The Buzzard''s Roost and Eustis mollusc sequences: comparison between the paleoenvironments of two sites in the Wisconsinan loess of Nebraska, USA

We investigated the Upper Pleistocene mollusc sequence in the loess deposit of Buzzard's Roost, Nebraska, and compared the results with those obtained for Eustis, 50 km to the west. Both sequences show similarities in: (i) stratigraphy and magnetic susceptibility implying identical fluctuations in loess deposition, (ii) biostratigraphy which indicates a succession of cold-and-moist to cold-and-dry conditions, and (iii) an increasing trend in mollusc abundance and diversity of species characterizing a climatic and/or environmental improvement. We interpret the variations in both sequences as related to the advance of the James and Des Moines ice lobes towards both sites, which was synchronous with the retreat of the Laurentide Ice Sheet. The occurrence of hygrophilous species in the mollusc record, from the upper sequences of Buzzard's Roost, suggests a moister environment than in Eustis. Our preliminary comparison of both malacological sequences suggests that regional or local, rather than global, influence prevailed over the Great Plains between 20 and 12 kyr BP.     

Enhanced lifetime of atmospheric circulation types over Europe: fact or fiction?

We analyse the lifetimes (persistence) of circulation types using 23 classifications (of which 18 are objective and 5 subjective) in the period 1957–2002 over Europe and its subregions. The objective catalogues are developed on the same gridded climatic data (ECMWF ERA-40 reanalysis) but differ in the classification method used and in the number of synoptic types. Significant seasonal trends in the lifetime (both positive and negative) are fairly scarce and are present in all seasons in the manual catalogues only. In the subjective Hess–Brezowsky catalogue, there is an abrupt shift toward higher persistence in 1986, whereas in the Hungarian manual catalogue, a smaller but significant negative shift took place in the same year. These statistical inconsistencies probably result from inhomogeneities in the subjective catalogues. Our results suggest that the increase in the persistence of circulation types reported recently in several papers for the Hess–Brezowsky catalogue is an artefact rather than a real feature.     

Differentiating natural and anthropogenic cloud condensation nuclei in the California coastal zone

Aerosol samples were collected and cloud condensation nuclei (CCN) concentrations at five supersaturations were measured along and off the central California coast within the cloud-topped, marine boundary layer from aircraft flights during August 2007. Receptor modelling has been applied to estimate the natural versus anthropogenic source contribution of cloud condensation nuclei in this region, a region of climatically important marine stratocumulus. The results suggest that anthropogenic CCN accounted for about 50% of the CCN active at 0.3% supersaturation in this region during the measurement period.     

Ensemble sensitivities of the real atmosphere: application to Mediterranean intense cyclones

Ensemble sensitivity has been recently proposed as an alternative cheap approach to sensitivity analysis. We adapt it to compute climatological sensitivity estimates of intense Mediterranean cyclones using a climatology based of the ECMWF ERA-40 fields. A catalogue of 1202 events, objectively detected and classified in 25 clusters, is used in this study. Sensitivity fields are derived for each intense Mediterranean cyclone type by correlating the precursor conditions with the mature cyclones depths. Corrections to the raw sensitivity estimates are applied by means of the correlation coefficient. Further, a normalization based on the climatological spatial variability of the variance of the precursor conditions is used to derive the final sensitivity fields. The 24 h sensitivity information derived for each intense Mediterranean cyclone type is easily interpretable both in amplitude and distribution. A synthetic result combining the sensitivity fields for all 25 intense Mediterranean cyclone classes shows that the evolution of these high-impact systems 24 h prior to its maturity stage depends largely on structures located over Western Europe, the Northern African lands and parts of east North Atlantic. These results are in agreement and complement with previous results obtained with the expensive adjoint model, although further work is needed to objectively verify the results.     

