Dr. Chris Westbrook
Room 2U04
Dept. Meteorology
University of Reading
Earley Gate
Reading RG6 6BB
Tel. (0118) 3 787381
I am a member of the radar meteorology group
Useful stuff:
Web-based calculator to estimate the fall speed of ice particles falling in air
- this page also has tools for calculating the mass and area ratio of particles of a given size.
Calculator to estimate the density and viscosity of glycerine / water mixtures
Research interests
Ice formation in supercooled clouds
Using the radars and lidars at Chilbolton we have quantified how often liquid water is present at the top of ice cloud layers: see the plot below. You can see that at temperatures > -22C or so, almost all ice particles seem to originate from a liquid layer. This is strong evidence that ice is formed primarily via freezing of liquid droplets at these temperatures, and the plot indicates that this is the primary nucleation pathway even in quite cold clouds. For more info, see our paper in GRL.
This work was part of the APPRAISE-CLOUDS project which aims to try and improve our understanding of supercooled clouds, production of ice, and their representation in models.
Observations of a glaciating 'hole-punch' cloud
We were lucky enough to make some remarkable remote sensing observations of a fallstreak hole or holepunch cloud. These are produced when an aircraft penetrates a supercooled liquid cloud layer. More details can be found in our paper.
Origin of the Parry arc
It has long been assumed that the rare Parry arc is produced by hexagonal column crystals which orient themselves in free fall such that a pair of prism faces are horizontal (falling 'flat-on', or 'Parry oriented'). However, simple experiments with hexagonal models falling steadily in a tank of water show that this is not in fact the preferred orientation: actually regular hexagonal columns in a still fluid preferentially orient with a pair of prism faces vertical ('edge-on'). However, if the hexagonal column is made scalene in its cross-section (hexagon is no longer regular), it Parry orientation can readily be achieved in certain cases. This suggests that scalene crystal growth is key to the formation of the Parry arc and associated optical effects, and this conclusion is supported by scrutiny of the crystal shapes sampled during Parry arc events by Tape and others. Two pairs of almond-shaped standing eddies are formed on the upper side of the column (see photos below). For more info, see this paper.
Fall speeds of ice particles
A number of modelling studies have shown that the simulated properties of ice clouds are rather sensitive to how fast the ice particles within them fall. Current standard practice is to use the approach of Mitchell (1996) which assumes a single drag coefficient - reynolds number (Cd-Re) curve can be applied to all ice particle shapes. We have tested this assumption against laboratory tank data and field observations of ice particles. We find good agreement where the area ratio of the particle is close to 1, but for more open particles (dendrites, needles, open rosettes) the Mitchell approach overestimates the terminal velocity by up to a factor of 2. We have developed a simple way of accounting for this variation of Cd-Re with area ratio based on the tank data, and this appears to correct the problem. This is important in models where we want to link the particle weight, size and shape directly to it's terminal velocity. See paper for more details.
Measurements of drizzle rates and latent cooling below stratus and stratocumulus clouds
Stratus and stratocumulus are well known to exert a strong influence on the earth's radiation budget, and drizzle is believed to be an important component of this (Wood 2008). However, measuring drizzle rates is very difficult: aircraft observations suffer due to small sample volumes, while remote sensing using radar alone has large uncertainties. I have developed a new technique to measure drizzle rates, using lidar measurements at two different wavelengths (0.9 and 1.5 microns). This takes advantage of the appreciable absorption of light within drizzle drops at 1.5 microns, whilst absorption at 0.9 microns is negligible. This differential absorption increases exponentially with drop size, and can be used to estimate the average drop size in the distribution; from this + the backscatter profile, the drizzle rate and other moments can be derived, and we can then infer the corresponding fluxes and cooling rates below cloud base. See paper for details. Here's an example retrieval for a drizzling stratus layer:
The capacitance of realistic ice particles
The `electrostatic analogy' has been around for 60-odd years, but to apply it we need to know the capacitance C for ice crystals and snowflakes. For columns and plates it seems likely that prolate and oblate spheroids are not too bad an approximation, and analytic formulas exist for these shapes. Progress on calculating C for more complex shapes has been slow however: the metal model experiments of the 60s were not very accurate because of the wire connecting the model to the capacitance meter; the challenge of solving Laplace's equation numerically in three dimensions has made theoretical progress difficult. We have shown that a good way to solve this problem is to use random walks to sample the trajectories of the diffusing water molecules, and simply count how many of these walkers hit the ice particle. This allows the capacitance for an arbitrary particle to be estimated rather accurately (the only real error is a sampling one, which can be reduced by simply sampling more trajectories). We have applied the method to a variety of realistic ice particle habits including hexagonal columns and plates, bullet-rosettes, stellars, dendrites, and aggregates (see our paper for details).
