Measuring tropospheric water vapour and surface emissivity using far-infrared radiances

This thesis describes two strands of work relating to the gathering of far-infrared (wavenumbers 10-667 cm−1 wavelengths 15-1000µm) radiance measurements, exploring the capabilities of the instruments involved and examining the data generated in the context of the relevant models. The first strand of the thesis explores the role of atmospheric water vapour which absorbs strongly in the far-infrared. The work described makes use of spectrally resolved far-infrared radiance measurements taken during the PIKNMIX-F airborne field campaign. On this field campaign co-incident upwelling mid- and far-infrared spectra were recorded in clear sky and overflying cirrus. The clear sky spectra from one flight are used to investigate the sensitivity of the far-infrared to the atmospheric water profile. Forward modelling is used to explore the changes to the expected radiance caused by changes in the surface properties and water vapour spectroscopy. Retrievals of water vapour and temperature profiles are also carried out. These show that the far-infrared radiance measurements contain more information about the atmospheric water vapour profile than the co-incident mid-infrared radiance measurements for this set of instruments. The second strand of the thesis describes the new FINESSE instrument and its first measurements of far-infrared surface emissivity. FINESSE combines a commercial Fourier transform spectrometer with a spectral range of 400-1600 cm−1 and a custom pointing and calibration system. The main purpose of FINESSE is to make in-situ measurements of surface emissivity extending into the far-infrared. As part of the development of FINESSE, a simulator is produced that allows for the emissivity retrieval to be tested under different environmental conditions. This thesis describes the first measurements of emissivity made by FINESSE. The emissivity of deionised water is retrieved using radiance measurements made from the rooftop of Imperial College London during summer and winter conditions. The measurements compare favourably to theoretical calculations and previous mid-infrared measurements.Open Acces

An evaluation of satellite-derived humidity and its relationship to convective development

An aircraft prototype of the High-Resolution Interferometer Sounder (HIS) was flown over Tennessee and northern Alabama during summer 1986. The HIS temperature and dewpoint soundings were examined on two flight days to determine their error characteristics and utility in mesoscale analyses. Random errors were calculated from structure functions while total errors were obtained by pairing the HIS soundings with radiosonde-derived profiles. Random temperature errors were found to be less than 1 C at most levels, but random dewpoint errors ranged from 1 to 5 C. Total errors of both parameters were considerably greater, with dewpoint errors especially large on the day having a pronounced subsidence inversion. Cumulus cloud cover on 15 June limited HIS mesoscale analyses on that day. Previously undetected clouds were found in many HIS fields of view, and these probably produced the low-level horizontal temperature and dewpoint variations observed in the retrievals. HIS dewpoints at 300 mb indicated a strong moisture gradient that was confirmed by GOES 6.7-micron imagery. HIS mesoscale analyses on 19 June revealed a tongue of humid air stretching across the study area. The moist region was confirmed by radiosonde data and imagery from the Multispectral Atmospheric Mapping Sensor (MAMS). Convective temperatures derived from HIS retrievals helped explain the cloud formation that occurred after the HIS overflights. Crude estimates of Bowen ratio were obtained from HIS data using a mixing-line approach. Values indicated that areas of large sensible heat flux were the areas of first cloud development. These locations were also suggested by GOES visible and infrared imagery. The HIS retrievals indicated that areas of thunderstorm formation were regions of greatest instability. Local landscape variability and atmospheric temperature and humidity fluctuations were found to be important factors in producing the cumulus clouds on 19 June. HIS soundings were capable of detecting some of this variability. The authors were impressed by HIS's performance on the two study days

VAS demonstration: (VISSR Atmospheric Sounder) description

The VAS Demonstration (VISSR Atmospheric Sounder) is a project designed to evaluate the VAS instrument as a remote sensor of the Earth's atmosphere and surface. This report describes the instrument and ground processing system, the instrument performance, the valiation as a temperature and moisture profiler compared with ground truth and other satellites, and assesses its performance as a valuable meteorological tool. The report also addresses the availability of data for scientific research

