NUCAPS views Saharan Air over the Atlantic

September 20th, 2018 |

Saharan Air Layer Analysis over the Tropical Atlantic, 0600 UTC on 20 September 2018 (Click to enlarge)

There have been many episodes of Saharan Air over the tropical Atlantic within the past months, and another episode is in progress on 20 September. The Saharan Air Layer (SAL) analysis, above, from the CIMSS Tropical Weather website (Direct Link), shows dry air north and east of the Caribbean. The Clean Window ABI Band 13 (10.3 µm) Full-Disk ABI infrared imagery, below, from 0500 UTC, overlain with NUCAPS sounding points, shows where data were available from that morning overpass of Suomi NPP.

GOES-16 ABI 10.3 µm Infrared Imagery at 0500 UTC along with NUCAPS Sounding Points at approximately the same time (Click to enlarge)

The stepping animation below shows NUCAPS Soundings at a selection of points that starts north of the Saharan Air Layer and ends up within the SAL. The underlying figure is the Dust RGB from AWIPS, an RGB that combines the Split Window Difference (12.3 µm -10.3 µm; Red Component), Split Cloud Top Phase Brightness Temperature Difference (11.2 µm – 8.5 µm; Green Component) and 10.3 µm Infrared Imagery (Blue Component). Typically, regions with dust as might accompany a SAL have a pink tinge. The soundings are annotated to include Total Precipitable Water measurements, and mid-level Relative humidity. NUCAPS soundings identify the region where the SAL is present.

Dust RGB at 0433 UTC north and east of the Caribbean, and NUCAPS Soundings at selected points along a transect (Click to enlarge)

The SAL air continued its movement west during the day on 20 September.  The toggle below shows the Dust RGB, ABI Band 3 (0.86 µm) and the Baseline Aerosol Detection Product (in blue) at about the same time as the afternoon NUCAPS Sounding overpass (from Suomi NPP).  Suomi NPP overflew the eastern half of the SAL air (the overpass from NOAA-20 was more centered on the SAL air approaching the Caribbean, but NOAA-20 NUCAPS soundings are not yet in AWIPS;  they should be by the end of the year).

GOES-16 ABI Dust RGB, “Veggie Band” (Near-Infrared at 0.86 µm), and Baseline Aerosol Detection Product (Blue points), 1615 UTC on 20 September 2018 (Click to enlarge)

NUCAPS Soundings at 3 points (North of the SAL, within the SAL, and south of the SAL), below, show much different thermodynamics within the SAL.

NUCAPS Profiles at ~1600 UTC on 20 September 2018 at three locations as noted (Click to enlarge)

NOAA’s G-IV flew through this outbreak, deploying dropsondes to sample the event. The path of the aircraft (with the dropsonde locations) is here. Sonde #26, below, in the heart of the SAL, is shown below, with a nearby NOAA-20 NUCAPS sounding. (Flight path and Sonde imagery courtesy Chris Barnet, STC/NOAA) Refer to the caption for details.  Recall that the Dropsonde shows values at a point.  The NUCAPS profile is sampling a volume that is approximately a 50-km cylinder!  There is nevertheless excellent agreement.

Dropsonde #26 data (raw data in light grey; values averaged into the 100 NUCAPS vertical layers in black); GFS sounding in magenta. NUCAPS Microwave-only sounding in green; NUCAPS Microwave and infrared retrieval (as might be seen in AWIPS) in Red. Time offset from the Dropsonde is noted (Click to enlarge)

SAL outbreaks cause a significant deterioration in air quality over the Caribbean. The image below, courtesy Ernesto Rodriguez, SOO for the National Weather Service office in San Juan, Puerto Rico, compares Air Quality before and during a SAL outbreak in July, and during the current outbreak.

The view outside of the National Weather Service office in San Juan on 20 September and 13 July 2018 (during SAL outbreaks) and on 12 July 2018 (before a SAL outbreak). Imagery courtesy Ernesto Rodriguez, NWS SJU.

GOES-16 ABI Legacy Profiles and Suomi NPP NUCAPS Profiles in AWIPS

April 8th, 2018 |

NUCAPS Sounding Availability points plotted over a VIIRS Visible (0.64 µm) image at 1815 UTC, 8 April 2018 (click to enlarge); NUCAPS Soundings from the point nearest Miami and Key West are shown below

NUCAPS vertical profile near Key West FL, 18Z on 8 April 2018 (Click to enlarge)

NUCAPS vertical profile near Miami FL, 18Z on 8 April 2018 (Click to enlarge)

NUCAPS (NOAA-Unique Combined Atmospheric Processing System) vertical profiles have been available in AWIPS for some time now (Click here to see how to access them in AWIPS; they are also available online at this site). Legacy Atmospheric Profiles (LAP) derived from ABI Channels (and GFS Information) are available in AWIPS now as well (horizontal fields derived from LAP soundings are available online as well). How do Legacy Atmospheric Profiles compare to NUCAPS profiles? Both are derived from satellite data. (Click here for a Quick Guide on NUCAPS; Click here for a Quick Guide on Legacy Atmospheric Profiles).

