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NCAS Weather – report of work for 2008-2009.

Introduction This year saw an expansion of NCAS Weather, implementing the strategy developed over the previous two years. New posts were created in the areas of Synoptic Storms, Convective Storms and Pollution Transport, and a joint post with FGAM created in lidar measurements and boundary-layer meteorology. This resulted in five new posts in all, two of which have started:

• Hugo Ricketts at Manchester on lidar measurements and boundary-layer meteorology, started April 2008 under the supervision of Prof G Vaughan.

• Dr John Marsham at Leeds was appointed to the Convective storms post in January 2009, with Dr Doug Parker as his PI.

Dr Charles Chemel at Hertfordshire joined NCAS to work on boundary-layer pollution transport and air quality applications in April 2009, and new posts in Synoptic Storms and long-range Pollution Transport have been awarded to Reading (PIs Drs John Methven and Sue Gray) and Lancaster (PI Dr Oliver Wild) respectively. Dr Connie Schwierz left Leeds to return to Zurich in January 2009. Collaboration has been a consistent theme this year. The Joint Climate Research Programme between the Met Office and NERC has expanded to involve the Weather area, and will be rebranded the Joint Research Programme. NERC have purchased some nodes on the new Met Office supercomputer which will facilitate collaborative use of the UM in specialist areas which cannot be done on HECTOR, and projects in the areas of convective scale ensembles and data assimilation (with NCEO) are planned. The Director, with Professor Stephen Mobbs and Dr Alan Gadian, visited NCAR in June 2008 to conclude a cooperative agreement between NCAS and NCAR. This will involve exchange of personnel between the two organisations, greater UK involvement in WRF (there will be a WRF Users’ workshop in the UK in September 2009) and collaborative field campaigns. A major field campaign in collaboration with American groups (including NCAR) was VOCALS – the VAMOS Ocean-Atmosphere-Clouds Study, which took place from Arica in Chile in autumn 2008. The UK part of this project was led by Professor Hugh Coe from Manchester, and is the first consortium grant to involve all parts of NCAS. The project studied the extensive stratocumulus sheet found over the South-Eastern Pacific, whose high albedo is important for the radiative balance of the Earth and yet whose stability as the climate changes is not understood. Prof Vaughan, Drs Gadian and Crosier and Mr Ricketts all took part in the campaign, flying on the Bae146 and the ARSF Dornier aircraft. Some details of their involvement are given below. Excellent measurements were obtained by both aircraft, and collaboration with the American groups is proceeding on joint studies and papers.

Research Achievements

Prof Geraint Vaughan, Manchester a) Aerosol and chemical transport in deep convection, ACTIVE. This is a NERC consortium grant to Manchester, Cambridge and York to take part in a major international field campaign in Darwin, Australia, during the wet season of 2005-6. Key findings over the past year have been:

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(i) Detailed trajectory analysis applied to the ozonesonde profiles and carbon monoxide measurements from the Egrett aircraft both show that the composition of the TTL over Darwin was largely controlled by long-range transport rather than local convective uplift, even in the active monsoon period. Results are shown in fig.1, where a clear pattern is seen both in ozone and CO across the Indonesian region. A paper has recently been published on ACPD (SCOUT-O3 special issue) describing this work. (Mr. W. Heyes)

(ii) Further model simulations of the effect of aerosol on deep convection has revealed that the sensitivity depends on meteorological conditions. WRF simulations of Hector storms on the Tiwi islands reveal the importance of local radiosonde profiles to initialise the model – standard global meteorological analyses (e.g. ECMWF) are too dry. Realistic simulations were possible with WRF for some Hector cases but in others adjustment of the initial conditions was required. (Drs P. Connolly, P. May and M. Zhu)

(iii) Measurements of aerosol in recently-evaporated anvils show, as expected, very high particle number concentrations. The time scales for aerosol growth following cloud evaporation have been derived from trajectory calculations and compared with published models – results look broadly consistent (Mr. D. Waddicor)

(iv) NO and NOx concentrations in the outflow of monsoon convection reached values around half those measured in land-based thunderstorms – contrary to some of the literature on this topic suggesting very little lightning activity. This work has been submitted to APCD (SCOUT-O3 special issue). (Drs. L. Labrador, A. Volz-Thomas)

Fig. 1b. Locations where back trajectories that experienced rapid uplift crossed the 500 hPa surface, colour-coded to meteorological period as before. The values represent the average background, out-of-cloud concentration of carbon monoxide recorded on the Egrett aircraft at the beginning of the back-trajectory.

Fig. 1a. Locations where back trajectories that experienced rapid uplift crossed the 500 hPa surface, colour-coded to meteorological phase. The values represent the concentration of ozone recorded by ozonesonde measurements at the beginning of the back-trajectory. b) Effect of upper-level PV anomalies on convective inhibition and initiation. This work, with Dr A. Russell, exploited data from the CSIP experiment in 2005 and the COPS experiment of 2007 to investigate the link between upper-level features and convection. One paper was published in QJRMS and another is in press in Tellus showing how the remnants of tropopause folds generated far upstream can both inhibit and promote convection. This work complements that of Dr G Allen on the ACTIVE project, who showed how tropopause folds resulting from breaking Rossby waves on the subtropical jet can have a profound influence on convection over Northern Australia, leading to tropical droughts and tropical plumes. These features are thin layers which cannot be represented in global meteorological models. The project continues with a detailed study of atmospheric lids using the COPS dataset. c) Observations of stratocumulus during VOCALS. During the VOCALS field campaign the NERC ARSF Dornier aircraft was used for remote sensing of the stratocumulus cloud deck, using

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an aerosol lidar, ARSF’s hyperspectral imagers, the SPECIM Eagle and Hawk, and a polarimeter from Dr Thomas Ruhtz at the Free University of Berlin. The aims were to measure the aerosol distribution above cloud, and to relate upwelling solar radiation to the in-situ measurements by the BAe146 and NCAR C130 aircraft in the cloud deck. The Dornier science team were Prof Vaughan, Dr Lorenzo Labrador and Mr Hugo Ricketts. Fourteen successful flights were conducted in all, with exciting measurements from all the instruments currently being analysed.

