Robert W. Fett

Environmental Phenomena of the Beaufort Sea Observed during the Leads Experiment

Abstract This paper describes unique environmental phenomena observed during LEADEX (Leads Experiment), a multidisciplinary investigation staged from an ice camp in the Beaufort Sea during March and April 1992. The paper focuses on phenomena observed by NOAA, DMSP, and the EuropeanERS-1 satellites. The opening and closing of a lead is studied using the synthetic aperture radar (SAR) data aboardERS-1. With a mesoscale model, the authors examine the three-dimensional nature of meteorological phenomena and their effect on the opening and closing of leads and show that this model is extremely useful for interpreting structures evident in satellite and in situ observations along northern Alaska. Storms or wind events, which result in leads and fractured ice, also cause ice floes to rotate; the authors document this rotation with automated weather stations anchored to the floes. Finally, the authors describe unique thermal streaks that appeared over a large area of the Beaufort Sea during strong northeasterly w...

Numerical simulation of cloud plumes emanating from Arctic leads

A two-dimensional, steady state boundary layer model is used to investigate the formation of cloud plumes over open Arctic leads. Satellite observations from the period of an intense storm in the Beaufort Sea during April 1992 are used to document lead plume formation [Fett et al., 1997]. These observations show a marked variability of open leads with and without cloud plumes dependent on synoptic weather conditions. Model simulations are conducted (using soundings before and after the storms passage) which show extensive plumes forming in the postfrontal environment but not in the prefrontal atmosphere. To better isolate important factors contributing to plume development and plume dynamics, a large set of idealized model experiments is performed. This matrix of model simulations permits exploration of the parameter space of plume formation. The model is found capable of forecasting the onset of steam fog over a lead in a manner which agrees with empirical relationships. Such steam fog, however, dissipates quickly upon advecting over the surrounding ice and does not therefore appear generally responsible for extensive plumes observed to emanate from leads. Instead, extended cloud plumes form at the top of the convective thermal internal boundary layer (TIBL) that develops over a lead. These elevated plumes are quite sensitive to the relative humidity of the environmental air. Unlike steam fog, the elevated plumes also are found to be sensitive to the lapse rate of the environmental air. Changes in wind speed have little impact on the integrated heat and moisture amounts transferred to the atmosphere from a lead (provided advection balances flux divergence) but do impact the cloud plumes by altering shear-driven entrainment. For wide leads, indications are that lead refreezing must be in an advanced stage before having a major impact on the plume formation process. Additionally, we investigate the conditions under which a cloud plume that is detached from its parent lead may form downwind of a lead. Often, the plumes are predominantly water clouds near the lead, with the ice crystal fraction increasing as the plume cools downstream. A simple formula for estimating plume penetration depth is found to agree reasonably well with the model simulations for 1 km wide leads but deviates considerably for wider leads.

Techniques for analyzing lead condition in visible, infrared and microwave satellite imagery

The condition of a lead or polynya, whether open, partially refrozen, or completely ice covered, is extremely important from many points of view. From a meteorological and climatological perspective the moisture and heat flux resulting from air and sea interaction over leads during cold weather periods is crucial. Enormous cloud plumes have been observed emanating from relatively small openings in leads and polynyas. Modeling studies have shown that plume development is extremely sensitive to changes in humidity, air temperature, and the nature of near-surface atmospheric lapse rates. The studies indicate that plumes can exist, even in the refreezing process, until the new ice becomes sufficiently thick so that its temperature approaches the temperature of the surrounding ice. Heat balance studies for modeling the arctic environment depend upon knowledge of such distinctions for accurate assessment of the radiation balance over the area at any given time. This paper describes a new visible channel differencing technique shown to have some capabilities in distinguishing between open and refrozen leads and, in larger leads, distinguishing the part that is open from that which is partially or newly refrozen. Output from this technique is correlated with infrared data in two examples over the Beaufort Sea and in a polynya example with additional microwave data over the Laptev Sea. Although in situ evidence for direct documentation of lead condition is limited, the existence of cloud plumes emanating from open leads provides some verification and establishment of remote sensing parameters indicative of an open or closed condition. A logical consistency of values from independent sensors over varying conditions of open water, mixed ice and water, new ice, and first or multiyear ice also provides contributing remote sensing evidence of lead condition.

Application of hypermedia and expert system technology to Navy environmental satellite image analysis

The Naval Research Laboratory Marine Meteorology Division, over a period of more than 15 years, has developed a series of satellite imagery training documents called the Navy Tactical Applications ...

Wind‐induced ice floe rotation and icequakes north of the Alaskan Beaufort Sea Coast

Linear sea ice canopy fractures (leads) and ice floe rotation appear to be interrelated. This study focused on a highly persistent storm wind event. As the wind stress increased over the ice pack, a separate rotation threshold for four floe-attached buoys was observed. The northwestern-most buoy recorded the start of clockwise ice-floe rotation just as hydrophones recorded increased oceanic ambient noise. Several days later, when the surface wind speeds and computed stress at all four buoys dropped to ∼60% and e -1 of their respective peak storm values, a counterclockwise rotation began at each buoy with a concomitant impulsive ambient noise peak. The rotationally-induced noise bursts are called icequakes in an analogy with earthquakes.