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)

Eruption of Popocatépetl in Mexico

January 9th, 2020 |

GOES-16 Low-, Mid- and Upper-level Water Vapor (7.3 µm, 6.9 µm and 6.2 µm), Split Window Difference (10.3-12.3 µm) and Cloud Top Height product [click to play animation | MP4]

GOES-16 Low-, Mid- and Upper-level Water Vapor (7.3 µm, 6.9 µm and 6.2 µm), Split Window Difference (10.3-12.3 µm) images [click to play animation | MP4]

Popocatépetl erupted at 1226 UTC on 09 January 2019 — GOES-16 (GOES-East) images of Low-level (7.3 µm), Mid-level (6.9 µm) and Upper-level Water Vapor (6.2 µm) and Split Window Difference (10.3-12.3 µm) (above) showed a higher-altitude ash plume moving rapidly south-southeastward, while ash at a lower altitude moved slowly north-northeastward.

The difference in speed and direction of ash transport was explained by plots of rawinsonde data from Mexico City and Acapulco at 12 UTC (below), which revealed stronger northwesterly winds within the 200-250 hPa pressure layer, with lighter southerly to southwesterly winds existing between 400 and 600 hPa.

Plots of rawinsonde data from Mexico City and Acapulco at 12 UTC [click to enlarge]

Plots of rawinsonde data from Mexico City (yellow) and Acapulco (cyan) at 12 UTC [click to enlarge]

At 1402 UTC a Mesoscale Domain Sector was positioned over Mexico — and 1-minute GOES-16 Ash RGB images created using Geo2Grid (below) tracked the distinct signature of the northern lower-altitude ash (brighter shades of pink to red) while the southern higher-altitude ash signature faded as it was more quickly dispersed by the stronger winds.

GOES-16 Ash RGB images {click to play animation | MP4]

GOES-16 Ash RGB images [click to play animation | MP4]

A GOES-16 Ash Height product from the NOAA/CIMSS Volcanic Cloud Monitoring site (below) indicated that the southern ash plume exhibited heights in the 6-8 km range, with similar heights seen for the slow-moving northern ash feature.

GOES-16 Ash Height product [click to play animation MP4]

GOES-16 Ash Height product [click to play animation MP4]