Fig. 2. Top left: Dornier at Arica airport. Note stratocumulus sky. Top right: Dornier flight path (colour-

coded with altitude) on 14 November 2008, overlaid on GOES image. Bottom: Lidar backscatter measured on this flight, showing aerosol layers in the free troposphere. Hugo Ricketts is currently processing these

data to retrieve aerosol backscatter. d) Remote sensing and turbulence. A Met Office CASE student, Mr Christopher Lee, is investigating how the spectra measured by wind profilers can be used to give more quantitative measures of atmospheric turbulence, particularly in the boundary layer. He has been comparing UFAM wind profiler measurements at Cardington with turbulence measurements from a tethered balloon. As part of this project, a campaign was conducted with the Dornier flying over Aberystwyth, where both the MST radar and the UFAM wind profiler were operating. Results look very encouraging, with radar spectral widths matching the wind variance measured by the aircraft turbulence probe. Unfortunately, full turbulence data were not collected because of a computer problem, and dissipation rates were not available from the aircraft. The campaign is being repeated in 2009 to rectify this deficiency. A second Met Office CASE student, Daniel Housley, started in October 2008 with the aim of synthesising different vertical profiling instruments to provide better information with which to initialise the forecast model

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Publications A. Russell, G. Vaughan, E. G. Norton, C. J. Morcrette, K. A. Browning and A. M. Blyth,

Convective inhibition beneath an upper-level PV anomaly. Quart. J. Roy. Meteorol. Soc., 134, 371–383, 2008.

P. T. May, J. H. Mather, G. Vaughan, C. Jakob, G. M. McFarquhar, K. N. Bower, and G. G. Mace. The Tropical Warm Pool International Cloud Experiment (TWPICE). Bull. Amer. Meteorol. Soc., 89, 632 - 645, 2008.

G. Vaughan, C. Schiller, A. R. MacKenzie, K. Bower, T. Peter, H. Schlager, N. R. P. Harris and P. T. May. Studies in a natural laboratory: High-altitude aircraft measurements around deep tropical convection. Bull. Amer. Meteorol. Soc., 89, 647 – 662, 2008.

G. Allen, G. Vaughan, K. Bower, P. Williams, J. Crosier, M. Flynn, P. Connolly, J. Hamilton, J. Lee, J. Saxton, N. Watson, M. Gallagher, H. Coe, T. W. Choularton, J. Allan and A. C. Lewis. Aerosol and trace-gas measurements in the Darwin area during the wet season. J. Geophys. Res, 113, D06306, doi:10.1029/2007JD008706, 2008.

J. F. Hamilton, G. Allen, N. M. Watson, J. D. Lee, J. E. Saxton, A. C. Lewis, G. Vaughan, K. N. Bower, M. Flynn, J. Crosier, G. D. Carver, N. R. P. Harris, R. J. Parker, J. J. Remedios and N. A. D. Richards. Observations of an Atmospheric Chemical Equator and its implications for the Tropical Warm Pool Region. J. Geophys. Res., 113, D20313, doi:10.1029/2008JD009940, 2008.

P. T. May, G. Allen, G. Vaughan, P. Connolly. Aerosol and thermodynamic effects on tropical cloud systems during TWP-ICE and ACTIVE. Atmos. Chem. Phys, 9, 15-24, 2009.

G. Allen, G. Vaughan, D. Brunner, P. T. May, W. Heyes, P. Minnis, J. K. Ayres. Modulation of tropical convection by breaking Rossby waves. Quart. J. Roy. Meteorol. Soc. 135, 125-137, 2009.

A. Russell, G. Vaughan, E. G. Norton, H. M. A. Ricketts, C. J. Morcrette, T. J. Hewison, K. A. Browning, A. M. Blyth. Convection forced by a descending dry layer and low-level moist convergence. Tellus Series A-Dynamic Meteorology And Oceanography, 61, 250-263, 2009.

Grants awarded NERC. OPEN CASE studentship award with Met Office ‘Towards a new generation of

atmospheric vertical profiling’ NE/F012950/1. £70,772.98. 1/10/2008 – 31/3/2012. G. Vaughan and P. J. Connolly with C. Gaffard, Met Office.

NERC. VAMOS Ocean-Cloud-Atmosphere-Land Study UK (VOCALS-UK) consortium grant. £1,364 k. NE/F019874/1. H. Coe, K. Bower, T. Choularton, J. Crosier, P. J. Connolly, M. W. Gallagher, G. Vaughan, G. McFiggans with colleagues from Reading, Leeds and the Met Office.

Mr Hugo Ricketts, Manchester As mentioned above, Hugo Ricketts contributed to the VOCALS project and also to the turbulence project at Aberystwyth. His PhD thesis was submitted in September 2008. (a) Ozone Fluxes across the Boundary Layer Top. The FGAM ozone and aerosol profiler is a mobile lidar system used for retrieving boundary layer profiles of ozone mixing ratio and aerosol backscatter. Combining these two datasets an algorithm was developed to retrieve fluxes across the top of the convective boundary layer. Using data from the Convective Storm Initiation Project (CSIP) in 2005 and the Leicester Air quality Measurement Project (LAMP) in 2007 it is possible to measure ozone fluxes across boundary layer tops. The work is ongoing and an example is shown in Fig.3 of ozone flux as calculated for 31/07/2007 during LAMP.

