3-D Cloud Morphology and Evolution Derived from Hemispheric Stereo Cameras

Clouds play a key role in the Earth-atmosphere system as they reflect incoming solar radiation back to space, while absorbing and emitting longwave radiation. A significant challenge for observation and modeling pose cumulus clouds due to their relatively small size that can reach several hundreds up to a few thousand meters, their often complex 3-D shapes and highly dynamic life-cycle. Common instruments employed to study clouds include cloud radars, lidar-ceilometers, (microwave-)radiometers, but also satellite and airborne observations (in-situ and remote), all of which lack either sufficient sensitivity or a spatial or temporal resolution for a comprehensive observation. This thesis investigates the feasibility of a ground-based network of hemispheric stereo cameras to retrieve detailed 3-D cloud geometries, which are needed for validation of simulated cloud fields and parametrization in numerical models. Such camera systems, which offer a hemispheric field of view and a temporal resolution in the range of seconds and less, have the potential to fill the remaining gap of cloud observations to a considerable degree and allow to derive critical information about size, morphology, spatial distribution and life-cycle of individual clouds and the local cloud field. The technical basis for the 3-D cloud morphology retrieval is the stereo reconstruction: a cloud is synchronously recorded by a pair of cameras, which are separated by a few hundred meters, so that mutually visible areas of the cloud can be reconstructed via triangulation. Location and orientation of each camera system was obtained from a satellite-navigation system, detected stars in night sky images and mutually visible cloud features in the images. The image point correspondences required for 3-D triangulation were provided primarily by a dense stereo matching algorithm that allows to reconstruct an object with high degree of spatial completeness, which can improve subsequent analysis. The experimental setup in the vicinity of the Jülich Observatory for Cloud Evolution (JOYCE) included a pair of hemispheric sky cameras; it was later extended by another pair to reconstruct clouds from different view perspectives and both were separated by several kilometers. A comparison of the cloud base height (CBH) at zenith obtained from the stereo cameras and a lidar-ceilometer showed a typical bias of mostly below 2% of the lidar-derived CBH, but also a few occasions between 3-5%. Typical standard deviations of the differences ranged between 50 m (1.5 % of CBH) for altocumulus clouds and between 7% (123 m) and 10% (165 m) for cumulus and strato-cumulus clouds. A comparison of the estimated 3-D cumulus boundary at near-zenith to the sensed 2-D reflectivity profiles from a 35-GHz cloud radar revealed typical differences between 35 - 81 m. For clouds at larger distances (> 2 km) both signals can deviate significantly, which can in part be explained by a lower reconstruction accuracy for the low-contrast areas of a cloud base, but also with the insufficient sensitivity of the cloud radar if the cloud condensate is dominated by very small droplets or diluted with environmental air. For sequences of stereo images, the 3-D cloud reconstructions from the stereo analysis can be combined with the motion and tracking information from an optical flow routine in order to derive 3-D motion and deformation vectors of clouds. This allowed to estimate atmospheric motion in case of cloud layers with an accuracy of 1 ms-1 in velocity and 7° to 10° in direction. The fine-grained motion data was also used to detect and quantify cloud motion patterns of individual cumuli, such as deformations under vertical wind-shear. The potential of the proposed method lies in an extended analysis of life-cycle and morphology of cumulus clouds. This is illustrated in two show cases where developing cumulus clouds were reconstructed from two different view perspectives. In the first case study, a moving cloud was tracked and analyzed, while being subject to vertical wind shear. The highly tilted cloud body was captured and its vertical profile was quantified to obtain measures like vertically resolved diameter or tilting angle. The second case study shows a life-cycle analysis of a developing cumulus, including a time-series of relevant geometric aspects, such as perimeter, vertically projected area, diameter, thickness and further derived statistics like cloud aspect ratio or perimeter scaling. The analysis confirms some aspects of cloud evolution, such as the pulse-like formation of cumulus and indicates that cloud aspect ratio (size vs height) can be described by a power-law functional relationship for an individual life-cycle.Wolken haben einen maßgeblichen Einfluss auf den Strahlungshaushalt der Erde, da sie solare Strahlung effektiv reflektieren, aber von der Erde emittierte langwellige Strahlung sowohl absorbieren als auch ihrerseits wieder emittieren. Darüber hinaus stellen Cumulus-Wolken wegen ihrer verhältnismäßig kleinen Ausdehnung von wenigen hundert bis einigen tausend Metern sowie ihres dynamischen