Archive for the ‘Passive Microwave’ Category

The SPoRT program has been collaborating with NOAA’s National Hurricane Center to transition passive microwave products to their operational system; the National Centers for Environmental Prediction Advanced Weather Information Processing system, or NAWIPS.  By viewing a storm in microwave wavelengths versus infrared, forecasters have the ability to observe storm structure that may be obscured by high clouds.  Many times, this ability is used to better determine the center fix on a tropical system.

One of the most recent missions to carry a passive microwave instrument is the joint NASA and Japan Aerospace Exploration Agency Global Precipitation Measurement (GPM) satellite.  The core observatory was launched on February 27, 2014 operational data from GPM’s microwave imager (GMI) was first available on May 29.  SPoRT has incorporated this into the data feed for the National Hurricane Center.  SPoRT is currently working to transition these observations to NAWIPS for the Central Pacific Hurrican Center in Honolulu, Hawaii.

The image below, taken by the National Hurricane Center in NAWIPS shows Hurricane Iselle in the Pacific Ocean several days before it struck the big island of Hawaii.  The image shows an RGB (red, green, blue) color composite of Hurricane Iselle from August 5, 2014 at 11:15 AM Eastern Daylight time.  The image is created by combining the horizontal and vertical polarization observations of the 89 GHz channel.  The resulting combination emphasizes strong convection / deep clouds in bright red.

Additional information on GPM can be found at: www.nasa.gov/gpm.

Hurricane Iselle - GMI

Hurricane Iselle as observed by the Global Precipitation Measurement Microwave Imager (GMI) with the 89 GHz RGB composite on August 5, 2014 at 11:15 AM Eastern Daylight Time.


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SPoRT continues to work with select NWS WFOs in evaluating the NESDIS SFR product.

One thing to take into consideration when using data from “whisk-broom” instruments on polar-orbiting satellites, such as the Advanced Microwave Sounding Unit (AMSU) used to generate the SFR product, is that data at the edge of the swath (i.e. limb) may provide misleading or erroneous observations. As the instrument scans farther from nadir, it is looking through more of the atmosphere, creating both a bigger observation field of view (i.e., larger pixel) and having the signal attenuated by more atmospheric constituents (e.g., in-cloud and falling snow).  As a result, when interpreting the SFR product, it is important to look for the extent of the swath (outlined in gray in the product in AWIPS) to determine whether the observed SFR is going to be limited by these limb effects.

Let’s take a look at an example over the NY Tri-State area for the post Super Bowl snow event.  In the first image, from Metop-A valid at 1458 UTC, there is a large area of snowfall across the area.  The heaviest SFRs appear to be around 1.2-1.5 in/hr (when multiplying the liquid equivalent by 10) across central and southern New Jersey.  However, an hour later (see second image from Metop-B valid at 1554 UTC), the shape of the heaviest SFR has expanded north and west and there are now readings over 2.0 +in/hr.  Other areas that had a SFR of less than 0.5 in/hr in the 1458 UTC image, appear to have a SFR of around 1.0 in/hr just an hour later, which is a large jump in intensity.

While it is certainly possible that the snow evolved and intensified in less than an hour, it is more likely that instrument limb effects are likely to blame for the larger SFRs in the second image.  Make sure to check that swath edge when using polar-orbiting satellite data!

NESDIS SFR Product from 1458 UTC on 3 February 2014 showing snow detected near nadir for Metop-A

NESDIS SFR Product from 1458 UTC on 3 February 2014 showing snow detected near nadir for Metop-A.

NESDIS SFR Product from 1554 UTC on 3 February 2014 depicting what are likely erroneous higher intensity SFR values along the swath edge from Metop-B.

NESDIS SFR Product from 1554 UTC on 3 February 2014 depicting what are likely erroneous higher intensity SFR values along the swath edge from Metop-B.

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SPoRT continues to work with select NWS WFOs in evaluating the NESDIS SFR product.

