Contrasting Observed Atmospheric Responses to Tropical Sea Surface Temperature Warming Patterns

AM was funded by a NERC doctoral training partnership grant (NE/L002558/1). This study was funded as part of NERC's support of the National Center for Earth Observation: HB and PIP were supported by grant number NE/R016518/1.Equilibrium climate sensitivity (ECS) is a theoretical concept which describes the change in global mean surface temperature that results from a sustained doubling of atmospheric CO2. Current ECS estimates range from ∼1.8 to 5.6 K, reflecting uncertainties in climate feedbacks. The sensitivity of the lower (1,000–700 hPa) and upper (500–200 hPa) troposphere to changes in spatial patterns of tropical sea surface temperature (SST) have been proposed by recent model studies as key feedbacks controlling climate sensitivity. We examine empirical evidence for these proposed mechanisms using 14 years of satellite data. We examine the response of temperature and humidity profiles, clouds, and top‐of‐the‐atmosphere radiation to relative warming in tropical ocean regions when there is either strong convection or subsidence. We find warmer SSTs in regions of strong subsidence are coincident with a decrease in lower tropospheric stability (−0.9 ± 0.4 KK−1) and low cloud cover (∼−6% K−1). This leads to a warming associated with the weakening in the shortwave cooling effect of clouds (4.2 ± 1.9 Wm−2K−1), broadly consistent with model calculations. In contrast, warmer SSTs in regions of strong convection are coincident with an increase in upper tropospheric humidity (3.2 ± 1.5% K−1). In this scenario, the dominant effect is the enhancement of the warming longwave cloud radiative effect (3.8 ± 3.0 Wm−2K−1) from an increase in high cloud cover (∼7% K−1), though changes in the net (longwave and shortwave) effect are not statistically significant (p < 0.003). Our observational evidence supports the existence of mechanisms linking contrasting atmospheric responses to patterns in SST, mechanisms which have been linked to climate sensitivity.Publisher PDFPeer reviewe

Self-Confirming Price Prediction for Bidding in Simultaneous Ascending Auctions

Simultaneous, separate ascending auctions are ubiquitous, even when agents have preferences over combinations of goods, from which arises the emph{exposure problem}. Little is known about strategies that perform well when the exposure problem is important. We present a new family of bidding strategies for this situation, in which agents form and utilize various amounts of information from predictions of the distribution of final prices. The predictor strategies we define differ in their choice of method for generating the initial (pre-auction) prediction. We explore several methods, but focus on emph{self-confirming} predictions. An agents prediction of characteristics of the distribution of closing prices is self-confirming if, when all agents follow the same predictor bidding strategy, the final price distributions that actually result are consistent with the utilized characteristics of the prediction. We extensively analyze an auction environment with five goods, and five agents who each can choose from 53 different bidding strategies (resulting in over 4.2 million distinct strategy combinations). We find that the self-confirming distribution predictor is a highly stable, pure-strategy Nash equilibrium. We have been unable to find any other Nash strategies in this environment. In limited experiments in other environments the self-confirming distribution predictor consistently performs well, but is not generally a pure-strategy Nash equilibrium

Self-Confirming Price Prediction for Bidding in Simultaneous Ascending Auctions

Simultaneous, separate ascending auctions are ubiquitous, even when agents have preferences over combinations of goods, from which arises the emph{exposure problem}. Little is known about strategies that perform well when the exposure problem is important. We present a new family of bidding strategies for this situation, in which agents form and utilize various amounts of information from predictions of the distribution of final prices. The predictor strategies we define differ in their choice of method for generating the initial (pre-auction) prediction. We explore several methods, but focus on emph{self-confirming} predictions. An agents prediction of characteristics of the distribution of closing prices is self-confirming if, when all agents follow the same predictor bidding strategy, the final price distributions that actually result are consistent with the utilized characteristics of the prediction. We extensively analyze an auction environment with five goods, and five agents who each can choose from 53 different bidding strategies (resulting in over 4.2 million distinct strategy combinations). We find that the self-confirming distribution predictor is a highly stable, pure-strategy Nash equilibrium. We have been unable to find any other Nash strategies in this environment. In limited experiments in other environments the self-confirming distribution predictor consistently performs well, but is not generally a pure-strategy Nash equilibrium

Interpreting Earth's top-of-the-atmosphere broadband radiation flux variability using observations and models

