The importance of temporal collocation for the evaluation of aerosol models with observations

This is the final version of the article. Available from European Geosciences Union (EGU) and Copernicus Publications via the DOI in this record.It is often implicitly assumed that over suitably long periods the mean of observations and models should be comparable, even if they have different temporal sampling. We assess the errors incurred due to ignoring temporal sampling and show that they are of similar magnitude as (but smaller than) actual model errors (20–60 %). Using temporal sampling from remote-sensing data sets, the satellite imager MODIS (MODerate resolution Imaging Spectroradiometer) and the ground-based sun photometer network AERONET (AErosol Robotic NETwork), and three different global aerosol models, we compare annual and monthly averages of full model data to sampled model data. Our results show that sampling errors as large as 100 % in AOT (aerosol optical thickness), 0.4 in AE (Ångström Exponent) and 0.05 in SSA (single scattering albedo) are possible. Even in daily averages, sampling errors can be significant. Moreover these sampling errors are often correlated over long distances giving rise to artificial contrasts between pristine and polluted events and regions. Additionally, we provide evidence that suggests that models will underestimate these errors. To prevent sampling errors, model data should be temporally collocated to the observations before any analysis is made. We also discuss how this work has consequences for in situ measurements (e.g. aircraft campaigns or surface measurements) in model evaluation. Although this study is framed in the context of model evaluation, it has a clear and direct relevance to climatologies derived from observational data sets.This work was supported by the Natural Environmental Research Council grant nr NE/J024252/1 (Global Aerosol Synthesis And Science Project). Computational resources for the ECHAM-HAM runs were made available by Deutsches Klimarechenzentrum (DKRZ) through support from the Bundesministerium für Bildung und Forschung (BMBF). The ECHAM-HAMMOZ model is developed by a consortium composed of ETH Zurich, Max Planck Institut für Meteorologie, Forschungszentrum Jülich, University of Oxford, the Finnish Meteorological Institute and the Leibniz Institute for Tropospheric Research, and managed by the Center for Climate Systems Modeling (C2SM) at ETH Zurich. P. Stier would like to acknowledge funding from the European Research Council under the European Union’s Seventh Framework Programme (FP7/2007-2013) ERC project ACCLAIM (grant agreement no. FP7-280025). HadGEMUKCA was run on the ARCHER UK National Supercomputing Service (http://www.archer.ac.uk). The development of the UKCA model (www.ukca.ac.uk) was supported by the UK’s Natural Environment Research Council (NERC) through the NERC Centres for Atmospheric Science (NCAS) initiative. MIROC-SPRINTARS was run on the SX-9 supercomputer at NIES (CGER) in Japan. The figures in this paper were prepared using David W. Fanning’s coyote library for IDL. The authors thank an anonymous reviewer and in particular Andrew Sayer for useful comments that helped improve the manuscript

Robust observational constraint of uncertain aerosol processes and emissions in a climate model and the effect on aerosol radiative forcing

The effect of observational constraint on the ranges of uncertain physical and chemical process parameters was explored in a global aerosol–climate model. The study uses 1 million variants of the Hadley Centre General Environment Model version 3 (HadGEM3) that sample 26 sources of uncertainty, together with over 9000 monthly aggregated grid-box measurements of aerosol optical depth, PM2.5, particle number concentrations, sulfate and organic mass concentrations. Despite many compensating effects in the model, the procedure constrains the probability distributions of parameters related to secondary organic aerosol, anthropogenic SO2 emissions, residential emissions, sea spray emissions, dry deposition rates of SO2 and aerosols, new particle formation, cloud droplet pH and the diameter of primary combustion particles. Observational constraint rules out nearly 98 % of the model variants. On constraint, the ±1σ (standard deviation) range of global annual mean direct radiative forcing (RFari) is reduced by 33 % to −0.14 to −0.26 W m−2, and the 95 % credible interval (CI) is reduced by 34 % to −0.1 to −0.32 W m−2. For the global annual mean aerosol–cloud radiative forcing, RFaci, the ±1σ range is reduced by 7 % to −1.66 to −2.48 W m−2, and the 95 % CI by 6 % to −1.28 to −2.88 W m−2. The tightness of the constraint is limited by parameter cancellation effects (model equifinality) as well as the large and poorly defined “representativeness error” associated with comparing point measurements with a global model. The constraint could also be narrowed if model structural errors that prevent simultaneous agreement with different measurement types in multiple locations and seasons could be improved. For example, constraints using either sulfate or PM2.5 measurements individually result in RFari±1σ ranges that only just overlap, which shows that emergent constraints based on one measurement type may be overconfident

Prominence oscillations and stability - Communicating the distant photospheric boundary

