Can you use NUCAPS soundings to determine the rain/snow line?

January 9th, 2020 |

NUCAPS Horizontal Temperature field, 925 hPa, at 1704 UTC on 9 January 2020, toggled with NUCAPS sounding observation points from the same orbit (Click to enlarge)

NOAA-Unique Combined Atmospheric Processing System vertical profiles of moisture and temperature are derived from retrievals that consider both infrared sounder data (from the Cross-track Infrared Sounder, CrIS) and microwave sounder data (from the Advanced Technology Microwave Sounder, ATMS). In AWIPS, the profiles are from NOAA-20 but they are also produced with data from Suomi-NPP. NUCAPS profiles from NOAA-20 and Suomi-NPP are available here (the site also includes NUCAPS profiles from MetOp satellites; those NUCAPS profiles use IASI infrared and MHS microwave data).

Polar2Grid software can be used to create horizontal fields of thermodynamic information from the vertical profiles (as discussed here). For the National Weather Service forecast offices, an extra step is taken that interpolates (in the vertical) the NUCAPS data from the pressure levels in the Radiative Transfer Model that is used in the retrievals to standard pressure levels. The toggle above compares the vertical profile points at 1648 UTC on 9 January 2020 to the  925-hPa temperature field. Note that derived field does extend outwards from the outermost NUCAPS profile: the sphere of influence for an individual NUCAPS point can be adjusted.  Note that the bounds of the temperature field have been adjusted from AWIPS defaults, and the color table has been modified so that 0º C occurs between the green and cyan values.  (A more intuitive color table for rain-snow discernment would include more color gradations in the -5º C to +5º C range).  Where the low-level thermal gradient occurs should help a forecaster determine where rain is more likely and where snow is more likely.

Dewpoint Depression at 925 hPa, 1704 UTC on 9 January 2020 (Click to enlarge)

Moisture fields are available as well at thermal fields.  Thus, the effects of evaporation might be considered.  The image above shows the dewpoint depression at 925 hPa.  Lapse rates derived from NUCAPS are also available (the one below shows the temperature change from 850 to 500 hPa). If strong vertical motion is forecast, the lapse rate and/or the dewpoint depressions fields can help you anticipate how much cooling might occur.

850-500 mb Lapse Rate, 1704 UTC on 9 January 2020 (Click to enlarge)

Note that horizontal fields as presented in NUCAPS include data from all NUCAPS profiles, thereby including points that may be ‘green’ (the infrared and microwave retrievals both converge to solutions), ‘yellow’ (the infrared retrieval failed, but the microwave retrieval converged) and ‘red’ (neither retrieval converged). It’s incumbent on the analyst to consider the impact of those profiles where convergence to a solution did not occur when using these fields.

(Thanks to Christopher Stumpf, WFO MKX, for assistance in getting these images)

Using NUCAPS soundings to nowcast convective evolution

August 15th, 2019 |

GOES-16 Visible (Band 2, 0.64 µm) Imagery, 1721 – 1946 UTC on 15 August 2019. NUCAPS Sounding Points — from 1926 UTC — are present over the image at 1946 UTC (Click to animate)

GOES-16 Visible Imagery, above (Click to animate), shows shower/thundershower development over eastern Oklahoma moving into Arkansas. At the end of the animation, 1946 UTC, NUCAPS Sounding profiles from 1926 UTC are shown, and they’re shown below too.

GOES-16 Visible (Band 2, 0.64 µm) Imagery, 1946 UTC on 15 August 2019. (Click to enlarge)

The time 1946 UTC is about the earliest you could hope to have NUCAPS profiles in an AWIPS system — and only if you had access to a Direct Broadcast antenna. The more conventional method of data delivery, the SBN, means NUCAPS will be available about an hour after they are taken, so by 2036 UTC. The visible imagery at 2036 UTC is shown below.

GOES-16 Visible (Band 2, 0.64 µm) Imagery, 1946 UTC on 15 August 2019. (Click to enlarge)

At 2036 UTC, which time is about when in the forecast office the NUCAPS soundings would become available, would you expect the convection in western Arkansas to move southward, or eastward, based solely on Satellite imagery? How could you use NUCAPS profiles to gain confidence in this prediction? Visible imagery alone suggests a moisture boundary; the southern quarter of Arkansas shows markedly less cumulus cloudiness. The animation shows motion mostly to the east, with higher clouds moving more west-northwesterly. The GOES-16 Baseline Total Precipitable Water product, below, shows a maximum in TPW over central Arkansas, with values around 1.5″;  values are around 1.3″ in southern Arkansas, and around 1.2-1.3″ in northwest Arkansas.  A corridor of moisture is indicated.

GOES-16 Baseline Level 2 Total Precipitable Water at 1946 UTC; Visible imagery is shown in cloudy regions. (Click to enlarge)

Baseline Total Precipitable Water, above, part of a suite of products that emerge from Legacy Profiles, is heavily constrained by model fields, however;  the image above could simply show the GFS solution.  In contrast, NUCAPS observations are almost wholly independent of models.  What do NUCAPS profiles show? The animation below steps through vertical profiles east and south of the developing convection.

