Showing papers by "Jim Haywood published in 2010"

The AMMA field campaigns: multiscale and multidisciplinary observations in the West African region

Abstract: AMMA--the African Monsoon Multidisciplinary Analysis--is the biggest programme of research into environment and climate ever attempted in Africa. AMMA has involved a comprehensive field experiment bringing together ocean, land and atmospheric measurements, on time-scales ranging from hourly and daily variability up to the changes in seasonal activity over a number of years. Many of the publications in this special issue make use of subsets of the AMMA measurements, collected from a diverse set of sensors. As a general introduction to the special issue, this paper provides a comprehensive overview of the AMMA observational programme, and summarises the scientific strategy which has defined the field deployment. The relationship between the existing observational monitoring networks of the region and the new sensors deployed for AMMA, and for the future, is described. Making use of regional and sub-regional maps, the main groups of sensors are described in terms of their deployment periods and their spatial co-ordination. The key linkages between different groups of measurements are also outlined, in terms of the strategy for their combined use and in terms of their interdependence. Some brief summaries of conditions sampled during the three years of the AMMA Extended Observing Period are also given.

Observations of the eruption of the Sarychev volcano and simulations using the HadGEM2 climate model.

Abstract: [1] In June 2009 the Sarychev volcano located in the Kuril Islands to the northeast of Japan erupted explosively, injecting ash and an estimated 1.2 ± 0.2 Tg of sulfur dioxide into the upper troposphere and lower stratosphere, making it arguably one of the 10 largest stratospheric injections in the last 50 years. During the period immediately after the eruption, we show that the sulfur dioxide (SO2) cloud was clearly detected by retrievals developed for the Infrared Atmospheric Sounding Interferometer (IASI) satellite instrument and that the resultant stratospheric sulfate aerosol was detected by the Optical Spectrograph and Infrared Imaging System (OSIRIS) limb sounder and CALIPSO lidar. Additional surface-based instrumentation allows assessment of the impact of the eruption on the stratospheric aerosol optical depth. We use a nudged version of the HadGEM2 climate model to investigate how well this state-of-the-science climate model can replicate the distributions of SO2 and sulfate aerosol. The model simulations and OSIRIS measurements suggest that in the Northern Hemisphere the stratospheric aerosol optical depth was enhanced by around a factor of 3 (0.01 at 550 nm), with resultant impacts upon the radiation budget. The simulations indicate that, in the Northern Hemisphere for July 2009, the magnitude of the mean radiative impact from the volcanic aerosols is more than 60% of the direct radiative forcing of all anthropogenic aerosols put together. While the cooling induced by the eruption will likely not be detectable in the observational record, the combination of modeling and measurements would provide an ideal framework for simulating future larger volcanic eruptions.

Geoengineering by stratospheric SO 2 injection: results from the Met Office HadGEM2 climate model and comparison with the Goddard Institute for Space Studies ModelE

Abstract: . We examine the response of the Met Office Hadley Centre's HadGEM2-AO climate model to simulated geoengineering by continuous injection of SO2 into the lower stratosphere, and compare the results with those from the Goddard Institute for Space Studies ModelE. Despite the differences between the models, we find a broadly similar geographic distribution of the response to geoengineering in both models in terms of near-surface air temperature and mean June–August precipitation. The simulations also suggest that significant changes in regional climate would be experienced even if geoengineering was successful in maintaining global-mean temperature near current values, and both models indicate rapid warming if geoengineering is not sustained.

Parameterization of contrails in the UK Met Office Climate Model

Abstract: Persistent contrails are believed to currently have a relatively small but significant positive radiative forcing on climate. With air travel predicted to continue its rapid growth over the coming years, the contrail warming effect on climate is expected to increase. Nevertheless, there remains a high level of uncertainty in the current estimates of contrail radiative forcing. Contrail formation depends mostly on the aircraft flying in cold and moist enough air masses. Most studies to date have relied on simple parameterizations using averaged meteorological conditions. In this paper we take into account the short-term variability in background cloudiness by developing an on-line contrail parameterization for the UK Met Office climate model. With this parameterization, we estimate that for the air traffic of year 2002 the global mean annual linear contrail coverage was approximately 0.11%. Assuming a global mean contrail optical depth of 0.2 or smaller and assuming hexagonal ice crystals, the corresponding contrail radiative forcing was calculated to be less than 10 mW m(-2) in all-sky conditions. We find that the natural cloud masking effect on contrails may be significantly higher than previously believed. This new result is explained by the fact that contrails seem to preferentially form in cloudy conditions, which ameliorates their overall climate impact by approximately 40%.