Lebenszyklus nach wie vor eine große Herausforderung für Beobachtung und Modellierung dar. Gegenwärtig für deren Erforschung im Einsatz befindliche Instrumente wie Lidar-Ceilometer, Wolkenradar, Mikrowellenradiometer oder auch satellitengestützte Beobachtungen stellen die für eine umfassende Erforschung dieser Wolken erforderliche räumliche und zeitliche Abdeckung nicht zur Verfügung. In dieser Arbeit wird untersucht, inwieweit eine bodengebundene Beobachtung von Wolken mit hemisphärisch projizierenden Wolkenkameras geeignet ist detaillierte 3-D Wolkengeometrien zu rekonstruieren um daraus Informationen über Größe, Morphologie und Lebenszyklus einzelner Wolken und des lokalen Wolkenfeldes abzuleiten. Grundlage für die Erfassung der 3-D Wolkengeometrien in dieser Arbeit ist die 3-D Stereorekonstruktion, bei der eine Wolke von jeweils zwei im Abstand von mehreren Hundert Metern aufgestellten, synchron aufnehmenden Kameras abgebildet wird. Beidseitig sichtbare Teile einer Wolke können so mittels Triangulation rekonstruiert werden. Fischaugen-Objektive ermöglichen das hemisphärische Sichtfeld der Wolkenkameras. Während die Positionsbestimmung der Kameras mit Hilfe eines Satelliten-Navigationssystems durchgeführt wurde, konnte die absolute Orientierung der Kameras im Raum mit Hilfe von detektierten Sternen bestimmt werden, die als Referenzpunkte dienten. Die für eine Stereoanalyse wichtige relative Orientierung zweier Kameras wurde anschließend unter Zuhilfenahme von Punktkorrespondenzen zwischen den Stereobildern verfeinert. Für die Stereoanalyse wurde primär ein Bildanalyse-Algorithmus eingesetzt, welcher sich durch eine hohe geometrische Vollständigkeit auszeichnet und auch 3-D Informationen für Bildregionen mit geringem Kontrast liefert. In ausgewählten Fällen wurden die so rekonstruierten Wolkengeometrien zudem mit einem präzisen Mehrbild-Stereo-Verfahren verglichen. Eine möglichst vollständige 3-D Wolkengeometrie ist vorteilhaft für eine darauffolgende Analyse, die eine Segmentierung und Identifizierung einzelner Wolken, deren raum-zeitliche Verfolgung oder die Ableitung geometrischer Größen umfasst. Der experimentelle Aufbau im Umfeld des Jülich Observatory for Cloud Evolution (JOYCE) umfasste zuerst eine, später zwei Stereokameras, die jeweils mehrere Kilometer entfernt installiert wurden um unterschiedliche Wolkenpartien rekonstruieren zu können. Ein Vergleich zwischen Stereorekonstruktion und Lidar-Ceilometer zeigte typische Standardabweichungen der Wolkenbasishöhendifferenz von 50 m (1.5 %) bei mittelhoher Altocumulus-Bewölkung und 123 m (7 %) bis 165 m (10 %) bei heterogener Cumulus- und Stratocumulus-Bewölkung. Gleichzeitig wich die rekonstruierte Wolkenbasishöhe im Durchschnitt meist nicht weiter als 2 %, in Einzelfällen 3-5 % vom entsprechenden Wert des Lidars ab. Im Vergleich zur abgeleiteten Cumulus-Morphologie aus den 2-D Reflektivitätsprofilen des Wolkenradars, zeigten sich im Zenit-Bereich typische Differenzen zwischen 35 und 81 m. Bei weiter entfernten Wolken (> 2 km) können sich Stereorekonstruktion und Reflektivitätssignal stark unterscheiden, was neben einer abnehmenden geometrischen Genauigkeit der Stereorekonstruktion in kontrastarmen Bereichen insbesondere mit einer oftmals unzureichenden Sensitivität des Radars bei kleinen Wolkentröpfchen erklärt werden kann, wie man sie an der Wolkenbasis und in den Randbereichen von Wolken findet. Die Kombination von Stereoanalyse und der Bewegungsinformation innerhalb einer Bildsequenz erlaubt die Bestimmung von Wolkenzug- und -deformationsvektoren. Neben der Verfolgung einzelner Wolkenstrukturen und der Erfassung von Wolkendynamik (beispielsweise der Deformation von Wolken durch Windscherung), kann im Fall von stratiformen Wolken Windgeschwindigkeit und -richtung abgeschätzt werden. Ein Vergleich mit Beobachtungen eines Wind-Lidars zeigte hierfür typische Abweichungen der Windgeschwindigkeit von 1 ms-1 und der Windrichtung von 7° to 10°. Ein besonderer Mehrwert der Methode liegt in einer tiefergehenden Analyse von Morphologie und Lebenszyklus von Cumulus-Wolken. Dies wurde anhand zweier exemplarischer Fallstudien gezeigt, in denen die 3-D-Rekonstruktionen zweier entfernt aufgestellter Stereokameras kombiniert wurden. Im ersten Fall wurde ein sich unter vertikaler Windscherung entwickelnder Cumulus von zwei Seiten aufgenommen, was eine geometrische Erfassung des stark durch Scherung geneigten Wolkenkörpers ermöglichte. Kennwerte wie Vertikalprofil, Neigungswinkel der Wolke und Durchmesser einzelner Höhenschichten wurden abgeschätzt. Der zweite Fall zeigte eine statistische Analyse eines sich entwickelnden Cumulus über seinen Lebenszyklus hinweg. Dies erlaubte die Erstellung einer Zeitreihe mit relevanten Kennzahlen wie äquivalenter Durchmesser, vertikale Ausdehnung, Perimeter oder abgeleitete Größen wie Aspektrate oder Perimeter-Skalierung. Während die Analyse bisherige Ergebnisse aus Simulationen und satellitengestützten Beobachtungen bestätigt, erlaubt diese aber eine Erweiterung auf die Ebene individueller Wolken und der Ableitung funktionaler Zusammenhänge wie zum Beispiel dem Verhältnis von Wolkendurchmesser und vertikaler Dimension