Improved simulation of aerosol, cloud, and density measurements by shuttle lidar

Data retrievals are simulated for a Nd:YAG lidar suitable for early flight on the space shuttle. Maximum assumed vertical and horizontal resolutions are 0.1 and 100 km, respectively, in the boundary layer, increasing to 2 and 2000 km in the mesosphere. Aerosol and cloud retrievals are simulated using 1.06 and 0.53 microns wavelengths independently. Error sources include signal measurement, conventional density information, atmospheric transmission, and lidar calibration. By day, tenuous clouds and Saharan and boundary layer aerosols are retrieved at both wavelengths. By night, these constituents are retrieved, plus upper tropospheric, stratospheric, and mesospheric aerosols and noctilucent clouds. Density, temperature, and improved aerosol and cloud retrievals are simulated by combining signals at 0.35, 1.06, and 0.53 microns. Particlate contamination limits the technique to the cloud free upper troposphere and above. Error bars automatically show effect of this contamination, as well as errors in absolute density nonmalization, reference temperature or pressure, and the sources listed above. For nonvolcanic conditions, relative density profiles have rms errors of 0.54 to 2% in the upper troposphere and stratosphere. Temperature profiles have rms errors of 1.2 to 2.5 K and can define the tropopause to 0.5 km and higher wave structures to 1 or 2 km

Atmospheric temperature, water vapour and liquid water path from two microwave radiometers during MOSAiC

The microwave radiometers HATPRO (Humidity and Temperature Profiler) and MiRAC-P (Microwave Radiometer for Arctic Clouds - Passive) continuously measured radiation emitted from the atmosphere throughout the Multidisciplinary drifting Observatory for the Study of the Arctic Climate (MOSAiC) expedition on board the research vessel Polarstern. From the measured brightness temperatures, we have retrieved atmospheric variables using statistical methods in a temporal resolution of 1 s covering October 2019 to October 2020. The integrated water vapour (IWV) is derived individually from both radiometers. In addition, we present the liquid water path (LWP), temperature and absolute humidity profiles from HATPRO. To prove the quality and to estimate uncertainty, the data sets are compared to radiosonde measurements from Polarstern. The comparison shows an extremely good agreement for IWV, with standard deviations of 0.08–0.19 kg m−2 (0.39–1.47 kg m−2) in dry (moist) situations. The derived profiles of temperature and humidity denote uncertainties of 0.7–1.8 K and 0.6–0.45 gm−3 in 0–2 km altitude

The aerosol-climate model ECHAM5-HAM

The aerosol-climate modelling system ECHAM5-HAM is introduced. It is based on a flexible microphysical approach and, as the number of externally imposed parameters is minimised, allows the application in a wide range of climate regimes. ECHAM5-HAM predicts the evolution of an ensemble of microphysically interacting internally- and externally-mixed aerosol populations as well as their size-distribution and composition. The size-distribution is represented by a superposition of log-normal modes. In the current setup, the major global aerosol compounds sulfate (SU), black carbon (BC), particulate organic matter (POM), sea salt (SS), and mineral dust (DU) are included. The simulated global annual mean aerosol burdens (lifetimes) for the year 2000 are for SU: 0.80 Tg(S) (3.9 days), for BC: 0.11 Tg (5.4 days), for POM: 0.99 Tg (5.4 days), for SS: 10.5 Tg (0.8 days), and for DU: 8.28 Tg (4.6 days). An extensive evaluation with in-situ and remote sensing measurements underscores that the model results are generally in good agreement with observations of the global aerosol system. The simulated global annual mean aerosol optical depth (AOD) is with 0.14 in excellent agreement with an estimate derived from AERONET measurements (0.14) and a composite derived from MODIS-MISR satellite retrievals (0.16). Regionally, the deviations are not negligible. However, the main patterns of AOD attributable to anthropogenic activity are reproduced

