The research activity deals with the participation of RSLAB in NEPTUNE project and tackles analysis and cross-examination of sea-motion effects in horizontal wind speed, horizontal wind direction, and vertical wind speed when measured from the lidar-instrumented buoy. Towards this aim, two different test campaigns are considered: One at LIM (Lab. Maritime Engineering)/UPC premises in Campus Nord and another at their premises at El Pont del Petroli pier. In both cases two lidars are used, one moving (the “floating” lidar) and non-moving one (the “fixed” lidar). In the first campaign, a lab-based motion-simulation platform is used to emulate pitch-and-roll sea movements while in the second campaign the “floating” lidar is assembled on a provisional buoy 40-m offshore. Different angular, position, and navigation sensors are used to cross-examine wind-retrieved data from the floating lidar against the fixed one (Fig. 1).

Fig. 1 NEPTUNE project. (Left) Calibration/validation (cal/val) of two Doppler lidars co-located at RSLAB premises (UPC, Campus Nord, Oct. 12, 2012). (Central) Horizontal Wind Speed scatter plot from the cal/val of the two lidars, 1-s resolution. (Right) El Pont del Petroli (PdP) campaign (May 22 to Jul. 12, 2013, WP2 partners: LIM, SWE, IREC, and RSLAB). The “floating” is the lidar buoy (yellow), the “fixed” lidar is on PdP pier on land.

Concerning ABL retrieval, 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 studied. 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.

Time-height plot of the range-corrected attenuated backscatter lidar signal (infrared channel, CL-31) showing time evolution of the ABL. (Magenta) ABL height estimated with a Kalman filter. Source: RSLAB-UMASS in TEC2012-34575 project on multi-spectral lidar observation).

This project deals with the study of autoconversion process, that is the process of transition between cloud droplets and precipitation: many connected open problems are currently object of study from the scientific community, like for example the need to be able to retrieve LWC for situations in which drizzle and cloud droplets are both present, the necessity to understand under which conditions clouds begin to precipitate and which are the corresponding threshold values for the onset of precipitation for LWP and other important variables. The aim of this PHD is therefore to try to study the process of formation of precipitation and derive such thresholds, verifying if they are consistent with models. Initially, the fellow will analyse LWP values with respect to the onset of precipitation as identified from different sources, then the analysis of temporal developments of Doppler spectra and its higher Doppler moments will be performed in order to identify thresholds values indicative of certain processes. Then, the results obtained will be implemented in the algorithm in order to improve the retrievals.

The proposed research project will investigate the “twilight zone” with multiple ground based remote sensors available at CNR-IMAA Atmospheric Observatory (CIAO). Clouds and aerosols play a significant role in the radiative balance of the Earth. One of the main key uncertainties in modeling climate is the aerosol indirect effect (AIE). A relevant part of the study of AIE is related to the effect of the “twilight zone”.

The “twilight zone” is described as a distinct zone (Charlson et al., 2007; Koren et al., 2009; Madonna et al., 2009) characterized by intermediate conditions associated with evaporating cloud fragments and enhanced aerosol. The “twilight zone” has been observed both close to the edge of visible cloud layers or also in apparently clear skies.

The importance of the “twilight zone” arises from the fact that it is estimated that between a significant part of the atmosphere previously considered as cloud-free in satellite observations might actually represented by the “twilight zone” of in-between particles. This fact could have a significant impact on the way the Earth’s radiative budget is estimated. Therefore it is important to study the microphysical processes that occur near clouds, but also in optically “clear” conditions, in order to better understand and characterize this area of the atmosphere.

The approach consists of an analysis of CNR-IMAA CIAO archive data from multiple sensors (MWR, multi-λ LIDAR, ceilometers and sun-photometer) for liquid water signatures in apparently cloud-free datasets (according to a sky camera). Analysis of key variables such as brightness temperatures, liquid water path, relative humidity , water mixing ratio, optical depth, sky radiances will be done in order to address the source (no visible clouds present).

Finally, an estimation of the TOA radiative forcing associated with the “twilight zone” through radiative transfer modeling will be done in cooperation with the University of Reading.

The research project will investigate the relation between microphysical properties of sub-cloud aerosols and clouds referring to droplet number and droplet size spectrum, which can contribute to a better understanding of the first indirect aerosol effect, one of the most concerning and significant uncertainties in climate change. The relation will be investigated by long term detailed in situ measurements of droplet activation in 120 m height and remote sensing observations (Sunphotometer, radar, microwave radiometer, and ceilometer).

By statistical analysis of the long term observations, suitable time periods of well-mixed planetary boundary layer conditions will be filtered out, in which droplet activation of the sub-cloud aerosols may be related to cloud microphysical properties. The goal is to improve the predictability of cloud microphysical properties (droplet number concentration, droplet size spectrum, and cloud optical properties) based on aerosol properties (critical parameters of droplet activation).

Figure 1: Research motivation. Figure 2: In-situ CCN measurement instruments: Cloud Condensation Nuclei Counter(CCN-C), Condensation Particle Counter (CPC), Differential Mobility Analyzer.