Jordi Tiana-Alsina

Atmospheric Boundary Layer Height Monitoring Using a Kalman Filter and Backscatter Lidar Returns

A solution based on a Kalman filter to trace the evolution of the atmospheric boundary layer (ABL) sensed by a ground-based elastic-backscatter tropospheric lidar is presented. An erf-like profile is used to model the mixing-layer top and the entrainment-zone thickness. The extended Kalman filter (EKF) enables to retrieve and track the ABL parameters based on simplified statistics of the ABL dynamics and of the observation noise present in the lidar signal. This adaptive feature permits to analyze atmospheric scenes with low signal-to-noise ratios (SNRs) without the need to resort to long-time averages or range-smoothing techniques, as well as to pave the way for future automated detection solutions. First, EKF results based on oversimplified synthetic and experimental lidar profiles are presented and compared with classic ABL estimation quantifiers for a case study with different SNR scenarios.

Distinguishing signatures of determinism and stochasticity in spiking complex systems

We describe a method to infer signatures of determinism and stochasticity in the sequence of apparently random intensity dropouts emitted by a semiconductor laser with optical feedback. The method uses ordinal time-series analysis to classify experimental data of inter-dropout-intervals (IDIs) in two categories that display statistically significant different features. Despite the apparent randomness of the dropout events, one IDI category is consistent with waiting times in a resting state until noise triggers a dropout, and the other is consistent with dropouts occurring during the return to the resting state, which have a clear deterministic component. The method we describe can be a powerful tool for inferring signatures of determinism in the dynamics of complex systems in noisy environments, at an event-level description of their dynamics.

Atmospheric Boundary Layer Height Estimation Using a Kalman Filter and a Frequency‐Modulated Continuous‐Wave Radar

An adaptive solution based on an extended Kalman filter (EKF) is proposed to estimate the atmospheric boundarylayer height (ABLH) from frequency-modulated continuous-wave S-band weather-radar returns. The EKF estimator departs from previous works, in which the transition interface between the mixing layer (ML) and the free troposphere (FT) is modeled by means of an erf-like parametric function. In contrast to lidar remote sensing, where aerosols give strong backscatter returns over the whole ML, clear-air radar reflectivity returns (Bragg scattering from refractive turbulence) shows strongest returns from the ML-FT interface. In addition, they are corrupted by “insect” noise (impulsive noise associated with Rayleigh scattering from insects and birds), all of which requires a specific treatment of the problem and the measurement noise for the clear-air radar case. The proposed radar-ABLH estimation method uses: 1) a first preprocessing of the reflectivity returns based on median filtering and threshold-limited decision to obtain “clean” reflectivity signal; 2) a modified EKF with adaptive range intervals as time tracking estimator; and 3) ad hoc modeling of the observation noise covariance. The method has successfully been implemented in clear-air, single-layer, and convective boundary-layer conditions. ABLH estimates from the proposed radar-EKF method have been cross examined with those from a collocated lidar ceilometer yielding a correlation coefficient as high as ρ = 0.93 (mean signal-to-noise ratio, SNR = 18 (linear units), at the ABLH) and in relation to the classic THM.

Interspike-interval correlations induced by two-state switching in an excitable system

We study correlations of intervals between pulses in an excitable system, using a semiconductor laser with optical feedback as an experimental model system. First we show, by means of a combination of experimental observations and theoretical analysis, that for an intermediate range of the lasers pump current the interspike intervals are positively correlated over a few lags, an effect that can be theoretically explained by an intrinsic two-state switching in the laser dynamics. The same theory can be also applied if the laser is externally driven by a dichotomous switching of the pump current, a scenario that allows for a controlled change of the spike rates of the two states over orders of magnitude. Varying one of the pump levels, we find experimentally that the correlation between adjacent intervals is maximized at a finite pump level corresponding to an optimal ratio of dropout rates in the two states. Our theory confirms these findings and reveals how the regularity of spiking in the two states shapes the correlation maximum.

Retrieval of boundary layer height from lidar using extended Kalman filter approach, classic methods, and backtrajectory cluster analysis

This contribution evaluates an approach using an extended Kalman filter (EKF) to estimate the planetary boundary layer height (PBLH) from lidar measurements obtained in the framework of the European Aerosol Research LIdar NETwork (EARLINET) at 12 UTC ± 30-min. for a 7-year period (2007-2013) under different synoptic flows over the complex geographical area of Barcelona, Spain. PBLH diagnosed with the EKF technique are compared with classic lidar methods and radiosounding estimates. Seven unique synoptic flows are identified using cluster analysis of 5756 HYSPLIT (HYbrid Single Particle Lagrangian Integrated Trajectory) three-day backtrajectories for a 16-year period (1998-2013) arriving at 0.5 km, 1.5 km, and 3 km, to represent the lower PBL, upper PBL, and low free troposphere, respectively. Regional recirculations are dominant with 54% of the annual total at 0.5 km and 57% of the total lidar days at 1.5 km, with a clear preference for summertime (0.5 km: 36% and 1.5 km: 29%). PBLH retrievals using the EKF method range from 0.79 - 1.6 km asl. Highest PBLH are observed in southwest flows (15.2% of total) and regional recirculations from the east (34.8% of total), mainly caused by the stagnant synoptic pattern in summertime over the Iberian Peninsula. Lowest PBLH are associated with north (19.6% of total) and northeast (4.3% of total) synoptic flows, when fresh air masses tend to lower PBLH. The adaptive nature of the EKF technique allows retrieval of reliable PBLH without the need for long time averaging or range smoothing, as typical with classic methods.