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Using the aerosol backscatter from the lidar the height of the convective boundary layer can be determined and its growth can be tracked over time. This growth can be quantified in terms of entrainment velocity. Combining this with ozone concentrations retrieved from the lidar in both the boundary layer and the free troposphere it is possible to determine the amount of ozone being entrained into the boundary layer. In this example, the growth of the boundary layer can be split into two regimes. Before midday the boundary layer grows at about 0.08 m.s-1 compared to the afternoon where the growth rate deceases to 0.01m s-1 (see below). The flux associated with both regimes is (-0.54 ± 0.27) ppbv m s-1 in the morning and (-0.061 ± 0.067) ppbv m s-1 in the afternoon. Papers containing these results are in preparation and due to be completed in 2009/10.

Fig. 3. Entrainment velocity and ozone fluxes derived from the lidar data

Fig. 4. Cloud top height derived from Leosphere lidar on a flight of the Dornier along 20ºS, showing increase

in cloud top with longitude. Black: outbound leg, Red: return leg (b) Cloud Top Height in the South-Eastern Pacific. During VOCALS, the Dornier 228 carried a Leosphere aerosol lidar as well as other remote sensing instruments. Using this lidar, aerosol backscatter profiles (see Fig.2) and cloud top height of the stratocumulus deck were derived. Results show that the cloud top height increased with distance from the coast of Chile as shown in fig.4. Processing data from other flights shows that the variation of cloud top height was not constant throughout the campaign and depended on the overall synoptic condition. Other ongoing

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work includes analysis of the available aerosol backscatter data combined with back trajectories to detect plumes being advected off the Andes as well as urban plumes from coastal towns. Publication A. Russell, G. Vaughan, E. G. Norton, H. M. A. Ricketts, C. J. Morcrette, T. J. Hewison, K. A.

Browning, A. M. Blyth. Convection forced by a descending dry layer and low-level moist convergence. Tellus Series A-Dynamic Meteorology And Oceanography, 61, 250-263, 2009.

Dr Jeffrey Chagnon, Reading Research conducted on the dynamics of mesoscale weather systems at the University of Reading has focussed on the potential vorticity and gravity waves generated by convective storms. The broad aim of this research is to elucidate the mechanisms by which flow disturbances associated with convection are communicated to and influence the evolution of the larger-scale systems in which convection is embedded. Following on from these studies, a new project will begin in 2009-2010 that will examine the effect of diabatic processes on the evolution and predictability of frontal systems in the North Atlantic, with a view towards supporting a UK consortium proposal to participate in the future observational campaign T-NAWDEX.

Fig. 5. Three-dimensional rendering of a horizontal potential vorticity (PV) dipole in a high resolution simulation of a convective storm using the UK Met Office Unified Model, version 6.1, with 1 km horizontal grid spacing. Plotted are isosurfaces of potential vorticity (red surface15PVU / blue surface -15 PVU) and vertical velocity (green surface 7 m s-1). For reference, contours of PV and vertical velocity at an elevation z = 6.5 km are projected onto the lower horizontal plane. Mid-tropospheric shear vectors (averaged from z = 2 to 7 km) are also plotted in the lower horizontal plane. a) Influence of diabatic processes on the structure and dynamics of convective storms. A novel modelling study was conducted examining the diabatic potential vorticity anomalies generated within convective storms. Analysis of potential vorticity clarifies the link between small-scale thermodynamic anomalies and system-scale dynamical anomalies. Several new and interesting results have emerged from this investigation which could inform both our theoretical understanding and our strategy for how to model diabatic processes in convective systems. First, we have identified the dominant mode of storm-scale potential vorticity in a sheared environment as a

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horizontally oriented dipole. This result challenges the typical assumption that storms strictly generate vertically oriented dipoles. Second, we have demonstrated that the maintenance of relative vorticity in the storm environment is dependent on moist processes. Third, we have shown that the representation of the potential vorticity dipoles in a numerical simulation is sensitively dependent on the model configuration (e.g., horizontal resolution). The investigative tools included a hierarchy of models of varying complexity, ranging from case study analysis with the high resolution version of the UK Met Office Unified Model, to a two dimensional nonlinear model of an idealised frontal zone, to analytical solutions of a linear model. A manuscript has been submitted to the Quarterly Journal of the Royal Meteorological society for publication. (See Figures 5 and 6).

Fig. 6. Longitudinal-height cross sections of potential vorticity across a line of convection, simulated using the UK Met Office Unified Model, that developed over Wales at 0600Z on 28 July 2005 showing the total PV in the a) 1 km run and b) 12 km run. Also shown are the primary sources of potential vorticity, accumulated over one hour, arising from c) the boundary layer and cloud schemes in the 1 km run, and d) the convection scheme in the 12 km run. b) Dynamics of gravity waves generated by convective storms. A previous study conducted in 2007-2008 identified the spectral properties of gravity waves above convection in high resolution numerical model simulations as well as observations from the NERC MST wind profiling Doppler radar located in Aberystwyth, Wales. We have devised a new theoretical approach to explain these spectral properties and, more generally, to characterize the interaction of gravity waves with their sources. The approach is based upon the concepts of “dynamical resistance” and “forcing efficiency” which consider the work done by wave induced perturbations upon a forcing, and vice versa. Calculations performed in an idealised two-dimensional model have demonstrated that there exist optimal configurations of the forcing, background flow, and stratification that minimize the dynamical resistance imposed by gravity waves upon the forcing. Results have been summarized in a manuscript submitted to the Journal of the Atmospheric Sciences for publication. c) A new modelling study will be initiated in 2009-2010 with the intention of supporting a UK consortium proposal to participate in the planned observational campaign T-NAWDEX (Thorpex North Atlantic Waveguide Disturbance Experiment) which will be conducted in 2011. The