A Statistical Approach to The Life Cycle Analysis of Cumulus Clouds Selected in A Virtual Reality Environment

In this study, a new method is developed to investigate the entire life cycle of shallow cumuli in large eddy simulations. Although trained observers have no problem in distinguishing the different life stages of a cloud, this process proves difficult to automate, because cloud-splitting and cloud-merging events complicate the distinction between a single system divided in several cloudy parts and two independent systems that collided. Because the human perception is well equipped to capture and to make sense of these time-dependent three-dimensional features, a combination of automated constraints and human inspection in a three-dimensional virtual reality environment is used to select clouds that are exemplary in their behavior throughout their entire life span. Three specific cases (ARM, BOMEX, and BOMEX without large-scale forcings) are analyzed in this way, and the considerable number of selected clouds warrants reliable statistics of cloud properties conditioned on the phase in their life cycle. The most dominant feature in this statistical life cycle analysis is the pulsating growth that is present throughout the entire lifetime of the cloud, independent of the case and of the large-scale forcings. The pulses are a self-sustained phenomenon, driven by a balance between buoyancy and horizontal convergence of dry air. The convective inhibition just above the cloud base plays a crucial role as a barrier for the cloud to overcome in its infancy stage, and as a buffer region later on, ensuring a steady supply of buoyancy into the cloud

Analytic studies of local-severe-storm observables by satellites

Attention is concentrated on the exceptionally violet whirlwind, often characterized by a fairly vertical axis of rotation. For a cylindrical polar coordinate system with axis coincident with the axis of rotation, the secondary flow involves the radial and axial velocity components. The thesis advanced is, first, that a violent whirlwind is characterized by swirl speeds relative to the axis of rotation on the order of 90 m/s, with 100 m/s being close to an upper bound. This estimate is based on interpretation of funnel-cloud shape (which also suggests properties of the radial profile of swirl, as well as the maximum magnitude); an error assessment of the funnel-cloud interpretation procedure is developed. Second, computation of ground-level pressure deficits achievable from typical tornado-spawning ambients by idealized thermohydrostatic processes suggests that a two-cell structure is required to sustain such large speeds

Exploratory Meeting on Airborne Doppler Lidar Wind Velocity Measurements

The scientific interests and applications of the Airborne Doppler Lidar Wind Velocity Measurement System to severe storms and local weather are discussed. The main areas include convective phenomena, local circulation, atmospheric boundary layer, atmospheric dispersion, and industrial aerodynamics

Characteristics And Variability Of Storm Tracks In The North Pacific, Bering Sea And Alaska

