David V. Guerra

A longer vernal window: the role of winter coldness and snowpack in driving spring transitions and lags

Climate change is altering the timing and duration of the vernal window, a period that marks the end of winter and the start of the growing season when rapid transitions in ecosystem energy, water, nutrient, and carbon dynamics take place. Research on this period typically captures only a portion of the ecosystem in transition and focuses largely on the dates by which the system wakes up. Previous work has not addressed lags between transitions that represent delays in energy, water, nutrient, and carbon flows. The objectives of this study were to establish the sequence of physical and biogeochemical transitions and lags during the vernal window period and to understand how climate change may alter them. We synthesized observations from a statewide sensor network in New Hampshire, USA, that concurrently monitored climate, snow, soils, and streams over a three-year period and supplemented these observations with climate reanalysis data, snow data assimilation model output, and satellite spectral data. We found that some of the transitions that occurred within the vernal window were sequential, with air temperatures warming prior to snow melt, which preceded forest canopy closure. Other transitions were simultaneous with one another and had zero-length lags, such as snowpack disappearance, rapid soil warming, and peak stream discharge. We modeled lags as a function of both winter coldness and snow depth, both of which are expected to decline with climate change. Warmer winters with less snow resulted in longer lags and a more protracted vernal window. This lengthening of individual lags and of the entire vernal window carries important consequences for the thermodynamics and biogeochemistry of ecosystems, both during the winter-to-spring transition and throughout the rest of the year.

Stimulated Raman scattering in hydrogen pumped with a tunable, high power, narrow linewidth alexandrite laser

The conversion efficiencies and the linewidth of the Stokes components which result from stimulated Raman scattering (SRS) in hydrogen gas pumped with a high power alexandrite laser have been studied. The pump laser was operated at 742 nm giving first and second Stokes components at 1074 nm and 1935 nm respectively. The pump laser could be operated either in the normal Q-switched configuration with a linewidth of the order of 0.2-0.3 nm or with injection seeding which narrowed the linewidth to 1-2 pm. Measurements of the Stokes components under these varying conditions reveal that there is no effect on the conversion efficiency by the narrowing of the pump linewidth and that the linewidth of the first Stokes component is broader than the expected linewidth of the injection seeded pump. Modelling of the conversion from the pump to the Stokes components shows a strong dependence of this conversion on a specific, resonant four wave mixing process. The study gives a complete set of information about the radiation resulting from SRS in molecular hydrogen pumped with a high power, narrow linewidth, tunable solid-state laser.

A Bernoulli's Law Lab in a Bottle

Bernoullis law is a fundamental relationship in fluid dynamics that is covered in most introductory physics courses. Basically a statement of conservation of energy for an open fluid system, Bernoullis law is often used in labs and examples to analyze the lift force on a fixed wing or the forces on objects in a fluid flow.1–4 Although these are legitimate uses of the law, they do not introduce students to the way in which several concepts, such as Bernoullis law and the conservation of mass, are used in combination to study the dynamics of fluid systems. Thus, to give students an easily understandable introduction to this method of analysis, we present a laboratory experience in which the drain time of water flowing out of an inverted soda bottle is measured and calculated for a set of easily interchangeable exit holes.

Prototype holographic atmospheric scanner for environmental remote sensing

A ground-based atmospheric lidar system that utilizes a holographic optical telescope and scanner has been developed and successfully operated to obtain atmospheric backscatter profiles. The Prototype Holographic Atmospheric Scanner for Environmental Remote Sensing is built around a volume phase reflection holographic optical element (HOE). This single optical element both directs and collimates the outgoing laser beam as well as collects, focuses, and filters the atmospheric laser backscatter while offering significant weight savings over existing telescope mirror technology. Conical scanning is accomplished as the HOE rotates on a turntable sweeping the 1.2 mrad field of view around a 42° cone. During this technology demonstration, atmospheric aerosol and cloud return signals have been received in both stationary and scanning modes. The success of this program has led to the further development of this technology for integration into airborne and eventually satellite Earth-observing scanning lidar telescopes.