In collaboration with Andy Heymsfield, I recently validated the capacitance-based growth rates against crystals grown in laboratory supercooled clouds between -2 and -22C: we found that they were in very good agreement, and also tested out various ventilation parameterisations for needles and dendrites. We also used the lab data to look at how realistic GCM parameterisations are at capturing the growth of ice at the expense of liquid water. See our paper in JAS for more details.
Influence of small (sub-60micron) crystals on ice spectra
There have been many in-situ observations of small ice crystals in very large concentrations, using forward scattering cloud droplet probes. Ice size spectra with a seperate small mode for these tiny, barely falling crystals have been parameterised for input into GCMs and remote-sensing retrievals (Ivanova et al 2001, Donovan 2003). However, these measurements are uncertain because of shattering of larger ice crystals on the probe inlet, with the detector then counting the many tiny fragments. In an attempt to determine if large quantities of small crystals are genuinely typical of cirrus or an artefact, we have forward modelled their effect on the measured lidar Doppler velocity, using Ivanova et al's parameterisation. Comparison with 17 months of continuous lidar observations strongly suggests that these small crystals are an artefact in most cases, with measured Doppler velocities significantly faster than predicted by the size spectra with small mode added. Forward modelling of the size spectra with the small mode removed leads to a much better comparison with the lidar observations. We conclude that the small mode should not be included in NWP/climate model parameterisations if cirrus is to be simulated realistically. See pubs below.
Specular reflection from planar ice crystals
Observations using a vertically pointing 1.5 micron Doppler lidar, and slightly off zenith 905nm ceilometer have allowed us to identify specularly reflecting planar ice crystals falling with their major axes aligned in the horizontal using the ratio of the two backscatters. Normally the backscatter is smaller at 1.5 due to increased absorption by ice. However, when oriented planar ice crystals are present the reverse is true (because of the anomalous return close to zenith). These crystals are found to be ubiquitous in mid-level mixed-phase and warm frontal clouds, and occur in around 45% of ice cloud at -15C. This is important for CALIPSO/EarthCARE retrievals where the inferred optical depth/microphysical properties will be unreliable in these cloud types, unless the lidar is pointed a few degrees off nadir. See pub 8.
Aggregation and radar scattering models
To accurately interpret dual-wavelength radar data, scattering models for realistic ice particles are required. Currently standard practice is to approximate complex ice particles such as aggregates as a homogeneous sphere, with an effective dielectric constant derived by inserting an assumed density relationship into a dielectric mixture prescription such as Maxwell-Garnett. However it is rather unclear how accurate this approach is, and there is an important ambiguity about what diameter the sphere should have to capture the non-rayleigh backscatter behaviour correctly (do you match maximum dimension, particle volume, projected area...?). To address this issue we are currently using the Rayleigh-Gans (see pub 3) and discrete dipole (in preparation) approximations to calculate the backscatter from some realistic aggregate geometries (these synthetic aggregates were produced from simple computer simulations of aggregation: see pubs 1 & 2 below). Good scattering models for large ice particles such as aggregates is key for interpreting radar data, since the radar sensitivity is so strongly weighted towards the largest particles in the size distribution.