The Atlantic inflow to the Saharan heat low: observations and modelling

Abstract: The inflow of relatively cold and stably stratified air from the Atlantic Ocean into western Mauritania and into the southwestern part of the Saharan heat low is studied using the mesoscale COSMO model. This model was used to provide operational forecasts for the GERBILS field campaign, which was conducted by the Met Office in West Africa in June 2007. The forecasts were validated against airborne measurements as well as satellite imagery and were found to represent the main synoptic features of the region accurately. A complex mesoscale feature in western Mauritania, which we call the Atlantic Inflow, was identified in the COSMO model output. The main component of the Atlantic Inflow is the sea breeze and associated front. The sea breeze interacts with larger-scale, higher-altitude fluctuations in the thermal and humidity advection. During the day the balance between horizontal advection of cool maritime air and turbulence in the convective boundary layer over land results in a stationary sea breeze front at the coast. Once turbulence dies down in the evening, the sea breeze front penetrates inland. Above the sea breeze layer, thermal advection in the Saharan Atmospheric Boundary Layer (SABL) also controls the structure of the Atlantic Inflow. A marked baroclinic zone was observed, in which the temperature and humidity made a relatively smooth transition from values typical of the Atlantic air to values characteristic of the SABL. Budget calculations showed that, through its cooling and occasional moistening at low levels, the Atlantic Inflow has an important impact on the regional heat and moisture budgets. Copyright © Royal Meteorological Society and Crown Copyright, 2009

Evidence of internal mixing of African dust and biomass burning particles by individual particle analysis using electron beam techniques

Abstract: [1] In this investigation, the interaction between aerosols derived from biomass burning and dust aerosol is the focus of interest. We have examined the morphology and chemical composition of dust and biomass burning aerosol particles sampled on airborne filters using an environmental scanning electron microscope equipped with both an energy dispersive X-ray analysis and an image analysis system. Using these techniques, we were able to study both mineral dust and biomass burning particles in samples containing significant amounts of both biomass burning and dust aerosol, compared to samples dominated by biomass burning aerosol and a sample dominated by dust aerosol. Thirteen particle types were classified in the samples. Aluminosilicates were the dominant particle type in the sample dominated by dust aerosol. Many more particles were found to be rich in S, Na, and K in the sample dominated by biomass burning aerosols, and particles in these samples were also found to be completely devoid of Cl due to aging of the aerosol and chemical conversion during atmospheric transport. We suggest that biomass burning particles containing all three characteristic elements are internal mixtures. Particle imaging showed that up to a third of soot particles were internally mixed with mineral dust particles in samples containing significant numbers of both biomass burning and mineral dust particles. This internal mixing would change the optical properties of the particle ensemble and the direct radiative forcing caused by biomass burning significantly.

Estimating the climate impact of linear contrails using the UK Met Office climate model

Abstract: The HadGEM2 global climate model is employed to investigate some of the linear contrail effects on climate. Our study parameterizes linear contrails as a thin layer of aerosol. We find that at 100 times the air traffic of year 2000, linear contrails would change the equilibrium global-mean temperature by +0.13 K, corresponding to a climate sensitivity of 0.3 K/(Wm(-2)) and a climate efficacy of 31% (significantly smaller than the only previously published estimate of 59%). Our model suggests that contrails cause a slight warming of the surface and, as noted by most global warming modelling studies, land areas are affected more than the oceans. Also, unlike the contrail coverage and radiative forcing, the contrail temperature change response is not geographically correlated with air traffic patterns. In terms of the contrail impact on precipitation, the main feature is the northern shift of the Inter-Tropical Convergence Zone. Finally, our model strongly indicates that the contrail impact on both the diurnal temperature range and regional climate is significantly smaller than some earlier studies suggested.