Signatures of the Alaska Range in GOES-17 Water Vapor imagery

February 28th, 2019 |

GOES-17 Low-level (7.3 µm), Mid-level (6.9 µm) and Upper-level (6.2 µm) Water Vapor images, with topography [click to play animation | MP4]

GOES-17 Low-level (7.3 µm), Mid-level (6.9 µm) and Upper-level (6.2 µm) Water Vapor images, with topography [click to play animation | MP4]

GOES-17 (GOES-West) Low-level (7.3 µm), Mid-level (6.9 µm) and Upper-level (6.2 µm) Water Vapor images (above) displayed subtle thermal signatures of some of the highest-elevation western and central portions of the Alaska Range on 28 February 2019.

Plots of GOES-17 Water Vapor weighting functions, calculated using 12 UTC rawinsonde data from Anchorage, are shown below. Even with a very large satellite viewing angle (or zenith angle) of 70.1 degrees — which would tend to shift the Water Vapor weighting functions to higher altitudes —  the presence of dry air within the entire mid-upper troposphere brought the weighting function peaks downward to pressure levels corresponding to those of the higher elevations of the Alaska Range.

GOES-17 Water Vapor weighting functions, calculated using 12 UTC rawinsonde data from Anchorage [click to enlarge]

GOES-17 Water Vapor weighting functions, calculated using 12 UTC rawinsonde data from Anchorage [click to enlarge]

The dry air aloft helped to provide a remarkably cloud-free day over much of south-central Alaska, as seen in GOES-17 “Red” Visible (0.64 µm) images (below). In addition, an example of the transient spikes in daytime solar reflectance seen during this time of year was evident in the Visible imagery — note the brief brightening of a few of the images centered at 2115 UTC. Additional details about this effect are available here.

GOES-17 "Red" Visible (0.64 µm) images [click to play animation | MP4]

GOES-17 “Red” Visible (0.64 µm) images [click to play animation | MP4]

Just for fun, a closer look at the GOES-17 Visible imagery (below) revealed the tidal ebb and flow of drift ice within Cook Inlet and Turnagain Arm in the Anchorage area — and a similar diurnal flow of ice was also seen on the following day (animated GIF | MP4).

GOES-17 "Red" Visible (0.64 µm) images [click to play animation | MP4]

GOES-17 “Red” Visible (0.64 µm) images [click to play animation | MP4]

===== 01 March Update =====

GOES-17 Low-level (7.3 µm), Mid-level (6.9 µm) and Upper-level (6.2 µm) Water Vapor images, with topography [click to play animation | MP4]

GOES-17 Low-level (7.3 µm), Mid-level (6.9 µm) and Upper-level (6.2 µm) Water Vapor images, with topography [click to play animation | MP4]

With a similar scenario to the previous day (but farther to the east), dry air aloft allowed thermal signatures of the highest summits of the eastern Alaska Range and the Wrangell Mountains to be apparent in GOES-17 Water Vapor imagery — even the Upper-level 6.2 µm band (above).

However, in this case the Water Vapor weighting functions derived using rawinsonde data from nearby Anchorage were not representative of the pocket of dry air farther east over the mountains. An increase in the moisture profile — especially within the mid/upper troposphere — shifted the weighting functions to higher altitudes, due absorption and re-emission by that higher-altitude (and colder) moisture. Note how the weighting function contributions centered around the 300 hPa pressure level increased during the 24 hours from 12 UTC on 28 February to 12 UTC on 01 March, while the relative contributions decreased within the 500-700 layer (below).

GOES-17 Water Vapor weighting functions, calculated using rawinsonde data from Anchorage on 28 February (12 UTC) and 01 March (00 and 12 UTC) [click to enllarge]

GOES-17 Water Vapor weighting functions, calculated using rawinsonde data from Anchorage on 28 February (12 UTC) and 01 March (00 and 12 UTC) [click to enllarge]

An overpass of the Suomi NPP satellite provided a number of NUCAPS sounding profiles across this region (below).

GOES-17 Upper-level Water Vapor (6.2 µm) image at 2045 UTC, with NUCAPS sounding points at 2049 UTC plotted in green [click to enlarge]

GOES-17 Upper-level Water Vapor (6.2 µm) image at 2045 UTC, with NUCAPS sounding points plotted in green [click to enlarge]

A comparison of a dry NUCAPS sounding over the mountains (Point D) with a moist sounding near Anchorage (Point M) is shown below. As shown here, the moisture profile of the NUCAPS Point M sounding was similar to that of the 12 UTC Anchorage sounding.