Parameter design of a biaxial lidar ceilometer

This paper presents parameter design methodology and related optomechanical engineering of a 905-nm diode-laser biaxial, eye-safe lidar ceilometer prototype for cloud-height monitoring. Starting with a brief review of the state-of-the-art ceilometer technology, acceptable parameter ranges are identified for the key system parts. Parameter tuning is achieved by imposing goal criteria on the simulated signal-to-noise ratio and laser-telescope overlap factor. The system is based on a low-cost pulsed semiconductor laser, low-cost Fresnel-lens telescope, a low-noise-equivalent power avalanche-photodiode optoelectronic receiver, and collimating/focusing adjustable parts. Finally, preliminary test measurements are presented.

Quantifying stochasticity in the dynamics of delay-coupled semiconductor lasers via forbidden patterns

We quantify the level of stochasticity in the dynamics of two mutually coupled semiconductor lasers. Specifically, we concentrate on a regime in which the lasers synchronize their dynamics with a non-zero lag time, and the leader and laggard roles alternate irregularly between the lasers. We analyse this switching dynamics in terms of the number of forbidden patterns of the alternate time series. The results reveal that the system operates in a stochastic regime, with the level of stochasticity decreasing as the lasers are pumped further away from their lasing threshold. This behaviour is similar to that exhibited by a single semiconductor laser subject to external optical feedback, as its dynamics shifts from the regime of low-frequency fluctuations to coherence collapse.

Experimental characterization of the transition to coherence collapse in a semiconductor laser with optical feedback

Semiconductor lasers with time-delayed optical feedback display a wide range of dynamical regimes, which have found various practical applications. They also provide excellent testbeds for data analysis tools for characterizing complex signals. Recently, several of us have analyzed experimental intensity time-traces and quantitatively identified the onset of different dynamical regimes, as the laser current increases. Specifically, we identified the onset of low-frequency fluctuations (LFFs), where the laser intensity displays abrupt dropouts, and the onset of coherence collapse (CC), where the intensity fluctuations are highly irregular. Here we map these regimes when both, the laser current and the feedback strength vary. We show that the shape of the distribution of intensity fluctuations (characterized by the standard deviation, the skewness, and the kurtosis) allows to distinguish among noise, LFFs and CC, and to quantitatively determine (in spite of the gradual nature of the transitions) the boundaries of the three regimes. Ordinal analysis of the inter-dropout time intervals consistently identifies the three regimes occurring in the same parameter regions as the analysis of the intensity distribution. Simulations of the well-known time-delayed Lang-Kobayashi model are in good qualitative agreement with the observations.

Motion compensation study for a floating Doppler wind LiDAR

This paper addresses a cardanic frame as mechanical-compensation device for a Doppler-wind lidar installed on a floating sea buoy as the “moving” lidar. From the methodological point of view, the horizontal wind speed (HWS) measured by both a reference lidar (i.e., “fixed”) and a floating lidar (i.e., “moving”) with and without a cardarnic frame is cross-examined by using standard statistical indicators and a compound pendulum model. Performance results are analysed both at laboratory level using a pitch/roll motion-simulation platform and at a sea-test measurement-campaign level some 250-m offshore Barcelona coast. Finally, simulations of the proposed pendulum-based model enables to tune-in and optimize cardanic-frame design parameters.

Experimental study of modulation waveforms for entraining the spikes emitted by a semiconductor laser with optical feedback

The entrainment phenomenon, by which an oscillator adjusts its natural rhythm to an external periodic signal, has been observed in many natural systems. Recently, attention has focused on which are the optimal conditions for achieving entrainment. Here we use a semiconductor laser with optical feedback, operating in the low-frequency fluctuations (LFFs) regime, as a testbed for a controlled entrainment experiment. In the LFF regime the laser intensity displays abrupt spikes, which can be entrained to a weak periodic signal that directly modulates the laser pump current. We compare the performance of three modulation waveforms for producing 1:1 locking (one spike is emitted in each modulation cycle), as well as higher order locking regimes. We characterize the parameter regions where high-quality locking occurs, and those where the laser emits spikes which are not entrained to the external signal. The role of the modulation amplitude and frequency, and the role of the dc value of the laser pump current (that controls the natural spike frequency) in the entrainment quality are discussed.