Transcript Slide 1

The effects of wind-shear on cirrus: a large eddy
model (LEM) and radar case study
John Marsham and Steven Dobbie
Institute for Atmospheric Science, School of Earth and Environment,
University of Leeds, Leeds, UK
Summary
• Wind-shear is an almost ubiquitous feature of the troposphere at cirrus altitudes, but there have been few studies focused on investigating its effects on cirrus clouds.
• There can be significant variations between results from different cirrus models1, which makes a comparison of the simulations with observations important. So, using
the Met Office LEM & the Fu-Liou radiation scheme2, we simulated a sheared frontal cirrus, observed by the 94 GHz radar at Chilbolton, UK
• Ice water contents (IWC) were output from the LEM in such a way as to mimic the radar observations. True and simulated IWC observations were compared, as well as
their Fourier transforms and probability density functions (pdfs). The LEM captured the horizontally averaged IWC profile reasonably well (although it was too bi-modal).
At upper levels the variability in the IWC field, at scales of less than approximately 14 km, was similar to the observations. At lower levels the IWC field was too
homogeneous. So, although the model intialisation is not sufficiently constrained for a validation of the LEM, this case-study has a reasonable basis in observations.
• Varying the shear within the LEM simulations showed that shear had little effect on the mean IWC profile, but increased mixing and so the homogeneity of the IWC field.
In this case shear had little effect on the top-of-atmosphere (TOA) and surface fluxes, or the within-cloud heating rate profile, but this would be significantly different for a
more patchy cloud.
• LEM results showed that the variation in the correlation between IWC with vertical separation is shear dependent and initially linear, with or without shear.
• A modified case-study showed that Kelvin-Helmholtz wave-breaking can significantly affect the microphysical processes, by increasing nucleation, deposition and
sublimation rates. These effects were most significant when there was no large-scale uplift and the vertical velocities from the wave-breaking formed a cirrus cloud, which
did not otherwise occur.
Figure 1: Radar retrievals from the 94 GHz radar on 27th Dec.
19993. Errors of less than a factor of two are expected3.
1 min ~ 1 to 3 km
(a)
(b)
Figure 4: (a) After 2 hours
the LEM IMR profile is
comparable to 10:00 or 10:30
UTC; although the vertical
distribution is too bimodal and
the upper peak is too large.
(b) These peaks correspond to
the distinct ice and snow
classes in the LEM. The profile
is very sensitive to doubling
the fallspeeds, which does not
halve the IMR in the cloud, but
decreases the magnitude and
height of the upper peak,
whilst the lower peak is less
affected. Doppler radar data,
to constrain the fallspeeds,
were not available for this
case.
Figure 7: The shear significantly
affects the two-dimensional
distribution of ice (not shown);
advecting the fallstreaks and
homogenising the high-shear
layer, but has little effect on the
IMR profile at 2 hours (Fig 7a).
The shear does, however, lead to
very limited Kelvin-Helmholtz
wave-breaking, which increases
vertical velocities between 5 and
8 km (Fig. 7b) giving
microphysical production of ice
and increasing the IMR at 7.5 to
8.0 km. The shear has little
effect on the net radiative
heating rates or surface or topof-atmosphere fluxes (< 0.5
W/m2).
Figure 2: Vertical profiles were sampled and averaged in
time to mimic the 94 GHz radar. The LEM was cooled at
0.001 K/s (to simulate frontal uplift) to give the 11:00 UTC
radiosonde profile after 2 hours. Note the fallstreaks below
7 km and the convecting region above (driven by longwave
cooling at the cloud-top).
Figure 5: Simulated radar
Model
data from 1.5 to 2.5 hours
Observations
model time is compared
with observations from
9:45 to 10:45 UTC. There
is less structure in the high
shear layer in the LEM than
the observations.
1 min ~ 3.5 km
Structures with scales less
than ~14 km are well
represented in the low
Model
Observations
shear layer.
Figure 3: Profiles from the 11:00 UTC
Herstmonceux radiosonde (~120 km from
Chilbolton). Note the stable high shear layer
(4.5 km to 7 km) and the less stable lower
shear layer (7.0 km to 10.5 km) and the near
neutral layer at cloud-base.
Figure 6: There is
less variance in the
LEM than the
observations at all
levels, unless the
largest IWC are
excluded. The LEM is
not capturing the
largest IWC at upper
levels and this leads
to a more
homogeneous
fallstreak region
(fractional variance=
variance/mean).
Figure 9: The trend
depends on shear
and is initially linear
with or without shear.
With shear
oscillations occur as a
gap is advected over
a streak or a streak
over a gap.
8(a) Total water: with/without shear (solid/dashed)
8(b)IWC: observed (solid) and modelled (dashed)
Figure 8: Pdfs at 2 hours. Skewed mono-modal
distributions4,5 provide a good fit. Shear induced
mixing has a significant effect on the pdfs. There is
too little structure at 5.5 km (see Figures 5 & 6).
Modelled pdfs are similar to observed, with too few
large IWCs.
Figure 10: Modifying the potential temperature profile
gave significant Kelvin Helmholtz wave-breaking, which
affected nucleation, deposition and sublimation. As the
shear was increased this wave-breaking initially occurred in
thin layers, of limited horizontal extent, which would not be
resolved by a global model. These wave-breaking effects
were most significant when no cooling was applied (to
simulate no large scale uplift) and the vertical velocities
from wave-breaking formed a cirrus cloud, which did not
otherwise occur.
Acknowledgments:
The authors would like to thank Robin Hogan (University of Reading, UK) for providing the radar data and also References: (1) Starr, D. O’C. et al, 2000, 13th Int. Conf. On Clouds and Precip. 1, 1-4, Reno. (2) Fu, Q., 1996, J,
Climate, 9, 2058-2082. Fu, Q. and Liou, K. N., 1992, J. Atmos. Sci., 49, 2139-2156. Fu, Q. and Liou, K. N., 1993, J. Atmos.
the two anonymous reviewers of “The effects of wind-shear on cirrus: a large eddy model and radar case study”, J. H. Marsham and S.
Sci. 50, 2008-2025. Fu, Q. et al, 1998, J. Climate, 11, 2223-2237. (3) Hogan, R. J. and Illingworth, A. J., J. Atmos. Sci. 60,
Dobbie, Q. J. R. Meteorol. Soc., Accepted 2005. This work was funded by the Natural Environment Research Council (NERC:
756-767. (4) Tompkins, A. M. 2002, J. Atmos. Sci., 59, 1917-1942. (5) Wilson, D. and Gregory, D. , Q. J. R.Meteorol. Soc.,
NER/M/S/2002/00127 & NER/T/S/2000/00983).
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For more information about this poster please contact Dr John Marsham, Environment, School of Earth and Environment, The University of Leeds, Leeds, LS2 9JT Email: [email protected] Tel:+44 (0)113 3437531