Compact scanning lidar systems using holographic optics

Two scanning lidar systems have been built using holographic optical elements (HOE) that function as a scanning telescope primary optic. One is a ground based lidar using a reflection HOE, and uses a frequency doubled Nd:YAG laser transmitter. The other system is an airborne/ground based system that uses a transmission HOE and operates at the 1064 nm fundamental of the Nd:YAG laser. Each HOE has a focal spot on the center- line, normal to the flat disk holding the hologram, and a field of view (FOV) that points approximately 45 degrees from the normal. Rotating the disk effects a conical scan of the FOV. In both systems, the same HOE is also used to collimate and steer the transmitted laser beam. The utility of using the HOEs to save weight and size in scanning lidars is evidenced by the atmospheric backscatter data collected with these systems. They also will lower the cost of commercial systems due to the low cost of replicating HOEs and the simplified mechanical scanning systems. Development of airborne scanning lidar altimeters and other lidars and passive instruments using holographic optics are underway, including the development of a one meter diameter, space qualified holographic scanning telescope for use in the ultraviolet.

Horizontal wind measurements using the HARLIE holographic lidar

We report the results of three campaigns in which the horizontal wind vector at cloud altitudes was measured using the holographic, conical-scan lidar HARLIE in its zenith-viewing mode. Measurements were made during the HOLO-1 and HOLO-2 tests in Utah and New Hampshire in March and June 1999, respectively, and at the DoE-ARM site in Oklahoma in September/October 2000. A novel algorithm facilitates the wind vector analysis of the HARLIE data. Observed wind velocity and direction were compared with radiosonde records and with other data obtained from video cloud imagery and independent lidar ranging. The results demonstrate good agreement between HARLIE data and the results of other methods. The conically scanning holographic lidar opens up new possibilities for obtaining the vertical profile of horizontal winds.

HOLO series: critical ground-based demonstrations of holographic scanning lidars

Results of two lidar measurement campaigns are presented, HOLO-1 (Utah, March 1999) and HOLO-2 (New Hampshire, June 1999). These tests demonstrate the ability of lidars utilizing holographic optical elements (HOEs) to determine tropospheric wind velocity and direction at cloud altitude. Several instruments were employed. HOLO-1 used the 1.064 mm transmission-HOE lidar (HARLIE, Goddard Space Flight Center), a zenith-staring 532 nm lidar (AROL-2, Utah State University), and a wide-field video camera (SkyCam) for imagery of clouds overhead. HOLO-2 included these instruments plus the 532 nm reflection-HOE lidar (PHASERS, St. Anselm College). HARLIE and PHASERS scan the sky at constant cone angles of 45° and 42° from normal, respectively. The progress of clouds and entire cloud fields across the sky is tracked by the repetitive conical scans of the HOE lidars. AROL-2 provides the altitude information enabling the SkyCam cloud images to be analyzed for independent data on cloud motion. Data from the HOE lidars are reduced by means of correlations, visualization by animation techniques, and kinematic diagrams of cloud feature motion. Excellent agreement is observed between the HOE lidar results and those obtained with video imagery and lidar ranging.

An Introduction to Dimensionless Parameters in the Study of Viscous Fluid Flows

It has been suggested that there is a need to deepen the understanding of fluid dynamics in the introductory physics course and to offer interesting experiments to do so.1 To address this need we have developed a laboratory experiment and the supporting analysis to demonstrate the role of viscosity and the interestingly mysterious use of dimensionless parameters in fluid dynamics.2 Since viscosity indicates the frictional dependence between the layers of a flowing fluid, a thoughtful student may ask why or when viscosity can be neglected. The laboratory experiment presented here uses common fluids to provide a concrete answer to this question and an easily understandable example of the role of dimensionless parameters in fluid dynamics.

Large aperture scanning lidar based on holographic optical elements

We have developed simplified conical scanning telescopes using Holographic Optical Elements (HOEs) to reduce the size, mass, angular momentum, and cost of scanning lidar systems. This technology enables wide-angle scanning and three-dimensional measurements of atmospheric backscatter when used in airborne instruments, and high temporal resolution observations of atmospheric dynamic structure, including wind profiles from ground-based facilities.

Emphasizing Environmental Concepts and Policies in an Introductory Meteorology Course

In an attempt to make the material covered in introductory meteorology courses more relevant to students, the concepts and policies central to the Clean Air Act have been integrated into the class. To further facilitate this integration of the material and to encourage class participation, each student is required to become the class expert in one facet of the development of this legislation. Assuming the roles of atmospheric scientists, legislators, regulatory personnel, economists, and lobbyists, students are required to comment on topics covered during the class from the perspective of their role and make a presentation as part of a classroom “Congressional” hearing on the Clean Air Act. An introductory meteorology textbook provides the underlying structure of the course, with material selected to furnish students with the necessary information to form an understanding of the environmental issues: global climate change, stratospheric ozone depletion, urban air pollution, and acid rain. The interplay of...