NUCAPS profiles from the ~1900 UTC overpass at points plotted over the 1946 UTC GOES-16 Band 2 Visible (0.64 µm) image (Click to enlarge)

AWIPS will soon (planned for shortly after Labor Day at the time of this post) include horizontal fields of information derived from NUCAPS vertical profiles. The images below show values computed within the NSharp AWIPS software for a variety of fields: Total Precipitable Water, MU Lifted Index, MU CAPE, MU CINH. All fields suggest that convection more likely to build eastward than to expand southward.

NUCAPS Sounding Points and derived quantities, as indicated, at 1926 UTC 15 August 2019; NUCAPS data are plotted over the 1946 UTC GOES-16 ABI Band 2 Visible 0.64 µm image. (Click to enlarge)

Convection did not move southward; motion and development was to the east. The timing of NUCAPS profiles means that they give a good estimate of atmospheric thermodynamics in mid-afternoon, a key time for assessing convective development.

GOES-16 Visible (Band 2, 0.64 µm) Imagery, 1721 UTC on 15 August 2019 to 0001 UTC on 16 August 2019 (Click to animate).

NUCAPS Soundings surrounding an isolated Thundershower

August 14th, 2019 |

GOES-16 ABI Band 2 (0.64 µm) at 1946 UTC on 14 August 2019 (Click to enlarge)

The GOES-16 Visible (0.64 µm) image above shows a weak thunderstorm over southeastern Oklahoma surrounding an decaying outflow boundary.  (Click here to see an animation of the visible imagery). The convection did not look particularly robust, but it did produce lightning that was detected by the Geostationary Lightning Mapper (GLM), as shown below.

GOES-16 ABI Band 2 (0.64 µm) and GLM observations of Flash Extent Density at 1946 UTC on 14 August 2019

Lightning requires charge separation in a cloud; typically lightning occurs after the cloud top glaciates. During daytime, glaciation can be detected with ABI Band 5, at 1.61 µm, the so-called Snow/Ice band. The toggle below shows the visible, snow/ice band, and the Baseline Cloud Phase product. Glaciation is indicated.

GOES-16 ABI Band 2 (0.64 µm), Band 5 (1.61 µm) and Baseline Cloud Phase at 1946 UTC on 14 August 2019

This case is interesting because NOAA-20 overflew the convection, and soundings were produced around the convection, as shown below.

GOES-16 ABI Band 2 (0.64 µm) at 1946 UTC on 14 August 2019 along with NUCAPS Sounding Points at 1945 UTC

The animation below steps north-south through seven profiles that surround the weak convection. Note that a profile near the convection has thermodynamic parameters more favorable for convection than at the other profiles.  For example, NUCAPS profiles show the convection at the northern edge of a precipitable water gradient, and also in a local minimum of inhibition.    Although the convection has initiated here, the fields do suggest that NUCAPS can be used to monitor thermodynamics at small scales before initiation.

NUCAPS Soundings at various points north, south and within convection at 1946 UTC on 14 August 2019 (Click to enlarge) Thermodynamic variables from the sounding are noted.

Horizontal gridded information derived from NUCAPS data will be in AWIPS shortly.  See this post from Emily Berndt at SPoRT!

NUCAPS Soundings and GOES-16 Derived Stability Index Lifted Index Comparison

March 6th, 2019 |

GOES-16 ABI Band 2 (0.64 µm) and Clear-Sky Lifted Index at 1730 UTC on 6 March 2019 (Click to enlarge)

The toggle above compares a CONUS Sector visible image (0.64 µm) over the Caribbean (at 1732 UTC) with a 1730 UTC Full Disk Legacy Atmospheric Profile (LAP) Derived Stability Lifted Index field.  (A “Veggie” Band 03 0.86 µm image, here, means that coastlines need not be drawn in because of the outstanding land/sea contrast at the wavelength).  The LAP Derived Stability Lifted Index shows modestly stable air southwest of the island of Jamaica;  the blue enhancement suggests positive lifted indices (stable air) vs. the yellow regions north and south where values range from -1 to -2.  Visible imagery over the stable region does show fewer clouds than to the north and south.  Does that help you believe the small variations in stability?

GOES-16 Legacy Atmospheric Profile Derived Lifted Index at 1730 UTC along with NUCAPS Sounding Locations, ~1730 UTC on 6 March 2019 (Click to enlarge)

Suomi NPP Overflew the region shortly before 1800 UTC, and NUCAPS soundings that are produced using data from the CrIS (Cross-Track Infared Sounder) and ATMS (Advanced Technology Microwave Sounder) on Suomi NPP are available at the points shown above. Points indicated in green show where both the Infrared and Microwave retrievals successfully completed, and the animation below shows NUCAPS Soundings (at 18.66ºN, 18.20ºN, 17.74ºN, 17.28ºN, 16.82ºN and 16.36ºN) that bisect the region diagnosed as stable by the LAP Lifted Index.  Note that the NUCAPS sounding with the smallest MU Parcel CAPE, at 17.28ºN, is in the middle of the GOES-16-diagnosed stable region.

NUCAPS Soundings over the Caribbean, location as indicated by the large Purple dot. Most Unstable Parcel CAPE is noted (and also available in the nSharp readout)