Observed broadband radiation fluxes at the top-of-the atmosphere (TOA) and at the Earth’s surface are determined by a complex network of atmospheric and surface processes. It is imperative that climate models are able to accurately simulate these observed variations and relationships in order to provide confidence in projections of our future climate. In this thesis I use a combination of observations, reanalysis fields and output from global circulation models (GCMs) to interpret radiation flux variability with respect to atmospheric properties and processes, in particular clouds, atmospheric water vapour and aerosols. I use observations and models in two ways. In Chapters 3 and 4 I evaluate model output using observations from satellite instruments and surface measurement stations to characterise the model ability to 1) recreate observed variability and 2) contrast TOA and surface radiation flux co-variability with atmospheric properties. In Chapter 5 I use satellite observations of atmospheric temperature and humidity profiles, as well as broadband radiation flux, to assess evidence of physical mechanisms which have recently been hypothesised using output from GCMs. The chapters are based on two regions of the tropics. I focus on the first of these, a region in western Africa, partly due to the presence of aerosols, such as Saharan mineral dust, and also the west African monsoon. Both of these factors have large impacts on the radiation balance and therefore make this region interesting from a radiation perspective. Additionally, west Africa is a region vulnerable to changes in climate, having already suffered from extended droughts in the last decades. My second focus region is the tropical ocean, where changes in tropical low clouds play an important role in the TOA radiation balance, and has therefore been linked to climate model sensitivity. The spatial and temporal scales used in the studies vary dramatically, which determines both the model output evaluated and also the methods I employ. In Chapter 3, I exploit the 2006 high frequency observational data at Niamey, Niger from the Atmospheric Radiation Measurement (ARM) Mobile Facility, the Geostationary Earth Radiation Budget (GERB) and Spinning Enhanced Visible and Infrared Imager (SEVIRI) instruments, and products from the Climate Mon itoring Satellites Applications Facility (CMSAF) to evaluate daily output from the European Centre for Medium-Range Weather Forecasts (ECWMF) Integrated Forecasting System 43r1. The data available include surface, atmospheric pro file and TOA measurements. By constructing multi-variate linear models of each component in the energy budget, I test their sensitivity to changes in atmo spheric properties, including 2m air temperature, aerosol optical depth (AOD), cloud properties and total column water vapour (TCWV). I find that the lack of ice in clouds, manifested as a reduced ice water path (IWP) in 43r1 with respect to the estimate from CMSAF, results in too much shortwave radiation passing through the atmosphere in 43r1, and therefore too much downwelling shortwave radiation (DSR) at the surface and too little reflected shortwave radiation (RSR) at the TOA. I also identify the use of an aerosol climatology in 43r1 as a cause of discrepancy between the observation and the model in the surface fluxes, with the lower aersol loading in the model leading to a reduction in downwelling longwave radiation (DLR) and an increase in DSR. This work is published in Atmospheric Chemistry and Physics as Mackie et al. (2017). In Chapter 4, I examine a wider region in western Africa, which I refer to as ‘west Africa’, which encompasses three distinct sub regions: the Sahel, the Sahara and the south-western coastal region. As observational references, I use a range of radiation data from the TOA and surface from satellite products and surface station measurements to construct mean annual cycles with which to evaluate output from GCMs submitted to the Intergovernmental Panel on Climate Change’s Coupled Model Intercomparison Project Phase 5 (CMIP5). This chapter has two aims: firstly, to compare the reference data and to establish the observational range in the targeted metrics, and secondly to evaluate how the CMIP5 multi-model mean and range fit with this range. By contrasting coupled and atmosphere-only model output, I link differences in radiation at the TOA to the models’ tendency to model the west African monsoon onset too late and to model the limit of its northwards progression to too far south. By contrasting the sensitivity of the models to changes in AOD and TCWV to that of the Clouds and the Earth’s Radiant Energy System Energy Balanced and Filled (CERES EBAF) product, I find some indication that DSR in the CMIP5 models may be too sensitive to changes in water vapour, and not sensitive enough to changes in AOD. This work is under review at the AGU journal Earth and Space Science. In Chapter 5, I evaluate observational evidence for a model-based hypotheses which links tropospheric temperature and humidity changes to patterns in tropical sea surface temperature (SST) warming. The hypothesis states that if SSTs in regions of strong ascent warm relative to the tropical ocean mean, the warming is efficiently lofted to the upper troposphere. In contrast, if warming is concentrated in regions of subsidence, the effects are limited to below the inversion which is characteristic of these regions. The subsequent effects of SST warming patterns are hypothesised to be key in determining the feedbacks from low cloud, and has thus been linked to climate sensitivity. To test this hypothesis I use co located Atmospheric Infrared Sounder (AIRS) temperature and humidity profiles and CERES radiation data, including window region data, and subset these data using vertical velocity at 500 hPa from ECMWF’s ERA-Interim reanalysis. I find some evidence which supports the hypothesised mechanism, specifically that if subsiding regions warm preferentially, there is a strong decrease in low cloud, with associated decrease in reflected shortwave radiation (RSR), and evidence that temperature increases are suppressed above the inversion. I also find small, but statistically significant, increases in humidity above the boundary layer inversion, though the origin of this is not clear. If regions of convection preferentially warm, the observations suggest that changes in relative humidity in the upper troposphere are due to changes in specific humidity rather than temperature, with temperatures in the upper troposphere relatively insensitive to relative warming. The largest changes in TOA radiation are in the longwave, which I hypothesise are linked to the observed increase in high cloud. This work is being prepared for publication