Evaluation and Application of Max-DOAS Methods for Monitoring Aerosols, NO2, and SO2 in Urban and Industrial Environments

The ideal measurement technique to effectively address an air quality problem depends on the chemical and physical properties of the species and its environment. Multi-Axis Differential Optical Absorption Spectroscopy (MAX-DOAS) techniques allow a diversity of applications for studying atmospheric species, including the determination of emissions, vertical profiles, and the tropospheric column loading of trace gases. Deployment of the MAX-DOAS instrument during the comprehensive air quality campaign in the Athabasca Oil Sands Region in 2013 provided a rare opportunity to evaluate the performance of multiple aspects of the MAX-DOAS retrievals. Retrievals of aerosol extinction, NO2, and SO2 were compared to data from lidar, sun photometer, Active-DOAS, and airborne in-situ measurements of trace gases. The MAX-DOAS retrievals performed well except under conditions of rapidly changing vertical profiles of pollution. Important elements required to achieve useful inter-comparisons of MAX-DOAS with other instruments (e.g., the lidar S-ratio) and advantages of the MAX-DOAS technique were identified. MAX-DOAS measurements of SO2 gas calibration cells were conducted to determine the optimal settings for fitting SO2 differential slant column densitities (dSCDs), currently absent in the literature. Fitting dSCDs of SO2 from solar measurements is challenging due to the effects of stray light, potential interference by O3 absorption, and low solar intensity in wavelength regions where SO2 absorption features are strong. Based on the experiments, the use of a short-pass filter and a fitting window of 307.5 < <319 nm are recommended. MAX-DOAS measurements in Toronto, Ontario, during 2015 quantified the impact of lake-breeze circulations on the tropospheric loading of NO2 and aerosol extinction. These first measurements of the total tropospheric loading of pollutants behind a lake breeze front on multiple days using MAX-DOAS confirms previously theorized 3-D structures of lake breezes. Finally, the mobile-MAX-DOAS technique of estimating NOx and SO2 emissions was improved by conducting simultaneous Mobile-MAX-DOAS and in-situ NO-NO2-NOx measurements and deploying a modular meteorological station while observing urban plumes in the industrial city of Sarnia, Ontario. These studies demonstrated the utility of MAX-DOAS techniques for monitoring tropospheric air quality in industrial and urban settings when in-situ and other remote sensing techniques are limited

Atmospheric water supply to the Atacama Desert from newly developed satellite remote sensing techniques and reanalysis