Comparison of a dry NUCAPS sounding over the mountains (Point D) with a moist sounding near Anchorage (Point M) [click to enlarge]

Comparison of a dry NUCAPS sounding over the mountains (Point D) with a moist NUCAPS sounding near Anchorage (Point M) [click to enlarge]

GOES-17 Default Mesoscale Domain Sector #2 is changing

February 28th, 2019 |

Default Mesoscale 2 Sector location starting 1500 UTC on 5 March 2019 (Click to enlarge)

The National Weather Service (NWS) and NESDIS are implementing a change to the default Mesoscale Domain Sector (MDS) locations for GOES-17. On March 5, at 1500 UTC, MDS #2 will have its default location changed to Alaska, to better serve the Alaska region of the National Weather Service. The image above shows the approximate location of the new default, centered at 56º N, 150º W (versus 35.5º N, 101.5º W). (Link). 

The default location before the switch for MDS #2 is shown in this image from AWIPS.  The toggle below, using SIFT, shows the approximate locations of current and future  default MDS #2.

Mesoscale sectors now and in the near future can be viewed at this site.

GOES-17 Default Mesoscale Sector #2 Locations from before 5 March 2019, 1500 UTC (over the central USA) and after 5 March 2019, 1500 UTC (over Alaska), shown using GOES17 ABI Band 13 10.3 µm Clean Window Infrared Imagery (Click to enlarge)

Standing wave west of Tropical Cyclone Pola in the South Pacific

February 26th, 2019 |

Himawari-8 Low-level (7.3 µm), Mid-level (6.9 µm) and Upper-level (6.2 µm) Water Vapor images [click to play animation | MP4]

Himawari-8 Low-level (7.3 µm), Mid-level (6.9 µm) and Upper-level (6.2 µm) Water Vapor images [click to play animation | MP4]

Himawari-8 Low-level (7.3 µm), Mid-level (6.9 µm) and Upper-level (6.2 µm) Water Vapor images (above) revealed an interesting standing wave west/northwest of Tropical Cyclone Pola in the South Pacific Ocean on 26 February 2019. The long-lived wave first became apparent just before 0800 UTC, and persisted until about 2330 UTC.

The standing wave feature was also apparent in Himawari-8 “Clean” Infrared Window (10.4 µm) images (below). The abrupt warming of cloud-top infrared brightness temperatures associated with the wave suggests that subsidence was lowering the cloud height. Also note the very cold cloud-top temperatures of -90ºC and colder (yellow pixels embedded within the darker purple enhancement) — this was colder than the tropopause temperature on 12 UTC rawinsonde data from both Nadi, Fiji (NFFN) to the southwest and Pago Pago, American Samoa (NSTU) to the northeast (the wave feature was located closer to the Nadi sounding).

Himawari-8 "Clean" Infrared Window (10.4 µm) images [click to play animation | MP4]

Himawari-8 “Clean” Infrared Window (10.4 µm) images [click to play animation | MP4]

Consecutive VIIRS Infrared Window (11.45 µm) images from Suomi NPP and NOAA-20, as viewed using RealEarth (below) showed a definitive bore-like structure with the wave, especially along the northern end.

VIIRS Infrared Window (11.45 µm) images from NOAA-20 and Suomi NPP [click to enlarge]

VIIRS Infrared Window (11.45 µm) images from NOAA-20 and Suomi NPP [click to enlarge]

Himawari-8 “Red” Visible (0.64 µm) images (below) showed the feature during daylight hours — a distinct shadow was being cast during local sunrise, which indicated a sharp drop-off in cloud height from east to west along the wave.

Himawari-8 "Red" Visible (0.64 µm) images [click to play animation | MP4]

Himawari-8 “Red” Visible (0.64 µm) images [click to play animation | MP4]

A HWRF-P model sounding for the latitude/longitude point 15.42ºS/179.75ºW valid at 18 UTC (source) showed directional wind shear at the 450 hPa pressure level — such a wind shear could have acted to initiate a horizontal roll circulation, creating a narrow zone of cloud-eroding subsidence. In addition, a sharp change in wind direction was seen above 150 hPa on the Paga Pago sounding — and the Nadi sounding showed speed shear with height — which also could have induced a horizontal roll circulation within the upper troposphere.