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modelling study will build upon the analysis of potential vorticity in convective storms that was conducted in 2008-2009. The purpose of the investigation will be to examine the source, structure, and magnitude of diabatic potential vorticity anomalies generated by small-scale and parameterized processes in mesoscale model simulations of developing midlatitude cyclones in the North Atlantic. Some of the questions considered during the course of investigation will include the following. What are the primary modelling sensitivities of upper-tropospheric potential vorticity lenses generated within warm conveyor belts? For example, to what extent are simulated upper-tropospheric potential vorticity sources sensitive to vertical resolution? What are the relative contributions from the various diabatic processes (e.g., microphysics, radiation, mixing/diffusion) to the total simulated diabatic potential vorticity, and how are they affected by model configuration? What sampling requirements are needed to observe these anomalies? The primary investigative tool will be the UK Met Office Unified Model (UM) in its mesoscale limited-area configuration, accommodating horizontal grid length of 1 km (for which convection is permitted explicitly). The primary method of analysis will involve the partitioning of the simulated total potential vorticity source/sink among the contributions from the various, sequentially-called sections of the model, including boundary layer mixing, cloud scheme, diffusion, convective scheme (where appropriate), radiation, among others. Publication Chagnon, J.M., and S. L. Gray, 2008: Analysis of convectively-generated gravity waves in

mesoscale model simulations and wind profiler observations. Quarterly Journal of the Royal Meteorological Society. 134, 663-676.

Dr. Jonathan Crosier, Manchester a) COPS. Laboratory characterization of the Manchester Differential Mobility Particle Sizer (DMPS) was conducted in collaboration with UFAM instrument scientist Dr P Williams to allow further data retrievals from the COPS dataset. This data has provided an insight into the influence of local/regional meteorological condition on the aerosol properties at the Hornisgrinde during COPS. For example, figure 7(a) shows the diurnal variation (5th, 25th, 50th, 75th and 95th percentiles) of equivalent potential temperature measured at the Hornisgrinde during COPS (excluding any contributions from cloud liquid water content). A strong diurnal cycle is seen and is related to both surface heating and also vertical lifting of the Rhine valley boundary layer upwards to encompass the Hornisgrinde site.

Fig. 7. Diurnal trends of (a) equivalent potential temperature (θe, excluding contributions from cloud water content) and (b) particle number concentration (N) measured by the nano-DMPS (5nm-30nm), showing 5th,

25th, 50th, 75th and 95th percentiles. Fig. 7b shows the diurnal cycle of total number concentration measured on the nano-DMPS (5nm-30nm) during COPS during all out of cloud conditions (RH measured > 97%). The two striking features of fig. 7(b) are a strong diurnal cycle which sees very large increases in number concentration from 1000 hours, reaching a maximum at around 1300 hours. Number concentrations reduce back to the low levels by around 2000 hours and have little variability as demonstrated by

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the percentiles. This demonstrates that either large numbers of aerosol particles are being nucleated in the region during the daytime, or boundary layer expansion is bringing fresh pollution up from the Rhine valley to the Hornisgrinde site. The night time number concentrations seen at the Hornisgrinde, while low and stable relative to day time concentrations, are still high (around 2,000 cm-3) and suggest that the Hornisgrinde site is a fairly polluted one even at night. In contrast daytime concentrations reached well over 10,000 cm-3 on several occasions. Similar diurnal plots of the same size range during In-Cloud conditions shows a near identical profile. This suggests that (a) clouds are poor at scavenging/removing nano-particle pollutants which may be of importance with respect to human exposure and (b) support the theory that these particles are transported from the boundary layer as particle nucleation in cloudy conditions is expected to be low. Diurnal profiles of accumulation mode aerosol mass/number/volume do not show such trends and suggests this mode of aerosol (which is dominated by organic species) is more regional than local. Accumulation mode aerosol was also found to be effectively removed during in cloud periods though activation and scavenging. A significant amount of modelling work based on the COPS measurements has also been conducted. Sensitivities of cloud droplet number concentration to aerosol physical and chemical properties has been examined using a cloud parcel model and will be compared to sensitivities in remote marine environment in he near future. b) VOCALS. Measurements of large aerosol, cloud droplets and precipitation were made during the VOCALS-UK field project. This was conducted on the FAAM BAe146 aircraft using a variety of cloud microphysics instrumentation including the CAPS probe (Cloud, Aerosol and Precipitation Spectrometer) and the 2D-S probe. This highly resolved data will provide a great insight into the lifecycle of stratocumulus in the south-east pacific, a region where predictability of cloud fields is poor. Fig. 8 shows 1 second average data from the FAAM cloud droplet probe (CDP) during a vertical profile through the stratocumulus deck. Cloud droplets clearly grow in size as you pass from the bottom to top of the cloud layer while number concentrations’ remain approximately constant.