The photosphere provides an important boundary condition for prominence support. The conservation of photospheric flux (sometimes called line tying) sets a serious constraint on the evolution of coronal magnetic fields. This boundary condition can only be communicated to the prominence by Alfvén and magneto-acoustic waves. As a result, the boundary condition as experienced by the prominence at height h lags behind a time h/υA (υA: Alfvénspeed) as compared to the instantaneous situation at the location of the photosphere. In this paper I study vertical oscillations and stability of prominences, taking retardation effects into account. An equation of motion for a Kuperus-Raadu prominence is derived, describing the prominence as a line current and the photosphere as a perfectly conducting plate. Solving this equation of motion implies solving the full time-dependent Maxwell equations, thus guaranteeing a realistic field evolution under the assumption of photospheric line tying. In terms of the currents that flow, such a description is equivalent to the corresponding MHD picture. The results indicate that the travel time h/υA is an important parameter of the system as it influences the decay or growth times of prominence oscillations greatly. A new kind of instability is found, whereby the prominence experiences oscillations growing in time, even in the nonlinear regime. This instability occurs when the travel time h/υA is comparable to or greater than the oscillation period. Also, forced oscillations can only be significant for rather precisely matched values of h/υA and the driving period

Vertical prominence oscillations and stability - A comparison of the influence of the distant photosphere in Inverse Polarity and Normal Polarity prominence models

All MHD models for prominence equilibrium to date can, in essence, be reduced to two simple current models that describe so-called Inverse (IP) and Normal (NP) Polarity topologies. Using these simple current models, I investigate the influence of the boundary condition provided by the distant photosphere (flux conservation) on vertical prominence oscillations. The fact that the photosphere is some distance z away from the prominence, implies that the Lorentz force acting on the prominence, due to the photospheric boundary condition, evolves with a delay z/vA (vA coronal Alfvénspeed). In an earlier paper (Schutgens 1997), it was shown that, in the case of a Kuperus-Raadu (IP) prominence, this delay can greatly influence the vertical stability properties of prominences, especially when ωτ0 ≳ 1 (ω: frequency of oscillation, τ0 = 2z/vA). In this paper a comparative study is made of this effect in IP and NP prominences. Because of a different force balance, NP and IP prominences have currents and oscillation periods of different magnitude. The influence of the distant photosphere on NP prominences is minimal, while it has a very pronounced effect on IP prominences. As a result, NP and IP prominences have widely different stability properties. Foot point shaking due to photospheric 5 min. oscillations will only excite IP prominences

Simulated Doppler radar observations of inhomogeneous clouds: Application to the EarthCARE space mission

A new simulation technique for spaceborne Doppler radar observations that was developed specifically for inhomogeneous targets is presented. Cloud inhomogeneity affects Doppler observations in two ways. First, line-of-sight velocities within the instantaneous field of view are unequally weighted. As the large forward motion of a spaceborne radar contributes to these line-of-sight velocities this causes biases in observed Doppler speeds. Second, receiver voltages now have time-varying stochastical properties, increasing the inaccuracy of Doppler observations. The new technique predicts larger inaccuracies of observed Doppler speeds than the traditional random signal simulations based on the inverse Fourier transform. The accuracy of Doppler speed observations by a spaceborne 95-GHz radar [as part of the proposed European Space Agency (ESA)/Japan Aerospace Exploration Agency (JAXA)/National Institute for Information and Communications Technology (NICT) EarthCARE mission] is assessed through simulations for realistic cloud scenes based on observations made by ground-based cloud-profiling radars. Close to lateral cloud boundary biases as large as several meters per second occur. For half of the cloud scenes investigated, the distribution of the in-cloud bias has an rms of 0.5 m s-1, implying that a bias in excess of 0.5 m s-1 will not be uncommon. An algorithm to correct the bias in observed Doppler observations, based on the observed gradient of reflectivity along track, is suggested and shown to be effective; that is, the aforementioned rms bias reduces to 0.14 m s-1. © 2008 American Meteorological Society

Simulating range oversampled Doppler radar profiles of inhomogeneous targets

A new technique for generating range oversampled profiles of Doppler radar signals that have been backscattered by distributed targets is presented in this paper. The technique was developed for spaceborne cloud radars, but it can just as well be used for ground-based precipitation or wind-profiling radars. The technique is more versatile than the traditional inverse FFT technique and faster than the individual hydrometeor simulation (Monte Carlo) technique. Doppler radar signals from backscattering hydrometeors are essentially correlated stochastic variables. The technique uses an accurate description of covariances between voltages measured for different pulses and at different positions (range gates) along a profile. A matrix formalism is developed to subsequently transform uncorrelated Gaussian noise into correlated receiver voltages with the appropriate covariances. In particular, the new technique deals with target variability in a physically consistent manner, accounting for the effects of inhomogeneity both within the instantaneous field of view and between subsequent pulses. The new technique is showcased with examples of simulated 95-GHz Doppler radar observations by the Earth Clouds, Aerosols and Radiation Explorer (EarthCARE) space mission. © 2008 American Meteorological Society