The strength of NUCAPS Soundings is that they are observation-based and independent of a model first guess (or background field). That is, they are retrieved from satellite measurements of emitted radiation at hundreds of different wavelengths in the infrared and in the microwave, with a statistical regression as first guess. In the case of NUCAPS from Suomi NPP (or NOAA-20), the data are from CrIS and from ATMS, infrared and microwave sounders, respectively. Compare the vertical profiles above to the 12 UTC soundings from Miami and Key West. There is better vertical resolution in the radiosondes, of course, but NUCAPS provides timely model-independent information at times when convective initiation might be starting.

NUCAPS Soundings Availability (plotted on top of the VIIRS 11.45 µm infrared image from 1815 UTC), a subset of GOES-16 LAP Sounding Availability points (plotted on top of the GOES-16 0.64 µm Visible Image), and then all GOES-16 LAP Sounding Availability points (Click to enlarge)

NUCAPS Soundings are produced in clear and partly cloudy conditions. This owes to the 3×3 CrIS field of regard that is incorporated into each NUCAPS profile, and to the ability of microwave imagery to produce a sounding in cloudy (but not precipitating) regions.

In contrast, LAP temperature and moisture profiles are produced only where the GOES-16 Clear Sky Mask identifies clear skies. LAP output is on a 10-km grid, however, so there are many possible soundings. The image below, zoomed in over southern Florida, shows in cyan the availability of LAP Vertical Profiles, and the availability in NUCAPS Soundings, color-coded Green, Yellow and Red (the meaning of the points is described in the Quick Guide). There are many LAP points, but they do not exist anywhere where clouds are present. The LAP grid is the same from one time to the next however.

GOES-16 ABI Visible (0.64 µm) Imagery at 1752 UTC, LAP locations at 1752 UTC (cyan points) and NUCAPS Sounding availablilty points (Green, Yellow, Red points) at 1758 UTC (Click to enlarge)

Because LAP are produced every 30 minutes at the CONUS scale, at the same point each time, their evolution can tell you something. Note, however: these profiles are very heavily constrained by the GFS (Global Forecasting System) 1/2-degree simulation that is used to create LAP information. That is, they are not independent of the model (as is the case for NUCAPS Vertical Profiles). GOES-R ABI tells very little about the temperature structure of the atmosphere in particular because it lacks the spectral resolution of, for example, the GOES-15 Sounder that has multiple channels around 4.4 µm and multiple channels around 14 µm. The GOES-R Series of satellites does not include a hyperspectral sounder such as CrIS (on Suomi-NPP and NOAA-20) or IASI (on Metop-A and Metop-B).

GOES-R ABI does have similar moisture information as the GOES-15 Sounder — both have 3 separate water vapor channels making observations between 6 µm and 7.5 µm. Thus, the Legacy profiles might alter the moisture in the vertical profiles from the GFS, but it is far less likely that GOES-16 ABI will cause a notable change in temperature profiles from the GFS. In contrast, as noted above, NUCAPS Vertical Profiles are satellite observations only created via a regression and a retrieval that uses as a tool a Radiative Transfer Model.

Thus, when you see a time animation of a series of LAP soundings, as shown below, you are likely seeing the evolution of the GFS vertical profiles with a modest change in mid-level moisture occurring because of GOES-16 ABI data. Note also that soundings will not be produced when the clear-sky mask indicates clouds. Thus, the sounding near Key West shows hourly values from 09 to 21 UTC (with some gaps); the sounding near Miami for the same time-span shows hourly values only at 13, 15 and 16 UTC — because more clouds are present. Changes in the LAP sounding temperature are likely the result of GFS information changing; changes in LAP moisture are from both GFS moisture changing and/or GOES ABI water vapor channel information changing.

GOES-16 LAP Vertical Profiles of Temperature and Moisture at from 09-21 UTC for a point near Key West (Click to enlarge)

GOES-16 LAP Vertical Profiles of Temperature and Moisture at 13, 15 and 16 UTC for a point near Miami (Click to enlarge)

LAP data are used to create Derived Stability Indices (CAPE, Total Totals, Lifted Index, K-index, Showalter Index) and Total Precipitable Water. These integrated quantities, also available in AWIPS, are likely to be more useful to forecasters than point data. This is especially true because the most reliable information from the LAP Soundings and the derived stability indices are gradients and time tendencies. (Here is an animation of Lifted Index from 1332 – 2147 UTC on 8 April 2018)

In 2019, AWIPS will included gridded horizontal fields derived from NUCAPS temperature and moisture profiles. This will allow visualization of convective parameters such as Lifted Index and CAPE. In addition, NUCAPS soundings from NOAA-20 and from Metop-A and Metop-B will flow to AWIPS at some point after early Summer 2018, greatly increasing the number of observation-based soundings available.