Fig. 8. Image plot of normalised number concentration of droplets measured during a profile through stratocumulus during VOCALS-UK, along with the calculated Mean Volume Diameter (MVD, µm) c) APPRAISE. Measurements of large aerosol, cloud droplets and ice crystals have been conducted as part of the APPRAISE-CLOUDS project. These have been done using state-of the art imaging probes which provide extremely high time resolution data over a wide range of ice-crystal and precipitation size particles. Aerosol size and composition has also been measured using the newly upgraded Aerosol Mass Spectrometer. Initial data processing and analysis is in the early phase and considerable progress is expected in the next 12 months. Publications

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J. Cozic, B. Verheggen, E. Weingartner, J. Crosier, K. N. Bower, M. Flynn, H. Coe, S. Henning, M. Steinbacher, S. Henne, M. C. Coen, A. Petzold, U. Baltensperger. Chemical composition of free tropospheric aerosol for PM1 and coarse mode at the high alpine site Jungfraujoch. Atmos. Chem. Phys, 8, 407-423, 2008.

J. F. Hamilton, G. Allen, N. M. Watson, J. D. Lee, J. E. Saxton, A. C. Lewis, G. Vaughan, K. N. Bower, M. J. Flynn, J. Crosier, G. D. Carver, N. R. P. Harris, R. J. Parker, J. J. Remedios, N. A. D. Richards. Observations of an atmospheric chemical equator and its implications for the tropical warm pool region. J. Geophys. Res., 113, doi: 10.1029/2008JD009940, 2008.

J. Haywood, M. Bush, S. Abel, B. Claxton, H. Coe, J. Crosier, M. Harrison, B. Macpherson, M. Naylor and S. Osborne. Prediction of visibility and aerosol within the operational Met Office Unified Model. II: Validation of model performance using observational data, Quart. J. Roy. Met. Soc., 134, 1817-1832, 2008.

S. Sjogren, M. Gysel, E. Weingartner, M. R. Alfarra, J. Duplissy, J. Cozic, J. Crosier, H. Coe, U. Baltensperger. Hygroscopicity of the submicrometer aerosol at the high-alpine site Jungfraujoch, 3580 m a.s.l., Switzerland. Atmos. Chem. Phys, 8, 5715-5729, 2008.

Grants Awarded NERC. VAMOS Ocean-Cloud-Atmosphere-Land Study UK (VOCALS-UK) consortium grant. £1,364 k. NE/F019874/1. H. Coe, K. Bower, T. Choularton, J. Crosier, P. J. Connolly, M. W. Gallagher, G. Vaughan, G. McFiggans with colleagues from Reading, Leeds and the Met Office. Dr Alan Gadian, Leeds Exciting scientific developments and weather studies this year have involved international collaborations and new challenges. The European Convective Orographic Precipitation study, has led to further research and improvement in our understanding of the processes that cause precipitation and follow on from the published work from COST 722 (Gadian et al, 2008 and Huang et al, 2008 below). The need to understand and predict hazardous weather events, and their hydro meteorological impacts is becoming more important in this time of rapid climate change, and this is being included in the work of Younger et al. (2008)

Fig. 9. Results from a climate model showing how modification of stratocumulus droplet numbers, can lead to increased cloud reflectivity and global cooling (Latham et al, 2008).

Fig. 10. An artists impression of ships that could be used to squirt sea water into the low atmosphere and stratus clouds

a) VOCALS. The major new activity this year has been the participation in VOCALS, a large international project to examine oceanic stratocumulus clouds. These clouds cover 25% of the ocean surface and the impact of their reflectivity on the global energy balance far exceeds the importance of CO2 greenhouse effects. The processes which determine how these clouds maintain

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their structure, how and why they dissipate and what controls their albedo is critical, and poorly understood. A greater understanding of these "weather" processes is needed to understand climate change. Latham et al (2008) had an important NCAS contribution in discussing these issues (fig. 9). Whether temporary mitigation of CO2 warming is a serious and viable stop gap, using technologies such as spraying sea water into the atmosphere (fig.10), is a matter of conjecture, but a better understanding of the important processes is needed over the coming years. b) New model development. Activity in the area of cut cells and other designs continues and is likely to be very important over the next five years. Cut cells have been implemented in the micro-scale model; Dr. Sarah-Jane Lock, is collaborating with NCAR to implement cut cells in WRF, and discussions with DWD to implement cut cells in the DWD LM model are leading to further European collaborations. Optimisation of existing codes on the HECToR supercomputer also continues in the background, via DCSE awards. A grand challenge in this area is to develop grid structures to implement NWP codes on massively parallel (MPP) architectures - the proposed new Hartree Centre will enable the development of MPP grids suitable for next generation computer architectures. Unstructured grids in atmospheric models are being investigated in collaboration with Dr. Joanna Smeltzer, Loughborough University. NE/G004358/1. c) The OJIMS / JISC project investigating the possibility of an overlay journal has reached its penultimate stage. This has been an interesting collaboration between NCAS and the Royal Meteorological Society. The project final report has been submitted to JISC and the RMS council and is anticipated to lead to the generation of a new overlay journal for data and technical reports and software. Publications Y. H. Huang, A. M. Blyth, P. R. A. Brown, T. W. Choularton, P. Connolly, A. M. Gadian, H. Jones,

J. Latham, Z. Q. Cui, K. Carslaw. The development of ice in a cumulus cloud over southwest England. New J. Phys., 10, doi: 10.1088/1367-2630/10/10/105021, 2008.

J. Latham, P. Rasch, C. C. Chen, L. Kettles, A. Gadian, A. Gettelman, H. Morrison, K. Bower, T. Choularton. Global temperature stabilization via controlled albedo enhancement of low-level maritime clouds. Phil. Trans. Roy. Soc. A, 366, 3969-3987, 2008.