Observed and CMIP5‐Simulated Radiative Flux Variability Over West Africa

We explore the ability of general circulation models in the Coupled Model Intercomparison Project (CMIP5) to recreate observed seasonal variability in top‐of‐the‐atmosphere and surface radiation fluxes over West Africa. This tests CMIP5 models' ability to describe the radiative energy partitioning, which is fundamental to our understanding of the current climate and its future changes. We use 15 years of the monthly Clouds and the Earth's Radiant Energy System Energy Balanced and Filled (EBAF) product, alongside other satellite, reanalysis, and surface station products. We find that the CMIP5 multimodel mean is generally within the reference product range, with annual mean CMIP5 multimodel mean—EBAF of −0.5 W m−2 for top‐of‐the‐atmosphere reflected shortwave radiation, and 4.6 W m−2 in outgoing longwave radiation over West Africa. However, the range in annual mean of the model seasonal cycles is large (37.2 and 34.0 W m−2 for reflected shortwave radiation and outgoing longwave radiation, respectively). We use seasonal and regional contrasts in all‐sky fluxes to infer that the representation of the West African monsoon in numerical models affects radiative energy partitioning. Using clear‐sky surface fluxes, we find that the models tend to have more downwelling shortwave and less downwelling longwave radiation than EBAF, consistent with past research. We find models that are drier and have lower aerosol loading tend to show the largest differences. We find evidence that aerosol variability has a larger effect in modulating downwelling shortwave radiation than water vapor in EBAF, while the opposite effect is seen in the majority of CMIP5 models.ISSN:2333-508

Targeted screening in the UK:A narrow concept with broad application

A recent report on screening in the UK proposed that the responsibility for recommendations on population and targeted screening programmes should be held by one new integrated advisory body. There is no wide international consensus on the definition of targeted screening. Our review identified and compared the defining components of screening terms: targeted, population, selective, and cascade screening, and case finding. Definitions of targeted screening and population screening were clearly demarcated by the eligible population; targeted and selective screening were found to be conceptually interchangeable; cascade screening, whilst conceptually similar to targeted screening across several components, was only used within the context of genetic diseases. There was little consensus between different definitions of case finding. These comparisons contributed to an updated definition of targeted screening. Considerable overlap between definition components across terms implies that a broad range of disease areas may fall into the remit of the new advisory body

Self-Confirming Price Prediction for Bidding in Simultaneous Ascending Auctions

Simultaneous ascending auctions present agents with the exposure problem: bidding to acquire a bundle risks the possibility of obtaining an undesired subset of the goods. Auction theory provides little guidance for dealing with this problem. We present a new family of decisiontheoretic bidding strategies that use probabilistic predictions of final prices. We focus on selfconfirming price distribution predictions, which by definition turn out to be correct when all agents bid decision-theoretically based on them. Bidding based on these is provably not optimal in general, but our experimental evidence indicates the strategy can be quite effective compared to other known methods.http://deepblue.lib.umich.edu/bitstream/2027.42/49509/1/ppsaa.pd

Bidding Strategies for Simultaneous Ascending Auctions

Simultaneous ascending auctions present agents with various strategic problems, depending on preference structure. As long as bids represent non-repudiable offers, submitting non-contingent bids to separate auctions entails an \emph{exposure problem}: bidding to acquire a bundle risks the possibility of obtaining an undesired subset of the goods. With multiple goods (or units of a homogeneous good) bidders also need to account for their own effects on prices. Auction theory does not provide analytic solutions for optimal bidding strategies in the face of these problems. We present a new family of decision-theoretic bidding strategies that use probabilistic predictions of final prices: \emph{self-confirming distribution-prediction} strategies. Bidding based on these is provably not optimal in general. But evidence using empirical game-theoretic methods we developed indicates the strategy is quite effective compared to other known methods when preferences exhibit complementarities. When preferences exhibit substitutability, simpler \emph{demand-reduction} strategies address the own price effect problem more directly and perform better.http://deepblue.lib.umich.edu/bitstream/2027.42/57741/2/saa-appendix.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/57741/1/ppsaa.pd