Spectral aerosol optical depth characterization of desert dust during SAMUM 2006

The aerosol optical depth (AOD) in the range 340–1550 nm was monitored at Ouarzazate (Morocco) during the Saharan Mineral Dust Experiment (SAMUM) experiment in May–June 2006. Two different sun photometers were used for this purpose. The mean AOD at 500 nm was 0.28, with a maximum of 0.83, and the mean Ångström exponent (AE) was 0.35. The aerosol content over the site changed alternatively from very low turbidity, associated to Atlantic air masses, to moderate dust load, associated to air masses arriving in the site from Algeria, Tunisia and Libya. The dusty conditions were predominant in the measurement period (78% of data), with AOD (500 nm) above 0.15 and AE below 0.4. The spectral features of the AOD under dusty conditions are discussed. Air mass back trajectory analysis is carried out to investigate the origin and height patterns of the dust loaded air masses. The advection of dust occurred mainly at atmospheric heights below 3000 m, where east flow is the predominant. At the 5000 m level, the air masses originate mainly over the Atlantic Ocean. Finally the Optical Properties of Aerosols and Clouds (OPAC) model is used to perform a set of simulations with different aerosol mixtures to illustrate the measured AOD and AE values under varying dust concentrations, and a brief comparison with other measurement sites is presented.     

On small particles in the Arctic summer boundary layer: observations at two different heights near Ny-Ålesund, Svalbard

Concurrent observations of particle number densities and size distributions observed at two different heights (near ocean level and 475 m above sea level) in Ny-Ålesund, Svalbard were studied with respect to the diurnal variation during a summer period in June 2004. The results show that observed variation in particle number density in the Arctic boundary layer may be strongly modulated by vertical mixing and dilution. The particles appeared to be formed in the early morning when solar intensity reached about 30% of the mid-day intensity. Based on differences in the observed number densities at the two heights it appears as if particles are formed in the lower part of the boundary layer. The formation rate of 10 nm diameter particles is estimated to be 0.11 cm⁻³ s⁻¹ and the growth rate is in a range between 1 and 2 nm h⁻¹.     

Using the ensemble Kalman filter to estimate multiplicative model parameters

This paper proposes a simple approach to estimating multiplicative model parameters using the ensemble square root filter. The basic idea, following previous studies, is to augment the state vector by the model parameters. While some success with this approach has been reported if the model parameters enter as additive terms in the tendency equations, this approach is problematic if the model parameters are multiplied by the state variables. The reason for this difficulty is that multiplicative parameters change the dynamical properties of the model, and in particular can cause the model to become dynamically unstable. This paper shows that model instability can be avoided if the usual persistence model for parameter update is replaced by a temporally smoothed version of the update model. In addition, the augmentation approach can be interpreted as two simultaneously decoupled ensemble Kalman filters for the model state and the parameter state, respectively. Implementation of the parameter estimation does not require changing the data assimilation algorithm—it just has to be supplemented by a parameter estimation step that is similar to the state estimation step. Covariance localization is found to be necessary not only for the model state, but also for augmented model parameters, if they are spatially dependent. The new formulation is illustrated with the Lorenz-96 model and shown to be capable of estimating additive and multiplicative model parameters, as well as the state, under relatively challenging conditions (e.g. using 20 observations to estimate 120 unknown variables).     

Spatial distribution and optical properties of Saharan dust observed by airborne high spectral resolution lidar during SAMUM 2006

Airborne measurements of pure Saharan dust extinction and backscatter coefficients, the corresponding lidar ratio and the aerosol optical thickness (AOT) have been performed during the Saharan Mineral Dust Experiment 2006, with a high spectral resolution lidar. Dust layers were found to range from ground up to 4–6 km above sea level (asl). Maximum AOT values at 532 nm, encountered within these layers during the DLR Falcon research flights were 0.50–0.55. A significant horizontal variability of the AOT south of the High Atlas mountain range was observed even in cases of a well-mixed dust layer. High vertical variations of the dust lidar ratio of 38–50 sr were observed in cases of stratified dust layers. The variability of the lidar ratio was attributed to dust advection from different source regions. The aerosol depolarization ratio was about 30% at 532 nm during all measurements and showed only marginal vertical variations.