4A.6 94-GHz DOPPLER RADAR OBSERVATIONS OF MAMMATUS

4A.6
94-GHz DOPPLER RADAR OBSERVATIONS OF MAMMATUS IN TROPICAL ANVILS
DURING CRYSTAL-FACE
Ieng Jo*, Bruce A. Albrecht and Pavlos Kollias
University of Miami, Miami FL
1. INTRODUCTION
In this paper unprecedented high-resolution
observations of mammatus from a profiling W-band
Doppler radar during Cirrus Regional Study of Tropical
Anvils and Cirrus Layers Florida Area Cirrus Experiment
(CRYSTAL-FACE) are presented. The recorded radar
Doppler spectra illuminate the microphysical processes
and modification of the distribution of ice crystals within
the descending mamma core below the cloud base. The
fine structure of individual mammatus clouds is
presented and significant velocity perturbations (-6 to +1
-1
ms ) are recorded. Strong evidences of upward motions
around the descending mammatus cores are presented.
Areas of turbulent mixing near the cloud base are
identified using the Doppler spectrum width. The role of
gravity waves on mammatus cloud formation is
investigated. The presence of large ice crystals near the
cloud base is necessary to support mammatus
formation. The results are discussed in the context of
suggested theories for mamma formation and
morphology.
Nyquist Velocity). The vertical resolution was 30 m and
the temporal resolution 1.7 sec. 512-FFT Doppler
spectra were recorded and the Doppler moments were
calculated. The sensitivity of the radar is -51 dBZ at 1
km for 1 s averaging. Fig. 1 shows a time-height
mapping of the cirrus cloud reflectivity from the ETL 35GHz Doppler radar operated a few meters from the
UMDCR.
2. BACKGROUND
Due to their notable visible appearance, mammatus
formed at the base of convective anvils often capture
the interest of scientists and photographers (e.g. Stith,
1995; Warner, 1973). Radar measurements of
mammatus characteristics have been extremely scarce
and often conducted by scanning airborne or ground
based radar that lack the resolution needed to resolve
the small scale features of mammatus clouds (e.g.
Martner, 1995; Winstead et al., 2001). Three processes
related to their formation have been identified:
subsidence of a cloud interface layer, fallout of
precipitation, and evaporation
of precipitation. W-band Doppler radar observations of
mammatus clouds were observed during the CRYSTALFACE experiment conducted in South Florida on July
22, 2002. The observations were taken as thick cirrus
clouds produced by deep convection over the ocean
advected over the ground radar site located at Tamiami
Airport in Miami. The University of Miami 94-GHz
Doppler Cloud Radar (UMDCR) operated in the
-1
vertically pointing mode with a PRF of 5 kHz (±4 ms
*Corresponding author address: Ieng Jo, University of
Miami, Rosenstiel School of Marine and Atmospheric
Science, Division of Meteorology and Physical
Oceanography;
4600 Rickenbacker Causeway, Miami, FL 33149-1098;
e-mail: [email protected]
Fig. 1 Reflectivity time-height mapping of the thick cirrus
cloud that produced the mammatus. The mammatus
area is indicated by the box.
The parent cirrus anvil had a cloud top around 14 km at
the beginning of the observations and gradually
descended to less than 10 km after six hour of
observations (Fig. 1). There is evidence of high
-1
sedimentation velocities (1-1.5 ms ) imbedded with
gravity wave activity within the cirrus clouds. The
mammatus form at the cirrus base when a broad cloud
mass with high reflectivity values reached the cloud
base.
3. HIGH RESOLUTION MAMMATUS OBSERVATIONS
Due to its excellent temporal and spatial resolution, the
UMDCR captured the fine structure of individual
mamma. Fig. 2 shows the first three Doppler spectra
moments (reflectivity, mean Doppler velocity and
Doppler spectrum width) from the UMDCR within the
box area indicated in Fig. 1. The observations cover a
25 min period.
spectrum width mapping is a good indicator or vigorous
turbulent mixing occurring near the cirrus cloud base.
The highest Doppler spectrum width values are highly
correlated with the spatial interfaces between upward
and downward motions. Further support for these
observations is given in Fig. 3 that shows a time cross
section of the UMDCR Doppler moments at the altitude
of 5.65 km.
Fig. 3 Time cross section of cloud reflectivity (solid) and
mean Doppler velocity (dashed) (top panel) and time
cross section of cloud reflectivity (solid) and Doppler
spectrum width (dashed) (bottom panel) from the
UMDCR at a height of 5.65 km.
Fig. 2 High resolution observations of mammatus cloud
reflectivity (top panel), mean Doppler velocity (middle
panel) and Doppler spectrum width (bottom panel) from
the UMDCR.
