min
data
– Not available in the upper
part
– 3km horizontal resolution
(when 15m/s speed)
– No prominent small-scale
structure even in 3-min data
3-min average
8-11km平均
8 and 20 Nov
case
W(cm/s)
• 8 Nov case
– Gentle upward
motion in the
later stratiform
period
Z(dBZ)
14
3
Stratiform
W(cm/s)
• 20 Nov case
– A little stronger,
but similar
gentle upward
motion
Z(dBZ)
14
3
Stratiform
Comparison of 3
cases
• Histogram in 8-11km
height range with 12
min and ΔZ=150m
resolution
• In all cases, high
rate of 0-40cm/s
upward motion
detected
• Similar gentle
upward motion
region in 2 and 15
Nov
+0～40cm/s
Down
+40cm/s<
Cause of gentle upward motion
•
Two factors
1. Generation of gentle
uniform upward motion
2. Suppression of strong
upward motion and
short-period variabilities
•
1. Generation of gentle
upward motion
a)Deposition
–
–
–
–
Saturated with ice, but
no with wate
Latent heat by
deposition of vapor to
ice
Mesoscale low pressure
This process can
product the uniform
upward motion if it
occurs in deep range
Houze (1989, QJRMS)
•
b) Gravity wave
(slow)
– The total effect of
cumulonimbi in the
convective region
– Slow gravity wave
– Upward motion
phase is placed to
the nimbostratus
Pandya and Durran
(1996,JAS)
• 2. Suppression of strong upward motion and
short-period variability
– Processes that make small scale convective motions
• Old cells generated in the convective part
• New convective motions by instability
– Riming (seeder-feeder), which brings latent heat only in the
lower part of nimbostratus
– (deposition, if it happens only in the lower part)
– Gravity wave with short time scale
• Generated in mainly convective region
We hope the present results are referred
for understanding dynamical and physical
processes in nimbostratus … and …
cirriform clouds through theoretical and
modeling approach
Summary
• 3 cloud clusters during November 2003
– Fine structure with 3min、150m(z) resolution data
– Gentle upward motion with small fluctuation during
the later stage of stratiform precipitation region
• 0-40cm/s, thickness3-5km, 2-3 hours
• No predominance of waves and convective cells
– Although the representative value of W in stratiform
region is 10-20cm/s, the fine structure has not been
so well-examined
– Deposition and gravity wave may be possibly causes
of this gentle upward motion