Kenneth W. Fischer

The Labrador Sea Deep Convection Experiment

In the autumn of 1996 the field component of an experiment designed to observe water mass transformation began in the Labrador Sea. Intense observations of ocean convection were taken in the following two winters. The purpose of the experiment was, by a combination of meteorological and oceanographic field observations, laboratory studies, theory, and modeling, to improve understanding of the convective process in the ocean and its representation in models. The dataset that has been gathered far exceeds previous efforts to observe the convective process anywhere in the ocean, both in its scope and range of techniques deployed. Combined with a comprehensive set of meteorological and air-sea flux measurements, it is giving unprecedented insights into the dynamics and thermodynamics of a closely coupled, semienclosed system known to have direct influence on the processes that control global climate.

Spirals on the sea

Spiral eddies were first seen in the sunglitter on the Apollo Mission 30 years ago; they have since been recorded on synthetic aperture radar (SAR) images and in the infrared. We present a small sample of images. The spirals are broadly distributed over the worlds oceans, 10–25 km in size and overwhelmingly cyclonic. Under light winds favourable to visualization, linear surface features with high surfactant density and low surface roughness are of common occurrence. The linear features are wound into spirals in vortices associated with horizontal shear instability, modified by rotation, in regions where the shear is comparable with the Coriolis frequency. Two models for concentrating shear are presented: a softened version of the classical sharp Margules front, and the time–dependent Lagrangian model of Hoskins & Bretherton. Horizontal shear instabilities and both frontal models favour cyclonic shear and cyclonic spirals, but for different reasons.

Visible wavelength Doppler lidar for measurement of wind and aerosol profiles during day and night

An incoherent (direct detection) Doppler lidar is developed that operates in the middle of the visible spectrum and measures wind and aerosol profiles during the day and night from the planetary boundary layer to the lower stratosphere. The primary challenge of making a lidar measurement in the visible spectrum during daylight hours is the strong presence of background light from the sun. To make a measurement of this type, the laser line must be isolated spectrally to the greatest extent possible. This has been accomplished through the use of a multiple etalon Fabry-Perot interferometer in combination with a narrow-band filter. The incoherent technique and system are a modified version of the Fabry-Perot interlerometer and image-plane detector technology developed for an earlier Doppler lidar developed at the University of Michigan and for the High-Resolution Doppler Imager (HRDI) now flying on the Upper Atmosphere Research Satellite. The incoherent Doppler analysis is discussed and sample measurements are shown. Winds are measured in the boundary layer with 100-m vertical resolution and 5-mm temporal resolution with 1 to 3 m s-1 accuracy.

Greenland Sea Odden sea ice feature: Intra‐annual and interannual variability

The “Odden” is a large sea ice feature that forms in the east Greenland Sea that may protrude eastward to 5°E from the main sea ice pack (at about 8°W) between 73° and 77°N. It generally forms at the beginning of the winter season and can cover 300,000 km2. Throughout the winter the outer edge of the Odden may advance and retreat by several hundred kilometers on timescales of a few days to weeks. Satellite passive microwave observations from 1978 through 1995 provide a continuous record of the spatial and temporal variations of this extremely dynamic phenomenon. Aircraft synthetic aperture radar, satellite passive microwave, and ship observations in the Odden show that the Odden consists of new ice types, rather than older ice types advected eastward from the main pack. The 17-year record shows both strong interannual and intra-annual variations in Odden extent and temporal behavior. For example, in 1983 the Odden was weak, in 1984 the Odden did not occur, and in 1985 the Odden returned late in the season. An analysis of the ice area and extent time series derived from the satellite passive microwave observations along with meteorological data from the International Arctic Buoy Program (IABP) determined the meteorological forcing associated with Odden growth, maintenance, and decay. The key meteorological parameters that are related to the rapid ice formation and decay associated with the Odden are, in order of importance, air temperature, wind speed, and wind direction. Oceanographic parameters must play an important role in controlling Odden formation, but it is not yet possible to quantify this role because of a lack of long-term oceanographic observations.