The UMDCR reflectivity shows several mammatus
clouds at different stages of development at the cirrus
base. They extend 0.5-1.0 km below the cirrus base
and there is a noticeable reflectivity decrease with
decreasing altitude. The temporal scale of the
mammatus oscillations is around 1 minute. The mamma
base and sides are characterized by low reflectivity
values. The UMDCR sensitivity is better than -30 dBZ at
this altitude. The mean Doppler velocity exhibits strong
downdrafts extending below the cirrus base within the
mamma core. Typical observed downward velocities are
-1
-3 to -5 ms . Near the mamma bases the velocities
decrease significantly, indicating the presence of either
updrafts or smaller ice crystals. The highest mean
Doppler velocities (slightly downward or even upward)
are observed at the mamma sides and are highly
correlated with low reflectivity values. The Doppler
The top panel shows a strong positive correlation
between cloud reflectivity and downward Doppler
velocities. This feature was well observed in previous
mamma observations (e.g. Martner, 1995). The bottom
panel shows the correlation of cloud reflectivity and
Doppler spectrum width. The Doppler spectrum width
has contributions from both ice crystal size variations
and small-scale turbulence unresolved by the UMDCR
resolution volume and large or resolved scale
turbulence associated with the sharp horizontal
gradients of the mean Doppler velocity across the
mamma.
Fig. 4 shows a different prospective of the mamma
interior. We selected the two most pronounced mamma
clouds (t=3.91 UTC and t=4.1-4.15 UTC) and displayed
the profiles of the cloud reflectivity and mean Doppler
velocity from the interior of the cirrus cloud to the base
of the mamma.
4. DISCUSSION
In this paper unprecedented high-resolution
observations of mammatus from a profiling W-band
Doppler radar during CRYSTAL-FACE are presented.
The observations demonstrate the capability of the
UMDCR and 94-GHz radars in general to capture the
fine structure of cloud structures and highlight important
cloud scale processes often under sampled by other
active sensors. The fine structure of individual
mammatus elements is presented and significant
velocity perturbations are recorded. The mamma exhibit
temporal scales of 1 minute and 0.5-0.7 km vertical
extend. The mean Doppler velocity measurements show
sharp horizontal gradients. Strong downward mean
-1
Doppler velocities (4-5 ms ) are observed in the
mamma cores and compensating upward motions (1-2
-1
ms ) are observed in the mamma peripheries. Areas of
mixing involving cloudy and subcloud air are identified
using the Doppler spectrum width.
5. ACKNOWLEDGMENTS
This research was supported under NASA Grant
NAG511508.
6. REFERENCES
Martner, B. E., 1995: Doppler radar observations of
mammatus. Mon. Wea. Rev., 123, 3115-3121.
Fig. 4 Profiles of cloud reflectivity and mean Doppler
velocity The top panel is from the interior of the mamma
cloud observed at t=3.91 UTC and the bottom panels
show two profiles of cloud reflectivity and mean Doppler
velocity collected by 30 sec apart at t=4.1 UTC at the
interior (solid) and side (dashed) of the mamma.
The top panel of Fig. 4 shows a rather extreme
-1
downward mean Doppler acceleration from -1 to -5 ms
within 0.7 km of descent, accompanied by a very sharp
deceleration near the mamma base. During the
downward acceleration the UMDCR reflectivity barely
varies, indicating that dynamics rather than particle size
is responsible for the sharp vertical gradient of the mean
Doppler velocity. However, near the mamma base the
deceleration of the mean Doppler is correlated with a
sharp decrease of the reflectivity. Moreover, mean
-1
Doppler velocities of -5 ms observed at ~ 5.5 km
cannot be attributed to ice crystal fall velocities or
melting (0 °C at ~ 4 km). The bottom panel shows
similar profiles of cloud reflectivity and mean Doppler
velocity. A sharp horizontal gradient of cloud reflectivity
from the interior to the side of the mamma (almost 15
dBZ) is present. Furthermore, sharp horizontal gradient
of the mean Doppler velocity (bottom right panel) is
observed. The mean Doppler velocity varies from -4
-1
-1
ms in the mamma interior to +1.5 ms at the side of
the mamma cloud.
Stith, J. L., 1995: In situ measurements and
observations of cumulonimbus mamma. Mon.
Wea. Rev., 123, 907-914.
Warner, C., 1973: Measurements of mamma. Weather,
28, 394-397.
Winstead, N. S., J. Verlinde, S. T. Arthur, F. Jaskiewicz,
M. Jensen, N. Miles and D. Nicosia 2001: HighResolution Airborne Radar Observations of
Mammatus. Mon. Wea. Rev., 129, 159-166.