P. M. Younger, A. M. Gadian, C. G. Wang, J. E. Freer, K. J. Beven. The usability of 25 m resolution data from the UK Meteorological Office Unified Model input data for a hydrological model. Met. Apps., 15, 207-217, 2008.

A. M. Gadian, A. Coals, A. M. Blyth, W. Grabowski, J.-L. Brenguier, J. Latham. The effect of mixing and initial CCN distribution on condensational growth of droplets in clouds. COST Action 722, ISBN 978-92-898-0038-9. Final Report on Short range forecasting methods of fog, visibility and low clouds, p168-177, 2008.

Grants awarded Carnegie Institute. PhD studentship, $50,000 p.a. for three years. $150,000 in total. NERC. VAMOS Ocean-Cloud-Atmosphere-Land Study UK (VOCALS-UK) consortium grant.

£588 k. NE/F019874/1. A. Gadian, A. Blyth and J. McQuaid with colleagues from Manchester, Reading and the Met Office.

DCSE projects (resources to optimise code on national supercomputer) 1. WOMPS (WRF optimisation for massively parallel systems): joint bid between Manchester and Leeds, work being carried out at Daresbury 2. LEM optimisation. Led by Manchester (Paul Connolly) with NCAS participation.

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Dr. Ralph Burton, Leeds Dr Burton is NCAS Weather’s WRF expert, and his research programme involves support for other groups is setting up and using WRF, as well as using it himself for studies of orographic processes and convection. a) COPS work: i) 15 July case. WRF was run with both NCEP GFS and ECMWF analyses for this case. The failure of both sets of analyses to produce deep cloud suggests that there is some fundamental inconsistency between analysis fields, WRF model physics and reality. This is perhaps due to some aspect of the surface field initialisation, e.g. insufficient moisture in the boundary layer. Comparison with observed 2m temperatures and 2m RH suggests that WRF, at higher observation altitudes, agrees well with observations during daytime hours but struggles during the night time (fig.11): implying that the BL scheme is not performing well at higher altitudes (at lower altitudes, agreement is good). I have recently implemented a manual modification of lower-level RH in the WRF boundary condition files: this approach "works" (in the sense that any underlying balance violations due to RH modification do not cause the model to fail.) Early examination of this work suggests that increasing the lower-level RH increases maximum CAPE values in the simulation – but no deep cloud is produced.

Fig. 11. The modelled (red) and observed (black) 2m temperatures for selected AWS sites during the COPS campaign; data are for 15th July 2007. At higher elevations, the agreement is poor during the early morning hours. At lower elevations, agreement is good throughout the day. This raises concerns about the behaviour of the boundary-layer scheme in WRF, particularly the modelled representation of the nocturnal boundary layer. WRF does not seem to be mixing enough warm air down to the ground at night. ii). 12th August case. WRF has been initialised with GFS analyses and the results look promising. Lindsay Bennett (ICAS, Leeds) is repeating the experiment using ECMWF analyses. A comparison with the dual-Doppler data (supplied by Lindsay) suggests that WRF captures the observed wind fields reasonably well at lower altitudes (fig. 12), but discrepancies between observed and modelled winds are seen at higher altitudes. This may be linked to issues concerning the WRF BL schemes (see above). Also, investigation of WRF latent heat releases for this case is included in a paper submitted to Meteorologisches Zeitschrift (by the Salford Group, Jenny Davis et al.) of which I am named as co-author.

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(a)

(b)

Fig. 12. Results from WRF-Dual-Doppler Radar comparison (radar plots courtesy of Lindsay Bennett, Leeds). (a) shows the WRF model wind vectors for the 12th August, 11Z at 1.27 km above MSL; (b) shows the radar winds for the same time. In the south, the results generally agree well with WRF: in the north the two sets of results are not in such good agreement. b) T-REX. A paper is nearing completion. This study deals with the concept of linking variability in the valley response to upstream forcing (i.e. the upstream profile) - an old problem, but treated in a novel way! In collaboration with Simon Vosper and Peter Sheridan of the Met Office, I have determined the "underlying structures" in the surface flow patterns observed during T-REX, by means of principal components analysis. Then, the underlying structures in the modelled UM upstream profiles are similarly computed. Given that the modelled profiles manage to capture the essential atmospheric features observed in sonde releases (as demonstrated in the paper), I then linked the underlying structures observed up- to those observed downstream (fig. 13).

(a)

(b)

Fig. 13. One of the underlying structures from the statistical analysis (via principal components analysis) of wind vector data for T-REX. (a) shows a feature relating to convergence in the valley – associated with rotor-type events; (b) the importance of this particular structure at different times of day (local time). As can be seen there is a peak at around 3LT: this agrees with observations from, for example, glider pilots, who have described how rotor events are at their maximum in the mid-afternoon.

