Fog in the Gulf of Maine

July 8th, 2008 |


Fog that formed in the Gulf of Maine on Tuesday July 8th developed when relatively moist air moved from the continent over the cooler ocean waters and was cooled from beneath by conduction to the dewpoint. In other words, Advection Fog.

An obvious question arises from the loop: Why does the fog persist over the ocean east of Nantucket and along the coast of Maine even as it clears out in the central part of the Gulf of Maine? The answer is shown in the color-shaded image of SST (available from the Applied Physics Lab at Johns Hopkins University) below. Warmer waters over the central Gulf of Maine will promote the faster erosion of ocean fog because of enhanced vertical mixing because of lower stability. Ocean fog that moves over land — as over southwestern Nova Scotia — also dissipates as heating over land promotes vertical mixing. Dryer air aloft mixed towards the surface reduces the boundary layer relative humidity and the fog evaporates. The dryer air just above the surface is readily apparent in Skew-T plots from Yarmouth, Nova Scotia and from Gray, Maine.


California fires: burn scar detection

July 8th, 2008 |

MODIS false color + true color images (Animated GIF)

Many large wildfires continued to burn in parts of California on 08 July 2008. A comparison of MODIS false color and true color images from the SSEC MODIS Today site (above) demonstrated the use of the false color imagery for determining the areal coverage of burn scars in the Los Padres National Forest south of Monterey, California; the burn scars show up as reddish-brown features, with active fire hot spots having a brighter light pink appearance on the false color imagery. In the true color imagery, it is difficult to discriminate between thick smoke from the fires and stratus clouds right along the coast, but the smoke exhibits a light blue tint on the false color imagery.

Bertha becomes a hurricane

July 7th, 2008 |

GOES-12 IR images (Animated GIF)

Hurricane Bertha became the first hurricane of the season in the Atlantic Basin on 07 July 2008,  setting a new record for the furthest-east named storm formation in the tropics. GOES-12 IR images from the CIMSS Tropical Cyclones site (above) showed increasing coverage of cold cloud top temperatures and the formation of an eye; microwave imagery from the polar-orbiting SSM/I instrument (below) began to show better evidence of an eye structure a few hours before the geostationary satellite imagery.

SSM/I microwave image

GOES-12 visible imagery (below) showed a closer view of the forming eye of the hurricane.

GOES-12 visible image

A plot of the CIMSS Advanced Dvorak Technique intensity estimate (below) showed that Bertha began a period of more rapid intensification during the early morning hours of 07 July.

Advanced Dvorak Technique intensity plot

Bertha had been moving northwestward over increasingly warmer Sea Surface Temperatures (below), which may have played a role in the intensification of the tropical cyclone.

Sea Surface Temperature data

UPDATE: Hurricane Bertha rapidly intensified into a Category 3 storm during the afternoon hours on 07 July, with the CIMSS ADT intensity estimation technique suggesting peak wind speeds near 115 knots. During this period of rapid intensification, Bertha also displayed a nice eye on satellite imagery (QuickTime animations: GOES-12 visible | GOES-12 IR). It is interesting to note that the MIMIC Total Precipitable Water product (below) indicated that dry Saharan Air Layer (SAL) air had wrapped completely around Bertha during the 05-08 July period — the presence of such dry air in close proximity to a tropical cyclone would normally be thought of as a negative factor for rapid intensification!

MIMIC Total Precipitable Water (Animated GIF)

Russian smoke over the Pacific Northwest?

July 7th, 2008 |

GOES-12 visible images (Animated GIF)

A suspicious area of “haziness” began to appear over the Pacific Northwest region (specifically, Washington state and adjacent parts of northern Oregon and southern British Columbia) on the late afternoon and early evening GOES-12 visible images (above) on 06 July 2008. A MODIS true color image from the SSEC MODIS Today site (below) also shows the haziness over the Pacific Northwest and the adjacent offshore waters, as well as thick smoke farther to the south due to active fires that continued to burn in parts of California.

MODIS true color image

My first thought was: “Wow, all that smoke from the California fires has moved that far north again today (like it did on the first 3 days of July)?” — but that didn’t seem like a meteorologically plausible scenario for this particular day. Then I recalled seeing MODIS true color imagery of thick smoke from fires burning in far eastern Russia that was moving across the Sea of Okhotsk and the Kamchatka Peninsula on 30 June and 02 July; could this haziness seen on the GOES-12 visible imagery possibly be smoke from those Russian fires? Backward airmass trajectories using the NOAA ARL HYSPLIT model (below) seem to support that idea — air parcels arriving over Washington state at 2000, 3000, and 4000 meters above ground level came from the region where the thick smoke was seen on MODIS true color imagery several days earlier.

HYSPLIT backward trajectories

It is also interesting to examine the corresponding GOES-11 visible imagery (below) from the same time period as the GOES-12 imagery shown above. Why is the haziness over the Pacific Northwest region not as apparent? The answer to that question is: forward scattering. The forward scattering of light by the relatively small smoke particles increases as the angle between the sun, the smoke particles, and the GOES-12 (GOES-East) satellite approaches 180 degrees — this forward-scattered light makes the smoke appear “brighter” during late afternoon and early evening. The GOES-11 (GOES-West) satellite is positioned much farther to the west (at 135º W longitude, compared to 35º W longitude for GOES-12), so there is no forward scattering geometry to enhance the appearance of the airborne smoke over the Pacific Northwest region.

However, note that the hazy signature of the airborne smoke is evident on GOES-11 visible imagery from the next morning, when the sun was illuminating the smoke from the east (thereby creating a favorable forward scattering geometry with respect to the GOES-West satellite).

GOES-11 visible images (Animated GIF)