Many facets of atmospheric water supply to the Atacama Desert are poorly understood. However, in-depth knowledge regarding water availability, moisture sources and the underlying mechanisms is required to investigate biological and geological processes and to identify potential mutual relationships. This thesis provides a comprehensive meteorological perspective on the atmospheric water supply to the Atacama Desert within the context of the recent climate. Spatial and temporal variability of moisture as well as their controlling mechanisms depend on the type of water supply, i.e. clouds, water vapor, fog or precipitation. To investigate the influence of the persistent stratocumulus cloud deck above the southeast Pacific on the desert region, a new cloud base height retrieval method is introduced. It allows to estimate the vertical position of these clouds, which can help to identify regions within the coastal desert that are potentially influenced by these clouds. A first application of this new method revealed a strong relation between stratocumulus properties and the isotopic composition of coastal Tillandsia populations. The proximity of the Atacama Desert to main acting zones of the El Niño-Southern Oscillation (ENSO) phenomenon and of the Pacific Decadal Oscillation (PDO) together with results from previous studies suggest that modes of climate variability have strong influence on the moisture supply to this region. As oscillating extreme phases of these climate modes have recurring periods on the order of a few years to decades, a long data record is needed to study their impact. Therefore, spatio-temporal variability of integrated water vapor (IWV) provided by a century-spanning reanalysis data set is studied in relation to ENSO and PDO. It is shown that the reanalysis represents IWV in a suitable manner to study its long-term variability. On a decadal time scale, the PDO revealed a stronger coupling to IWV compared to ENSO. According to a seasonal analysis, identified relationships between ENSO and IWV are in line with findings reported for precipitation in the northeastern Atacama. This suggests that IWV has the potential to serve as a proxy for precipitation. The ENSO signal is opposite for summer and winter season. The negative phase (La Niña) favors wetter summers and drier winters, whereas the positive phase (El Niño) is associated with drier summers and wetter winters. Besides, it is shown that enhanced IWV under La Niña conditions is not constrained to the northeastern part of the Atacama Desert but can reach even offshore regions near the west coast. This effect can be typically observed in the summer season. Thus, the moisture can be supplied to the Atacama Desert from easterly or westerly sources depending on season and ENSO phase with regionally varying impacts. Water vapor is a key variable which controls fog formation. While a few studies demonstrate the impact of fog on the coastal desert based on in-situ measurements as well as spatially and temporally limited satellited-based observations, this thesis introduces a novel satellite-based fog detection method which allows a region-wide assessment. An application of the algorithm for a 3-year period shows the spatial distribution of fog frequencies across the Atacama Desert. Aside from the coastal maximum, high fog frequencies are also revealed for isolated locations farther inland, which often coincide with salt flats within the central valley. The mechanisms driving fog formation within these inland regions remain unclear. The novel fog detection method creates the opportunity to further investigate this issue in future research. Aside from westerly moisture sources associated with the Pacific Ocean and episodic easterly inflow from the continental interior, a third scenario is identified in this thesis. By investigating the role of atmospheric rivers for the Atacama Desert, it is revealed that moisture can be transported from the Amazon Basin across the Andes and the southeast Pacific towards the Atacama Desert. Furthermore, fractional precipitation rates of more than 50 % for various regions within the Atacama Desert demonstrate the importance of atmospheric rivers for this hyperarid environment

Cloud condensation nuclei concentrations from spaceborne lidar measurements – Methodology and validation