Major conferences
Invited talk at the Met Office in 2002
Theoretical Physics seminar at the University of Warwick in 2003
Lunchtime seminar, Reading Meteorology dept. 2008
Talk, APPRAISE annual meeting, Leeds 2008
Invited talk, Space and Atmospheric Physics Group, Imperial College 2009
Talk and poster at APPRAISE annual meeting, Manchester 2009
Talk, cloud physics across scales meeting, NCAR, 2009
Lecture, Royal Meteorological Society AGM, London, 2009
Talk, RMetS Atmospheric Processes specialist group meeting, Met Office, Exeter, 2011.
Award
LF Richardson prize 2008, Royal Met Soc.
Blog posts
Post on snow crystals for BBC's '23 degrees' series
Post on observing ice clouds using radar and lidar for BBC's '23 degrees' series
Post on Parry arc for Department's Weather and Climate Discussion blog
Publications
1. 'Universality in snowflake aggregation' (2004), Geophys. Res. Lett., 31 L15104
2. 'A theory of growth by differential sedimentation with application to snowflake formation' (2004), Phys. Rev. E, 70 021403
3. 'Radar scattering by
aggregate snowflakes' (2006) Q. J. R. Meteorol. Soc. 132 897
3b. Correction to the above paper, QJ 134 457-458
4. 'Theory and observations of ice particle evolution in cirrus using Doppler radar: Evidence for aggregation' (2007) Geophys. Res. Lett. 34 L02824
5. 'The capacitance of pristine ice crystals and aggregate snowflakes' (2008) JAS 65 206-209
6. 'The fall speeds of sub-100 micron ice crystals' (2008) QJ 134 1243-1251
7. 'Testing the influence of small crystals on ice size spectra using Doppler lidar observations' (2009) GRL 36 L12810
8. 'Doppler lidar measurements of oriented planar ice crystals falling
from supercooled and glaciated layer clouds' (2010) QJ 136 260-276
9. 'Observations of a glaciating hole-punch cloud' (2010) Weather 65 176-180
10. 'Advancements in the estimation of ice particle fall speeds using laboratory and field measurements', Heymsfield and Westbrook
(2010) JAS 67 2469-2482
11. 'A method for estimating the turbulent kinetic energy dissipation rate from a vertically pointing Doppler lidar, and independent evaluation from balloon-borne in-situ measurements', O'Connor et al (2010) JTECH 27 1652-1664.
12. 'Estimating drizzle drop size and precipitation rate using two-colour lidar measurements' (2010) Atmos.
Meas. Tech. 3 671-681
13. 'Observations of ice multiplication in a weakly convective cell embedded in supercooled
mid-level stratus' Crosier et al
(2011) ACP. 11 257-273
14. 'Origin of the Parry arc' (2011), QJ, 137 538-543
15. 'Ice crystals growing from vapor in supercooled clouds between -2.5 and -22°C: testing current parameterisation methods' (2011), JAS, 68 2416-2429.
16. 'Radar scattering from ice aggregates using the horizontally aligned oblate spheroid approximation' Hogan et al
(2011), JAMC, in press.
17. 'Evidence that ice forms primarily in supercooled liquid clouds at temperatures > -27°C' (2011),
GRL 38 L14808 doi: 10.1029/2011GL048021
Comments:
Comment on small crystal paper by Cooper and Garrett (2011) Atmos. Meas. Tech. Discuss. 4 C1065-C1068
I have refereed work for publication in:
PhD Thesis
'Universality in snowflake formation' University of Warwick (2005)
Employment & Education
2005 on, Research Assistant, Radar Group, Meteorology Dept., Reading.
2001-2004 Research Student, Theoretical Physics, Warwick.
1998-2001 BSc Physics, Warwick, Class I.
Teaching
Supervision of MSc and BSc projects.
Remedial Atmospheric Physics tutorial classes.
In 2004-2005 I worked half-time teaching 'A' level Further Maths to students from local Coventry schools as part of the'Access to Further Mathematics' scheme, a charity-funded pilot for what has now become a national, government-funded scheme. Part of this time was also spent producing enrichment material for the students, and maintaining the project website.
Between 2001-2004 I worked part-time as a teaching assistant in the Physics department at Warwick University: 3 years (~300 hours) experience taking problems classes and demonstrating in first year Electronics and second year Physics labs.