Thesis (Ph.D.) University of Alaska Fairbanks, 2009Storm activity in the North Pacific, Bering Sea and Alaska regions is investigated using various automated storm tracking and parameter extraction algorithms. Specific, novel details of storm activity throughout the year are presented. The influence of major climatic drivers is considered, including the Pacific/North American Index and sea ice variability. Details of synoptic-scale forcing on a specific, severe storm event are considered in the context of how different tracking algorithms are able to depict the event. New storm climatology results show that the inter-seasonal variability is not as large during spring and autumn as it is in winter. Most storm variables exhibited a maxima pattern that was oriented along a zonal axis. From season to season this axis underwent a north-south shift and, in some cases, a rotation to the northeast. Barotropic processes have an influence in shaping the downstream end of storm tracks and, together with the blocking influence of the coastal orography of northwest North America, result in high lysis concentrations, effectively making the Gulf of Alaska the "graveyard" of Pacific storms. Summer storms tended to be longest in duration. Temporal trends tended to be weak over the study area. Sea surface temperature did not emerge as a major cyclogenesis control in the Gulf of Alaska. Positive sea-ice anomalies in the Sea of Okhotsk were found to decrease secondary cyclogenesis, shift cyclolysis locations westward, and alter the North Pacific subtropical jet. In the Atlantic, a negative North-Atlantic-Oscillation-like pattern is observed; these results were confirmed by experiments on the ECHAM5 Atmospheric Global Circulation Model driven with sea-ice anomalies in the Sea of Okhotsk. The destructive west Alaska storm of autumn 1992, which flooded Nome, was investigated using two storm tracking algorithms: NOAA's (National Oceanic and Atmospheric Administration) current operational algorithm and the Melbourne algorithm. Manual tracking was performed as a control. The main storm location features were captured by both algorithms, but differed in the genesis and lysis location. The NOAA algorithm broke the event into two. This storm was shown to have been affected by a blocking high that influenced how the tracking algorithms handled the event

Vortex structure and dynamics of Florida Keys waterspouts: 1974 field experiment, final report

Includes 2 different reprints of the article: Waterspout wind, temperature and pressure structure deduced from aircraft measurements by Verne H. Leverson and Peter C. Sinclair, Jospeh H. Golden.Includes in its entirety Verne H. Leverson's M.S. thesis, "Waterspout structure and dynamics."Includes bibliographical references.From direct penetrations of the waterspout funnel by specially instrumented aircraft, a quantitative description of the dynamic-thermodynamic structure of the waterspout has been developed. The Navier-Stokes equations of motion for the waterspout vortex are simplified by an extensive order of magnitude analysis of each term in the equations. The reduced set of equations provides a realistic mathematical model of the waterspout vortex. Further simplification shows that the cyclostrophic-Rankine combined vortex model accounts for, on the average, approximately 63% of the measured pressure drop from the environment to the waterspout core. The penetration measurements show that the waterspout funnel consists of a strong rotary and vertical field (radial component is smaller) of motion which results in a combined flow pattern similar to that of a helical vortex. In general, the measurements indicate that this one-cell vortex structure is the dominate configuration. However, several penetrations suggest reduced positive vertical velocities near the funnel core, and in one case, a downdraft core with vertical velocity of -0.8 msec-1. These measurements indicate that waterspout vortex may in some stages of development have a structure more closely described by the two-cell vortex such as discovered by Sinclair (1966, 1973) for the dust devil vortex. The temperature and pressure structure show that the waterspout, like the dust devil, is a warm core (ΔT = 0.1 to 0.5°C), low pressure (ΔP = -0.6 to -8.4 mb) vortex. All aircraft penetrations of the visible funnels were made within 150 m of cloud base at speeds of 55-65 msec-1

Doppler-radar observation of the evolution of downdrafts in convective clouds

A detailed analysis of the 20 July 1977 thunderstorm complex which formed and evolve over the South Park region in Central Colorado is presented. The storm was extensively analyzed using multiple Doppler radar and surface mesonet data, developed within an environment having very weak wind shear. The storm owed its intensification to the strength of the downdraft, which was nearly coincident with the region where the cloud had grown. The noteworthy features of this storm were its motion to the right of the cloud-level winds, its multicellular nature and discrete propagation, its north-south orientation, and its relatively large storm size and high reflectivity factor (55 dBZ). This scenario accounts for the observed mesoscale and cloud-scale event. A line of convergence was generated at the interface between the easterly upslope winds and westerly winds. During stage II, the convergence line subsequently propagated down the slopes of the Mosquito Range, and was the main forcing mechanism for the development of updraft on the west flank of the storm. The formation of downdraft on the eastern side of updraft blacked surface inflow, and created a detectable gust front. As the original downdraft intensified, the accumulation of evaporatively-chilled air caused the intensification of the mesohigh, which likely destroyed the earlier convergence line and created a stronger convergence line to the east, which forced up-lifting of the moist, westerly inflow and caused the formation of updraft to the east. An organized downdraft circulation, apparently maintained by precipitation drag and evaporational cooling, was responsible in sustaining a well-defined gust front. The storm attained its highest intensity as a consequence of merging with a neighboring cloud. The interaction of downdrafts or gust fronts from two intense cells appeared to be the primary mechanism of this merging process as suggested by Simpson et al. (1980). The merging process coincided with more rain than occurred in unmerged echoes