Measurement of aerosol loading profiles and mixing heights in Atlanta, Georgia during the 1992 SORP-ONA field study

The Southern Oxidants Research Program on Ozone Non-Attainment (SORP-ONA) field study was held in Atlanta, Georgia, during the summer of 1992. SORP-ONA was the first in a series of intensive studies to characterize urban ozone in the South as a part of the Southern Oxidants Study (SOS). The University of Michigan Doppler Lidar was stationed on the Georgia Tech Campus during the study and measured aerosol profiles with approximately 15-minute temporal resolution. A study of mixing in the urban boundary layer determined that aerosol and presumably chemical constituents are not always well-mixed as expected and that some structure does exist. A technique for separation of aerosol and molecular scattered signal for retrieval of aerosol profiles is described. Additionally, a technique is introduced to estimate boundary layer mixing height which shows excellent correlation with rawinsonde potential temperature profile estimates of mixing height.

Satellite observations of deep-water convection

A key part of the global thermohaline circulation and hence the worlds heat balance, deep ocean convection is the process by which the deep waters of the North Atlantic are renewed. This paper details the results of a study to identify remotely sensible surface signatures for deep ocean convection. Remote sensing efforts have focused on the microwave part of the electromagnetic spectrum due to the all-weather capability. In particular, the high resolution imaging capability of synthetic aperture radar is explored for the existence of convective signatures. Key findings are the existence of a definite identifiable radar surface signature in convective regions and a set of conditions under which one would expect to observe these signatures.

Design for a compact field-deployable tropospheric ozone lidar

It has become clear that in order for lidar technologies to gain wider acceptance outside the research community, they must be smaller, less expensive, and more autonomous. ERIM International has conducted a design study to determine the minimum package size for a fieldable tropospheric ozone lidar. After considering several different wavelength pairs for the differential absorption lidar measurement, a design was selected based on Raman shifting the 4th harmonic of an Nd:YAG laser from 266 nm to 289 nm using deuterium and from 266 nm to 299 nm using hydrogen. Model results indicate that the three wavelengths used in concert will allow measurements of ozone out to a range of nearly 5 km with an accuracy of 5 ppb or better with a one hour integration time. The overall system design consists of a sensor head mounted inside a small shipping crate with the laser, Raman shift tubes, receiving telescope, and detectors and separate data collection/control module in a rugged case. It is anticipated that the system could be built in a combined package occupying less than 2 m3.

First results: ERIM elastic backscatter lidar

The Environmental Research Institute of Michigan (ERIM) has developed a simple elastic backscatter lidar system using multiple aperture incoherent detection to study radiative properties of clouds and aerosols. The capabilities of the system are elastic backscatter measurement of cloud height, aerosol spatial density distribution, atmospheric mixed layer height and optical thickness of thin aerosol and cloud layers. The ERIM lidar system utilizes an Nd:YAG laser and collects return signal using a several aperture telescope array to demonstrate the concept of multiple aperture incoherent detection. Initial tropospheric measurements of clouds and aerosols made with the system are presented.

Visible wavelength Doppler Lidar for measurement of wind and aerosol profiles during day and night

The University of Michigans Space Physics Research Laboratory has constructed a mobile high-spectral-resolution Doppler lidar capable of measuring wind and aerosol loading profiles in the troposphere and lower stratosphere. The system uses a 3-W pulsed frequency-doubled Nd:YAG laser operating at 532 nm as the active source. Backscattered signal is collected by a 44.4-cm-diameter Newtonian telescope. A two axis mirror scanning system allows the instrument to achieve full sky coverage. A pair of Fabry-Perot interferometers in combination with a narrowband (0.1nm) interference filter are used to filter daylight background and provide a high spectral resolving element to measure the Doppler shift. In addition, the aerosol and molecular scattered components of the signal can be separated, giving a measure of the relative aerosol loading. Measurements have been made day and night in the boundary layer with vertical resolution of 100 m and a temporal resolution of approximately 5 minutes. Accuracy of the wind velocity is on the order of 1 to 2 m/s in the boundary layer.

Recent field results from the ERIM International M10 portable lidar

Wider use of lidar systems requires that they continue to become smaller, less expensive, and more reliable. To address this need, ERIM International has developed the M10 portable atmospheric lidar to measure aerosol profiles and cloud optical and physical properties. The system is based on a compact Nd:YAG laser and operates at either 532, 1064, or 1574 nm. The 1574 nm wavelength was added recently to address eye safety concerns. The system has been ruggedized for unattended field use and has been on several data collection campaigns recently. Date and analyses are presented from the Program for Regional Oxidants: Photochemistry, Emissions, and Transport Summer 1997 field study in Pellston, Michigan, and also from the Hyperspectral Day/Night Radiometry Assessment field study at Eglin AFB in Florida.