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(a)

(b)

Fig.14 (a) The locations of observing stations in Cumbria supplying hourly rainfall data. (b) The observed (black) and modelled (red) rainfall amounts for the six stations of (a) for the 25th October storm event. As can be seen, there is much variability in rainfall amount across stations - and in the ability of the model to capture the timing, and amount, of precipitation well. Sites 5 and 6 seem to do reasonably well, even though they are spatially quite far apart; site 4 does not agree well – even though it is geographically close to site 6. c) Lake district storm. WRF has been used to simulate the storm that occurred on the 25th Oct. of this year in the Lake District - a storm which made headlines as many competitors in a sporting event were "lost" for some time. Heavy amounts of very localised rainfall were observed. WRF seems to capture this (fig. 14) - and thus this makes an interesting case study. Early results suggest the presence of mountain waves in the Lake District extending to great heights - did this enhance the updraughts as the system moved over? Dr. Bharathi Boppana and Dr Zheng-Tong Xie, Southampton With contributions from Mr. Jean Claus and Prof Ian Castro a) Large Eddy Simulation for heat transfer in urban environments: The core component of the urban energy balance is the sensible heat flux and estimating its value accurately is crucial in parameterisation of urban areas. There exists a plethora of 2-D CFD models for investigating thermal effects in urban canyons. But a better insight of heat transfer effects on flow and dispersion in urban canopies and consequently its parameterisation demands robust 3-D simulations. To meet this objective, large eddy simulation (LES) of dispersion of a passive scalar from an area source in urban-like roughness has been studied as a first step. In particular, the laboratory experiment of Pascheke et al (2008), who measured scalar concentrations from an area source using naphthalene sublimation technique, has been modelled using LES. The computational domain is of size Lx × Ly × Lz = 16h × 16h × 6h where h is the height of the cube and a part of this domain consists of a passive scalar on the surface whose size is 8h×8h and is shown as a shaded surface in Fig. 15.

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Figure 15: Schematic view of the com- putational domain (shaded surface repre- sents a scalar with a fixed mass fraction of Cs=0.01)

Figure 16: Comparison of CN (obtained from each row of roughness elements) with experiments at different heights above the source area

Key findings:

1. From our systematic grid checks, we identified that the resolution needed for the scalar needs to be significantly finer than the resolution required for the flow as given by Xie and Castro (2006)

2. The contours of the concentrations at different heights shown in Fig. 17 are found to be in good agreement with experiments. From Fig. 17(a), it is observed that a large amount of concentration is retained behind the cubes due to the presence of large recirculation regions. Also, from Fig. 17(b), we observe that higher concentration values are located at (x/h, y/h)=(5.5, -0.5) and (5.5, 1.5). This is caused by the streamwise as well as the vertical dispersion of the larger concentrations that are present within the canopy on the leeward side of the cubes A and B shown in Fig. 17(a).

3. Figure 16 shows good quantitative agreement between the LES simulations and the laboratory experiments for area-averaged concentrations above the source area.

(a) z/h=0.3 (b) z/h=1.2

Figure 17: Contours of CN at different heights (square box represents the location of scalar on the surface)

4. It is found that the vertical flux across the plane z/h=1.5 is about 50% of the total vertical flux input at z/h=0 while the other 50% is convected downstream. The total surface flux from the simulations and the experiments differ by less than 4%.

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Further Work: More simulations with random height roughness elements are under way, for comparison with experiment. An appropriate scalar wall model will be incorporated so that the steep concentration gradient near the scalar surface can be accurately estimated with coarser resolution. This will provide the main additional building block necessary to allow similar computations of heat transfer, at least for small temperature differences. We will then progress to situations in which the heat transfer (from or to building surfaces) leads to dynamical influences on the flow. b) Large-eddy simulation for pollutant dispersion in London’s Marylebone Rd region More applications of the efficient inflow generation technique developed at Southampton [2] have been performed on simulating the wind tunnel experiments conducted under the DAPPLE programme, which provided data on flow and dispersion in the Marylebone area of London (in collaboration with Dr Hayden and Prof Robins, Surrey EnFlo lab). Some of this work has been published in the journal Atmos. Environ. [3]. Another paper regarding various wind directions over the DAPPLE region is under preparation. c) Study of wind direction effect on flows over a generic urban geometry: A comprehensive study of the effect of wind direction on flows over generic geometries (via a NERC PhD studentship) is being performed both computationally and experimentally. Preliminary results indicate that the wind direction strongly affects the flow in terms of drag coefficient, vertical pressure distributions on the obstacles and directional variations of the mean flow within the urban canopy layer. Initial results have been presented at conferences (e.g. [1]). Publications [1] J. M. Claus, Z.-T. Xie and I. P. Castro. Wind direction effects in urban environments: an LES

study. 8th UK Conference on Wind Engineering, Guildford, UK, 2008. [2] Z.-T. Xie and I. P. Castro. Efficient generation of inflow conditions for large-eddy simulation of

developed turbulent flows. Flow Turbulence and Combustion , 81, 449–470, 2008. [3] Z.-T. Xie and I. P. Castro. Large-eddy simulation for flow and dispersion in urban streets.

Atmospheric Environment, 43, 2174–2185, 2009. Dr Omduth Coceal, Reading An important problem in modelling winds in urban areas concerns the estimation of the drag induced by a group of buildings. Simplified urban canopy models have been developed, which treat the urban area as a porous medium modelled in terms of a distributed drag force. These canopy models require as input the sectional drag coefficient Cd(z ) of buildings in an array, ideally for a range of building packing densities and geometries. These are difficult to obtain experimentally, and their values are not generally known even for the simplest building configurations. Building-resolving simulations were performed and analysed to provide for the first time detailed quantitative information on these parameters for a range of packing densities (Fig 18). A formal analysis of the effect of spatial and time averaging of data has been performed, and applied to a comprehensive dataset generated previously from direct numerical simulations over idealized buildings. This analysis will be extended to the operation of a lidar in a recent urban field experiment. As part of an effort to validate and develop a street network model for short-range dispersion in urban areas, numerical datasets have been generated and analysed for flow at different angles over idealized building arrays using direct numerical simulations. The results have shown that a sideways 'lift' force is generated for angles for which the flow is asymmetrical, and that the wind

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direction changes down into the building canopy. This new finding could have important implications for dispersion in urban areas.