A pathway analysis of global aerosol processes

We present a detailed budget of the changes in atmospheric aerosol mass and numbers due to various processes: emission (including instant condensation of soluble biogenic emissions), nucleation, coagulation, H2SO4 condensation and in-cloud production, aging and deposition. The budget is created from monthly averaged tracer tendencies calculated by the global aerosol model ECHAM5.5-HAM2 and allows us to investigate process contributions at various length-scales and timescales. As a result, we show in unprecedented detail what processes drive the evolution of aerosol. In particular, we show that the processes that affect aerosol masses are quite different from those that affect aerosol numbers. Condensation of H2SO4 gas onto pre-existing particles is an important process, dominating the growth of small particles in the nucleation mode to the Aitken mode and the aging of hydrophobic matter. Together with in-cloud production of H2SO4, it significantly contributes to (and often dominates) the mass burden (and hence composition) of the hydrophilic Aitken and accumulation mode particles. Particle growth itself is the leading source of number densities in the hydrophilic Aitken and accumulation modes, with their hydrophobic counterparts contributing (even locally) relatively little. As expected, the coarse mode is dominated by primary emissions and mostly decoupled from the smaller modes. Our analysis also suggests that coagulation serves mainly as a loss process for number densities and that, relative to other processes, it is a rather unimportant contributor to composition changes of aerosol. The analysis is extended with sensitivity studies where the impact of a lower model resolution or pre-industrial emissions is shown to be small. We discuss the use of the current budget for model simplification, prioritization of model improvements, identification of potential structural model errors and model evaluation against observations

A novel approach to the polarization correction of spaceborne spectrometers

We present a new polarization correction algorithm for polarization sensitive spaceborne spectrometers like Global Ozone Monitoring Experiment (GOME) or Scanning Imaging Absorption Spectrometer for Atmospheric Cartography (SCIAMACHY). These instruments measure polarization with less spectral resolution than the radiance. As a consequence, the current GOME polarization correction algorithm allows radiance errors of 10% in the UV and 3% in the visible (VIS) and near-IR. Using simulated spectra, we show that the continuum Stokes elements Q and U over a large wavelength range (300 to 800 nm) can be reliably retrieved from only a limited set of measurements (three to five). The interpolation to other wavelengths is made using numerically calculated polarization spectra. These polarization spectra can be calculated for Rayleigh scattering atmospheres, but we show that the retrieval is just as effective for cloudy and aerosol scenes as for clear scenes. For GOME the remaining radiance errors are less than 3% (UV) and 1% (VIS, near-IR), and often smaller than 0.5%. The lack of actual UV (300-330 nm) polarization measurements prevents further reduction of the radiance errors. We also show how the ozone Huggins absorption band may be accounted for through a parameterization and a single polarization broadband measurement and that strong but narrow absorption bands like the O2A band maybe ignored without detrimental effect on the algorithm

Assessment and improvement of the polarisation correction algorithm of GOME

We have studied the errors in GOME radiances resulting solely from the operational polarisation correction of GOME signals. The assessment was made by simulating on a computer both the observation of various Earth scenes and the subsequent calibration of signals, as relevant to GOME. For 290-350 nm, errors are typically larger than 1%, sometimes even as large as 10 %. Especially for 305-315 nm, errors will usually be 2-5 %. For longer wavelengths (350-790 nm), errors are normally smaller than 1%, allthough they may reach 2% for certain wavelengths and Earth scenes. The large UV errors are due to lack of UV polarisation data, either from theory or observation. We supply additional theoretical data in the form of parametrisations of the polarisation in the UV and show that a polarisation correction algorithm capable of using this additional information significantly reduces UV radiance errors. The correct retrieval of e.g. ozone profiles relies strongly on well-calibrated absolute UV radiances. Our results are also relevant to future instruments like SCIAMACHY and GOME-2

Characterising top-of-atmosphere ultra-violet polarisation from radiative transfer calculations

We have constructed a database of ultra-violet earthshine Stokes parameters for various solar and viewing geometries as well as surface albedos and atmospheric profiles, through radiative transfer modelling. For each atmospheric profile, we show that it is possible to parametrise certain aspects of the UV polarisation in solar zenith angle only. The effect of different total ozone columns can also be accounted for. These parametrisations lead to significant reductions in the radiance errors of polarisation sensitive instruments like GOME, GOME-2 and SCIAMACHY. This is important for the retrieval of data products like the ozone profile