The effects of wind-shear on cirrus: a large eddy model (LEM) and radar case study

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 3: Profiles fromthe 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 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 1: Radar retrievals from the 94 GHz radar on 27th Dec. 19993. Errors of less than a factor of two are expected3. 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 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 5: Simulated radar data from1.5 to 2.5 hours 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. Structures with scales less than ~14 km are well represented in the low shear layer. Model Observations 1 min ~ 1 to 3 km 1 min ~ 3.5 km Model Observations 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. 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 top-of-atmosphere fluxes (< 0.5 W/m2). (a) 8(a)Total water: with/without shear (solid/dashed) 8(b)IWC:observed (solid) and modelled (dashed) 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. (b) 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. Acknowledgments:The authors would like to thank Robin Hogan (University of Reading, UK) for providing the radar data and also 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. Dobbie, Q. J. R. Meteorol. Soc., Accepted 2005. This work was funded by the Natural Environment Research Council (NERC: NER/M/S/2002/00127 & NER/T/S/2000/00983). 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. 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, 756-767. (4) Tompkins, A. M. 2002, J. Atmos. Sci., 59, 1917-1942. (5) Wilson, D. and Gregory, D. , Q. J. R.Meteorol. Soc., 129, 967-986 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: jmarsham@env.leeds.ac.uk Tel:+44 (0)113 3437531