HWRF-P model sounding for the location 15.42ºS 179.75ºW at 18 UTC [click to enlarge]

HWRF-P model sounding for the location 15.42ºS/179.75ºW at 18 UTC [click to enlarge]

An interesting phenomenon was the apparent “shedding” of high-altitude cloud material from the higher/colder cloud canopy of Pola immediately east of the wave feature, as seen in Himawari-8 Shortwave Infrared (3.9 µm) images (below). The westward direction and velocity of this cloud material motion had good agreement with GFS model winds at 150 hPa. Note that this shed cloud material appeared warmer (darker gray) in the 3.9 µm imagery — the shearing of cirrus cloud may have acted to fracture the ice crystals, making them smaller in size and therefore more efficient reflectors of incoming solar radiation.

Himawari-8 Shortwave Infrared (3.9 µm) images, with plots of GFS 150 hPa winds [click to play animation | MP4]

Himawari-8 Shortwave Infrared (3.9 µm) images, with plots of GFS 150 hPa winds [click to play animation | MP4]

A toggle between GOES-17 (GOES-West) Infrared and Water Vapor images from the CIMSS Tropical Cyclones site (below) showed that the feature was aligned with a couplet of low-level convergence and upper-level divergence at 15 UTC — such an environment could also support a vertically-propagating gravity wave.

GOES-17 Infrared and Water Vapor images, with contours of low-level convergence and upper-level divergence at 15 UTC [click to enlarge]

GOES-17 Infrared and Water Vapor images, with contours of low-level convergence and upper-level divergence at 15 UTC [click to enlarge]

Another analysis of this feature is available from the Australian Bureau of Meteorology Training Centre.

Super Typhoon Wutip

February 25th, 2019 |

Himawari-8

Himawari-8 “Clean” Infrared Window (10.4 µm) images [click to play MP4 animation]

After previously reaching Category 4 intensity on 23 February, Super Typhoon Wutip underwent an eyewall replacement cycle (MIMIC-TC) and emerged to reach Category 5 intensity at 06 UTC on 25 February 2019 (ADT | SATCON) — becoming the strongest (and only Category 5) February tropical cyclone on record for the Northwest Pacific basin (and also for the Northern Hemisphere). Rapid scan (2.5-minute) Himawari-8 “Clean” Infrared Window (10.4 µm) images (above) displayed a well-defined eye with an annular to axisymmetric eyewall structure; mesovortices could also be seen circulating within the eye. Of particular interest were the series of gravity waves propagating radially outward from the eye during the first few hours of the animation.

In addition, note the arc of cooling cloud tops south of the eye beginning around 1530 UTC. A comparison of Himawari-8 Infrared and Infrared-Water Vapor brightness temperature difference (BTD) images from the CIMSS Tropical Cyclones site (below) revealed increasing BTD values within that arc of colder clouds — an indication of convective overshooting tops that were likely penetrating into the stratosphere.

Himawari-8 Infrared and Infrared-Water Vapor brightness temperature difference (BTD) images [click to enlarge]

Himawari-8 Infrared and Infrared-Water Vapor brightness temperature difference (BTD) images [click to enlarge]

Himawari-8 “Red” Visible (0.64 µm) images (below) provided a clearer view of the mesovortices within the eye.

Himawari-8 "Red" Visible (0.64 µm) images [click to play MP4 animation]

Himawari-8 “Red” Visible (0.64 µm) images [click to play MP4 animation]

Satellite-derived deep-layer wind shear in the vicinity of Wutip was very light — in the range of 5-10 knots — surrounding the time period when Wutip peaked at Category 5 intensity at 06 UTC (below).

Himawari-8 Water Vapor (6.9 µm) images, with contours of deep-layer wind shear [click to enlarge]

Storm-centered Himawari-8 Water Vapor (6.9 µm) images, with contours of deep-layer wind shear [click to enlarge]

Wutip continued to exhibit a well-defined poleward outflow channel (below), although mid-upper level outflow was good in all quadrants of the storm (which aided the intensification process).

Storm-centered Himawari-8 Water Vapor (6.9 µm) images, with plots of satellite-derived winds [click to enlarge]

Storm-centered Himawari-8 Water Vapor (6.9 µm) images, with plots of satellite-derived winds [click to enlarge]

Although Ocean Heat Content was modest, Sea Surface Temperature values around 28ºC were favorable (below).

Ocean Heat Content and Sea Surface Temperature [click to enlarge]

Ocean Heat Content and Sea Surface Temperature [click to enlarge]