Sequential NUCAPS Profiles at Higher Latitudes

October 20th, 2016 |
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Suomi NPP Day/Night Band (0.70 µm) and NUCAPS Sounding Locations, 0538 and 0724 UTC on 20 October 2016. Green Dots represent soundings that have passed quality control; Yellow Dots represent soundings for which the infrared retrieval failed; Red dots represent soundings for which both infrared and microwave retrievals failed (Click to enlarge)

The orbital geometry of Suomi NPP is such that regions north of about 43º N latitude can occasionally receive NUCAPS (NOAA-Unique Combined Atmospheric Processing System) Vertical Profiles of moisture and temperature on sequential orbital passes, meaning a given location could have vertical profiles separated by less than 2 hours. This occurred early on 20 October 2016 over Maine and Cape Cod, as shown above: Suomi NPP NUCAPS Vertical Profile locations are indicated over Day/Night Band Visible imagery. Two soundings at approximately the same location are circled in cyan in this small image and are shown below. There are two sequential profiles over Cape Cod, and then the two sequential profiles north of Maine. The atmosphere over Cape Cod was quiescent on this date, and little change between soundings is evident. In contrast, slight cold air advection was occurring north of Maine (Surface analysis from 0900 UTC, 500-mb analysis from 00 UTC), and the NUCAPS Sounding shows mid-level cooling.

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NSharp depictions of NUCAPS Vertical Profiles near 42N, 70W at 0500 and 0700 UTC on 20 October 2016 (Click to enlarge)

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NSharp Depictions of NUCAPS Vertical Profiles near 48 N, 68 W at 0500 and 0700 UTC on 20 October 2016 (Click to enlarge)

For stations in the northern Plains, or in Canada, sequential soundings overnight or perhaps more importantly in the mid-afternoon (Suomi NPP typically overflies the Plains a bit after Noon local time) could give important information about destabilization.

Previous CIMSS Satellite Blog Entries referencing NUCAPS Vertical Profiles are available here.


=============== Added 2100 UTC on 20 October 2016 ===============
The toggle below shows two soundings, from 1700 and 1800 UTC in central Pennsylvania in the region between Harrisburg and Williamsport (click here to see the Sounding Locations), just east of a slight risk issued by the Storm Prediction Center. The time evolution suggests upward motion (the top of the inversion rises) and a weakening in the cap. Severe Thunderstorm Watch #499 was issued 1945 UTC on 20 October for counties just to the west of these NUCAPS Profile locations — and damaging winds were reported in central Pennsylvania beginning at 2154 UTC.

NSharp depictions of NUCAPS Vertical Profiles near 41N, 77W at 1700 and 1800 UTC on 20 October 2016 (Click to enlarge)

Displaying NUCAPS data from CLASS

November 12th, 2014 |

NUCAPS data have been flowing into AWIPS 2 for months; in the recent past, these data started flowing into the NOAA CLASS data archive as well (click here for a tutorial on accessing the data). How can the NOAA CLASS output be displayed? This post will compare McIDAS-V plots to the data displayed using AWIPS-1, below.

GOES Sounder Total Column Ozone DPI Values Plotted with NAM 500-mb heights and NAM Pressure on the 1.5 PVU surface (click to enlarge)

GOES Sounder Total Column Ozone DPI Values Plotted with NAM 500-mb heights and NAM Pressure on the 1.5 PVU surface (click to enlarge)

Suomi NPP overflew the central United States at about 0850 UTC on 12 November, and ozone concentrations from the NUCAPS soundings at three different levels (~500, 300 and ~200 mb) are shown below. Note that the color scaling is not quite the same in the three plots as the range for each pressure level is different. Maxima in Ozone at all levels occur in the same region — the Dakotas — as indicated by the GOES Sounder Total Column Ozone DPI, above. NUCAPS soundings also show data in cloudy regions because microwave data from ATMS is used in the NUCAPS processing. Note that values at the edge of the color shading have been extrapolated outwards; values in western Nevada and Indiana, for example, are not from direct NUCAPS observations. This plot of 500-mb temperatures (that includes the actual values) shows the horizontal extent of data and the amount of interpolation at the edge.

Contours of Ozone Mixing Ratio (parts per billion) from NUCAPS Soundings at ~0848 UTC on 12 November 2014 (click to enlarge)

Contours of Ozone Mixing Ratio (parts per billion) from NUCAPS Soundings at ~0848 UTC on 12 November 2014 (click to enlarge)