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Environmental Conditions Along Long Island Sound, July 16-17, 1999

 NAAPS Optical Depth SeaWIFS Diagram
NAAPS Aerosol Optical Depth SeaWIFS Visible Image Ball Diagram of Vertical Structure

Introduction

A reduction in visibility due to haze has been cited by NTSB as a possible environmental effect leading to the crash of the plane flown by John F. Kennedy Jr. at around 0130 UTC July 17 (9:30 pm EDT July 16). Prof. Joseph Prospero (Univ. Miami) suggested that a study using NRL modeling capabilities might shed some light on the possible haze conditions during this time. Here we present radiosonde data and numerical simulations for the period. The radiosonde data are available to the general community, as are numerical weather simulations. The unique product presented here is a global sulfate aerosol simulation which is run in near-real-time by the Naval Research Laboratory in cooperation with the Navy's Fleet Numerical Oceanographic and Meteorological Center.

First, the radiosonde data are presented to show the vertical structure of the atmosphere. The corresponding model soundings are shown to verify that the global weather prediction model NOGAPS accurately captures the features observed in the soundings. This is important since the NOGAPS fields are used by the aerosol model.

Then the aerosol simulations from NAAPS are shown in plan view and in cross-sectional view. The plotted variables are aerosol optical depth (AOD, a vertical integral of the light-extinction properties of the aerosol) and extinction (the local measure of the light-extinction properties).

Lastly, the NAAPS simulation is verified qualitatively by comparison with the aerosol optical depth retrieved from satellite data by NOAA/NESDIS and imagery from NASA/GSFC SeaWIFS.

A brief discussion of the environment is given at the end. Details of the models can be found at the links in the preceding paragraphs.

Chatham Radiosonde Soundings These two balloon soundings were taken at Chatham MA.  They show several distinct layers.  A shallow marine boundary layer (MBL) occupies the lowest 300 m. It is relatively cool with high humidities (70-90%) and south-southwest winds (i.e. roughly paralleling the eastern seaboard).  This over-water fetch has probably led its formation at the base of the otherwise hot and dry air leaving the continent.  The MBL is capped by an inversion at 300 m. A weakly stable layer is found between the MBL inversion and an inversion at 3 km (700 mb).  This layer is drier (40-70% RH) than the MBL.  RH increases with height in the 12Z sounding.  Winds are westerly. These features indicate that this layer was recently a planetary boundary layer (PBL) . Above this layer is a more stable and much drier layer (< 10% RH), with west-northwesterly winds and stability typical of the free troposphere.

NOGAPS Chatham Soundings This is a model sounding from NOGAPS for a location near Chatham.  It shows the same layered structure seen in the radiosonde soundings.  Some small differences include a drier MBL and a shallower elevated mixed layer. This sounding is shown as evidence that NOGAPS has captured the characteristics of layered structure.

NAAPS Aerosol Optical Depth Simulation NAAPS shows a tongue of moderate sulfate aerosol optical depths (AOD > 0.2) leaving the continent with the Connecticut coastline just north of the axis of the plume.  This is the sulfate AOD.  Nearly all of the sulfate in NAAPS is of anthropogenic origins.  (The only other sulfate in the model comes from weak oceanic sources.)  Other aerosols, both natural and anthropogenic, were undoubtedly present in the atmosphere: photochemical smog, smoke, mineral dust, etc. and the actual AOD and extinction values (see below) were probably higher than the those provided by NAAPS.  The NOAA/NESDIS analysis of AOD (below) indicates values of 0.3 to 0.6 in the area and the NASA/GSFC/SeaWIFS image (below) shows heavy haze in the area.

3-day loop shows tongue of high sulfate optical depth leaving continent, with the axis slowly sagging south over the period. (Plot Info)

NAAPS aerosol extinction (colored, 1/km) with NOGAPS RH (solid contours, %), winds (compass winds, one full barb=5m/s), and potential temperature (dashed contours, K) for 00Z July 17. Note the locator map in the upper-right corner of the plots:
These cross-sections of NAAPS and NOGAPS fields illustrate the location of the sulfate aerosol relative to the three layers identified in the Chatham soundings.  High extinction values are found in the elevated mixed layer and at the top of the MBL.  Low values are found above the elevated mixed layer in the free troposphere.

NOAA/NESDIS Aerosol Optical Depth NESDIS AOD shows an aerosol plume emanating from the U.S. in the same location as NAAPS.

SeaWIFS Images

SeaWIFS visible wavelength imagery show a plume of aerosol (the lighter colors over the ocean) leaving the continent with a heavy layer in the vicinity of the Sound on July 18.

Discussion

The environmental conditions along the Connecticut coastline at the time of the final flight by John F. Kennedy Jr. are typical for the time of year. A strong high over the southeast states (not shown) had begun to build over the previous two days and now dominated the region causing westerly or west-northwesterly (offshore flow) at most levels and clear skies. (This is the beginning of the high that caused drought conditions on the east coast and in mid-west during the second half of July 1999.) Weak along-shore flow at the surface led to moistening and cooling of the lowest 300 m of the atmosphere to form a shallow marine boundary layer. Above this layer was an elevated mixed layer, extending to 3 km and laden with pollutants. Moderate humidity in the elevated mixed layer caused an enhancement of the aerosol optical effects. Above the elevated mixed layer, the free troposphere was relatively clean and dry. These features are summarized in this figure: Diagram of vertical structure


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Point of Contact:
Dr. Douglas L. Westphal
Naval Research Laboratory
Marine Meteorology Division
7 Grace Hopper Avenue, Stop 2
Monterey, CA 93943-5502
(831) 656-4613