The maximum likelihood ensemble filter performances in chaotic systems

The performance of the maximum likelihood ensemble filter (MLEF), is investigated in the context of generic systems featuring the essential ingredients of unstable dynamics and on a spatially extended system displaying chaos. The main objective is to clarify the response of the filter to different regimes of motion and highlighting features which may help its optimization in more realistic applications. It is found that, in view of the minimization procedure involved in the filter analysis update, the algorithm provides accurate estimates even in the presence of prominent non-linearities. Most importantly, the filter ensemble size can be designed in connection to the properties of the system attractor (Kaplan–Yorke dimension), thus facilitating the filter setup and limiting the computational cost by using an optimal ensemble. As a corollary, this latter finding indicates that the ensemble perturbations in the MLEF reflect the intrinsic system error dynamics rather than a sampling of realizations of an unknown error covariance.     

Toward a phase‐space cartography of the short‐ and medium‐range predictability of weather regimes

We compare the short‐ and medium‐range predictability of weather regimes of a quasi‐geostrophic model as defined by a hierarchical cluster algorithm and a Lyapunov‐based clustering method recently introduced in the literature. Both procedures lead to weather regimes displaying very different predictability properties on the short and medium range bases. While the former does not distinguish between stable and unstable weather regimes, the latter leads to clusters which do not display a good medium range predictability. We introduce a new clustering method taking advantages of the two previous techniques. Its application in the context of the quasi‐geostrophic model gives rise to regimes possessing at the same time a good medium range skill and well separated instability properties, indicating the possibility to build a systematic cartography of the short‐term predictability of weather fields in phase space.     

Inter‐annual variability of southerly winds in a coastal area of the Atacama Desert: implications for the export of aeolian sediments to the adjacent marine environment

The analysis of the aeolian content of marine cores collected off the coast of the Atacama Desert (Mejillones Bay, Chile) suggests that marine sediments can record inter‐annual to inter‐decadal variations in the regional southerly winds responsible for particle entrainment at the surface of the nearby desert. However, the establishment of a simple and direct correlation between the sediment and wind records is complicated by the difference of time scales between the erosion and accumulation processes. The aim of this work is to: (i) assess the inter‐annual variability of the surface winds responsible for the sand movements; and (ii) determine whether the integration over periods of several months completely smoothes the rapid changes in characteristics of the transported and deposited aeolian material. To accomplish this aim, 14 years of 10 m hourly wind speed, measured at the Cerro Moreno (Antofagasta) Airport between 1991 and 2003 and at the Orica Station between 2000 and 2004, were analyzed. For each year, the wind speed statistical distribution can be represented by a combination of two to three Weibull functions. Winds of the lowest Weibull mode are too weak to move the sand grains at the surface of the pampa; this is not the case for the intermediate mode and especially for the highest speed mode which are able to erode the arid surface and transport particles to the bay. In each individual year of the period of study, the highest speed mode only accounted for a limited number of strong erosion events. Quantitative analysis of the distribution of the friction velocities and of their impact on erosion using a saltation model suggests that, although all wind speeds above threshold produce erosion events, values around 0·45 m sec^?1 contribute less to the erosion flux. This gap allows separation of the erosion events into low and high saltation modes. The correlation (r = 0·997) between the importance of the third Weibull mode and the extent of higher rate saltation indicates that the inter‐annual variability of the erosion at the surface of the pampa, as well as the transport of coarse particles (>100 μm), are directly related to inter‐annual variations in the prevalence of the strongest winds. Finally, a transport and deposition model is used to assess the possible impact of the wind inter‐annual variability on the deposition flux of mineral particles in the bay. The results suggest that inter‐annual differences in the wind speed distributions have a quantifiable effect on the intensity and size‐distribution of this deposition flux. This observation suggests that a detailed analysis of the sediment cores collected from the bay could be used for reconstructing the inter‐annual variability of past winds.