Fig. 18: Variation of sectional drag coefficient within an urban-like canopy for different packing densities (from Santiago, Coceal, Martilli & Belcher, 2008). Left panel: The usual drag coefficient Cd(z) varies in an irregular way for dense building arrays, and is therefore not a good way to characterise the drag. Right panel: A modified version of the drag coefficient that takes into account spatial and temporal fluctuating velocity scales has a much more regular variation. A comprehensive review has been written on the literature and data on mean flow and turbulence in the urban canopy and roughness sublayers. This review was commissioned by the Met Office towards 'urbanizing' the NAME dispersion model and is available as a Met Office report. A journal review paper is also being written based on this report. There is a scarcity of reliable models of urban roughness sublayers, despite the wealth of experimental and CFD data gathered over recent years. A 2-day workshop was organized on 30-31 March 2009 at the University of Reading (http://www.met.reading.ac.uk/urb_met/workshop), and brought together leading scientists working in the subject and in related fields, with the aim of reviewing recent experimental and numerical results, discussing about the nature of turbulence in urban roughness sublayers, and generating ideas of how the urban roughness sublayer may be better modelled on the basis of the available results. A summary paper will be written. Publications Coceal, O., T.G. Thomas and S.E. Belcher. Spatially-averaged flow statistics within a canopy of

large bluff bodies: results from direct numerical simulations. Acta Geophysica 56, 3, 862-875, 2008.

Manes, C., D. Pokrajac, O. Coceal and I. McEwan. On the significance of form-induced/dispersive stress in rough wall turbulent boundary layers. Acta Geophysica 56, 3, 845-861, 2008.

Santiago, J.L., O. Coceal, A. Martilli and S.E. Belcher. Variation of the sectional drag coefficient of a group of buildings with packing density. Boundary-Layer Meteorol. 128, 445-457, 2008.

Xie, Z., O. Coceal and I.P. Castro. Large-Eddy Simulation of flows over random urban-like obstacles. Boundary-Layer Meteorol. 129, 1-23, 2008.

Barlow, J.F. and O. Coceal. A review of urban roughness sublayer turbulence, UK Met Office, Technical Report No. 527, 68p, 2008.

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Dr. John Marsham, Leeds Dr Marsham joined NCAS Weather in January 2009, assuming the Convective Storms post described above. Much of his work during 2008 was focussed on results from the GERBILS field campaign in West Africa, in particular:

(i) exploring the role played by cold-pool outflows from convective storms in lifting dust in the Sahel, and the possible contribution to the seasonal dust cycle

(ii) showing how surface albedo and flux anomalies can contribute to the layering of dust and water-vapour observed in the uniquely deep Saharan convective boundary layer.

His plans for 2009 are focused on the downdraughts from elevated storms, and the role they play in controlling such storms. (1) Complete CSIP IOP3 observational work (one paper submitted, one paper in preparation), as well as CSIP IOP12 rolls work. CSIP IOP3 has provided unique observations of: (i) an elevated mesoscale convective system (MCS) with two rear inflows (ii) the wave generated in the low-level stable air beneath the MCS, apparently generated by the lower rear-inflow, which could not penetrate the stable air. (2) Complete a paper in preparation on the observed transition from elevated to surface-based convection, where the storm's rear-inflow was observed to gradually build downwards to the surface (IHOP 13th June observations and WRF modelling run by Stan Trier, NCAR). This work was mainly done in a three month visit to NCAR funded by the British Council (Jan/Feb/Mar 2009). Follow-up work will explore microphysical sensitivities of the downdraughts in more detail and the interactions of the downdraughts with the stable nocturnal boundary-layer, perhaps using George Bryan’ CRM and/or WRF. (3) April: Attend Met Office MEVEX campaign in Oman for two weeks (for desert convection/meteorology and dust uplift). Future student projects on dust uplift in cold pools/dust transport from Arabia are possible. (4) A proposal is planned on the two-way coupling between maritime deep convection and aerosols, with both aerosols affecting cloud microphysics and the convection and its outflows affecting the aerosol (c.f. Devine et al, GRL, 2006, for the high windspeeds from cold-pools affecting the surface aerosol source). Publications Haywood, J. M., and 29 others inc. J. Marsham. Overview of the Dust and Biomass-burning

Experiment and African Monsoon Multidisciplinary Analysis Special Observing Period-0. J. Geophys. Res., 113, D00C17, doi:10.1029/2008JD010077, 2008.

J. H. Marsham, D. J. Parker, C. M. Grams, B. T. Johnson, W. M. F. Grey and A. N. Ross. Observations of mesoscale and boundary-layer scale circulations affecting dust transport and uplift over the Sahara, Atmos. Chem. Phys., 8, 6979-6993, 2008.

J. H. Marsham, D. J. Parker, C. M. Grams, C. M. Taylor and J. M. Haywood. Uplift of Saharan dust south of the inter-tropical discontinuity. J. Geophys. Res. Atmos., 113, D21102, doi:10.1029/2008JD009844, 2008.

Q. Huang, J. H. Marsham, D. J. Parker, W. S. Tian, T. Weckwerth. A Comparison of Roll and Nonroll Convection and the Subsequent Deepening Moist Convection: An LEM Case Study Based on SCMS Data. Mon. Wea. Rev., 137, 350-365, 2009.

J. Cuesta, J. H. Marsham, D. J. Parker, C. Flamant. Dynamical mechanisms controlling the vertical redistribution of dust and the thermodynamic structure of the West Saharan atmospheric boundary layer during summer. Atmospheric Science Letters, 10, 34-42, 2009.