Aerosol-cloud interactions are the most uncertain component of the anthropogenic radiative forcing. A substantial part of this uncertainty comes from the limitations of currently used spaceborne CCN proxies that (i) are column integrated and do not guarantee vertical co-location of aerosols and clouds, (ii) have retrieval issues over land, and (iii) do not account for aerosol hygroscopicity. A possible solution to overcome these limitations is to use height-resolved measurements of the spaceborne lidar aboard the CALIPSO (Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation) satellite. This thesis presents a novel CCN retrieval algorithm based on Optical Modelling of CALIPSO Aerosol Microphysics (OMCAM) that is designed particularly for CALIPSO lidar measurements, along with its validation with airborne and surface in-situ measurements. \noindent OMCAM uses a set of normalized size distributions from the CALIPSO aerosol model and modifies them to reproduce the CALIPSO measured aerosol extinction coefficient. It then uses the modified size distribution and aerosol type-specific CCN parameterizations to estimate the number concentration of CCN (nCCN) at different supersaturations. The algorithm accounts for aerosol hygroscopicity by using the kappa parametrization. Sensitivity studies suggest that the uncertainty associated with the output nCCN may range between a factor of 2 and 3. OMCAM-estimated aerosol number concentrations (ANCs) and nCCN are validated using temporally and spatially co-located in-situ measurements. In the first part of validation, the airborne observations collected during the Atmospheric Tomography (ATom) mission are used. It is found that the OMCAM estimates of ANCs are in good agreement with the in-situ measurements with a correlation coefficient of 0.82, an RMSE of 247.2 cm-3, and a bias of 44.4 cm-3. The agreement holds for all aerosol types, except for marine aerosols, in which the OMCAM estimates are about an order of magnitude smaller than the in-situ measurements. An update of the marine model in OMCAM improve the agreement significantly. In the second part of validation, the OMCAM-estimated ANC and nCCN are compared to measurements from seven surface in-situ stations covering a variety of aerosol environments. The OMCAM-estimated monthly nCCN are found to be in reasonable agreement with the in-situ measurements with a 39 % normalized mean bias and 71 % normalized mean error. Combining the validation studies, the algorithm outputs are found to be consistent with the co-located in-situ measurements at different altitude ranges over both land and ocean. Such an agreement has not yet been achieved for spaceborne-derived CCN concentrations and demonstrates the potential of using CALIPSO lidar measurements for inferring global 3D climatologies of CCN concentrations related to different aerosol types.:1 Introduction . . . . . . . . . . . . . . . 1 1.1 Background: Aerosols in the climate system . . . . . . . . . . . . . . . . . 1 1.1.1 Aerosol-induced effective radiative forcing . . . . . . . . . . . . . . 3 1.1.2 Significance of aerosol-cloud interactions . . . . . . . . . . . . . . . 3 1.2 Observation-based ACI studies . . . . . . . . . . . . . . . . . . . . . . . . 4 1.2.1 In-situ studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 1.2.2 Spaceborne studies . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 1.3 Spaceborne CCN proxies and their limitations . . . . . . . . . . . . . . . . 8 1.4 CCN concentrations from lidars . . . . . . . . . . . . . . . . . . . . . . . . 10 1.5 Objective: CCN from spaceborne lidar . . . . . . . . . . . . . . . . . . . . 11 2 Paper 1: Estimating cloud condensation nuclei concentrations from CALIPSO lidar measurements . . . . . . . . . . . . . . . 15 2.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 2.2 Data and retrievals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 2.2.1 CALIPSO . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 2.2.2 MOPSMAP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 2.2.3 POLIPHON . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 2.3 Methodology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 2.3.1 Aerosol size distribution . . . . . . . . . . . . . . . . . . . . . . . . 20 2.3.2 Aerosol hygroscopicity . . . . . . . . . . . . . . . . . . . . . . . . . 23 2.3.3 CCN parameterizations . . . . . . . . . . . . . . . . . . . . . . . . 23 2.3.4 Application of OMCAM to CALIPSO retrieval . . . . . . . . . . . 23 2.4 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 2.4.1 Sensitivity analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 2.4.2 Comparison with POLIPHON . . . . . . . . . . . . . . . . . . . . . 30 2.4.3 Case study . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32 2.5 Summary and conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . 35 3 Paper 2: Evaluation of aerosol number concentrations from CALIPSO with ATom airborne in situ measurements . . . . . . . . . . . . . . . 39 3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40 3.2 Data, retrievals, and methods . . . . . . . . . . . . . . . . . . . . . . . . . 42 3.2.1 ATom 3.2.2 CALIOP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43 3.2.3 Aerosol number concentration from CALIOP . . . . . . . . . . . . 44 3.2.4 Data matching and comparison . . . . . . . . . . . . . . . . . . . . 48 3.3 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52 3.3.1 Example cases . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52 3.3.2 General findings . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55 3.4 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59 3.5 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61 3.6 Appendix . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63 4 Paper 3: Assessment of CALIOP-derived CCN concentrations by in situ surface measurements . . . . . . . . . . . . . . . 65 4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66 4.2 Data and methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68 4.2.1 In situ observations . . . . . . . . . . . . . . . . . . . . . . . . . . . 68 4.2.2 CALIOP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69 4.2.3 Comparison Methodology . . . . . . . . . . . . . . . . . . . . . . . 71 4.3 Comparison of CCN Concentrations . . . . . . . . . . . . . . . . . . . . . . 73 4.4 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76 5 Summary and conclusions . . . . . . . . . . . . . . . 79 6 Outlook . . . . . . . . . . . . . . . 83 References . . . . . . . . . . . . . . . 88 List of Abbreviations . . . . . . . . . . . . . . . 107 List of Variables . . . . . . . . . . . . . . . 109 List of Figures . . . . . . . . . . . . . . . 111 List of Tables . . . . . . . . . . . . . . . 113 A List of Publications . . . . . . . . . . . . . . . 115 B Acknowledgements . . . . . . . . . . . . . . . 11