A rare winter storm impacted much of the deep South Tuesday morning and afternoon.  Areas of Central and Northeastern Alabama only received a couple of inches of snow, but this was enough to cause major headaches as roadways iced over resulting in highways across Alabama and Georgia being shut down, stranding thousands of motorists.  Most reports from late Tuesday morning indicated that the worse of the winter weather was falling south of Cullman, AL, through Birmingham, AL to Montgomery, AL and then eastward into areas like Fort Payne, AL.  I-65 north of Birmingham and I-20 east of Birmingham were particularly troublesome in the state of Alabama.

The AWIPS image below depicts the SFR product in AWIPS with overlaid interstate highways.  This image was taken from the AMSU on Metop-B at 1618 UTC (10:18 local Alabama time) right about the time when the heaviest snow was impacting the state.  The SFR Product indicates liquid water equivalent precipitation rates between 0.04 and 0.08 in/hr, which if you multiply by a factor of 10 to get the solid snowfall rate equates to around 0.4 and 0.8 in/hr.  Snowfall totals across this region were generally in the 1-3 inch range, so the rates that were detected by the product were consistent with what actually fell.

NESDIS SFR Product from 1618 UTC (around 10:00 A.M. Central) on 28 January 2014

NESDIS SFR Product from 1618 UTC (around 10:00 A.M. Central) on 28 January 2014

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On Jan 26 2014, an upper level shortwave caused an area of light snow across Ohio, western Pennsylvania and the northern counties of West Virginia. Surface temperatures were quite cold with readings generally in the teens. Even at these cold temperatures, the SFR product did indicate snowfall across the far northern counties of our forecast area.

The maximum snowfall rates indicated on the 1605 UTC product was about 0.3 to 0.4 inches per hour. Based on reports, these numbers appear to be representative of what actually was occurring.

While this is just one case, the SFR product appears to work reasonably well at temperatures below 22 DegF.

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On Jan 25 2014, a mid-level shortwave moved across the region generating light to moderate snow. I have included screen captures of the 1118 UTC regional radar mosaic and surface observations…along with a 1120 UTC Snowfall Rate Product and surface observations.

It looks like the SFR product did not detect all of the snow that was falling around 11 UTC. But the misses can generally be described as either (1) the surface temperatures being too cold or (2) the probabilistic model, that is part of the calulations, indicating probabilities that were too low to determine if there was snow.

Once you know all of the details on how the product is calculated, I think this product did a good job at detailing where the snowfall was occurring.

The highest snowfall rates indicated by this image was around 0.3 to 0.5 inches which seems to be representative of what was occurring.

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When I examined the 1522 UTC SFR product, I noticed there was an absence of snow across our forecast area. Radar and surface observations indicated that light to moderate snow was continuing across most of our counties.

Per the Quick Guide, I checked the surface observations to see if the temperatures were about 22 DegF or colder. The temperatures across our northern and western counties were actually 22 DegF or colder. So the SFR product was behaving as it should across those counties.

However, the temperatures across the remainder of our region were above 22 DegF. The snow is definitely not lake effect as the current snow was still related to a shortwave which had pushed to our east.

What could be causing the lack of indicated snow across the portions of our area that still had surface temnperatures above 22 DegF?

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I have attached a screen capture of the SFR product from 1024 UTC on 1/21/14.  The label on the image is wrong.  It states the units of the product are in/hr.  But they are actually mm/hr.

During this time, we were having widespread light to moderate snow as an upper level disturbance moved across our forecast area.  Reports around 2 inches of snow were common around the time of the product.   We had received reports of snow coming down around an inch per hour.  The maximum SFR detected in the product was 1.6 mm/hr…or 0.06 in/hr.  Using a ratio of 15:1 yields a maximum snowfall rate around 0.9 inches per hour.

While we had several surface observations from which we could estimate precipitation rates, our WSR-88D was not operating correctly.  The legacy precipitation were okay.  But the Dual Pol precipitation products were not totally reliable due to equipment issues.  So the additional information from the SFR product should have helped estimate the precipitation rates.


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