Category Archives: Radar

Radar Struggles with Low Density Snow

The total snowfall through 7 am this morning on the Alta-Collins stake sits at 6".  With a snow water content of 0.36", that makes for a water content of 6%.  Perhaps not quite cold smoke, but close to it. 

Alta continues to see some light snow.  This stuff is barely discernible in the radar loop where all one sees are some stronger returns over the high terrain that are ground clutter and some very light returns in a couple of bands that move from east to west across the Wasatch Range, consistent with precipitation in the "wrap around" part of the storm. 

It's very difficult to determine precipitation rates at Alta and in other parts of the Wasatch Range during such conditions using radar.  There are a couple of reasons for this.  First, the precipitation is quite shallow and often the radar beam partially or totally overshoots the precipitation. 

Source: Secrets of the Greatest Snow on Earth
Second, snow scatters a lot less radar energy back to the radar compared to rain.  This is especially true for low density dendritic snow.  As a result, returns are limited and the correlation between the radar reflectivity and snowfall rate can be poor.

Ski Tourers: Note that the Supreme Area at Alta is now closed to uphill traffic.

A Great Month of Outreach, Education, and Radar Meteorology with DOW7

Professor Powder attempts to keep DOW7 from leaving
After more than a month in Salt Lake City, DOW7 departed the University of Utah campus and began its trip home to Boulder this morning.

It had a great run in northern Utah, despite an uncooperative Mother Nature who was quite stingy providing storms.  We exhibited the DOW for 1500 visitors at the Natural History Museum of Utah and 300 at the University of Utah.

Meteorological outreach at the Natural History Museum of Utah.  DOW in the background. 
In the DOW during sidewalk exhibit at the University of Utah

Several graduate students are now fully trained DOW operators.  Students in our cloud microphysics, synoptic meteorology, mountain meteorology and a radar special topics class were able to participate in operations either on campus or in the field.  My mountain meteorology class is presenting results this afternoon from their initial analysis of a precipitation event in the Ogden Valley.

Seven people stuffed in the DOW.  A common scene during field operations. 
Officially, we did eight "intensive observing periods", or IOPs:

IOP0- Practice IOP scanning some weak snow showers over the northern Wasatch (Location: Antelope Island Marina)

IOP1- Leeside precipitation in the Ogden Valley (Location: Huntsville)

IOP2- Frontal precipitation over the Salt Lake Valley and mountain-induced precipitation over the northern Wasatch (Location: Fielding Garr Ranch, Antelope Island)

IOP3- Exploratory effort to examine precipitation over Ben Lomond (Location: Just south of Willard Bay)

IOP4- Exploratory effort for eared grebe migration (Location: Lakepoint)

DOW near Lakepoint in the Tooele Valley scanning for eared grebes on November 25th.  National Weather Service radar imagery showed the first signs of migration last night, so the truck returned to Boulder just a little too soon. 
IOP5- Cold front with topographic interactions over Tooele Valley (Location: Stansbury Island Causeway)

IOP6- Cold front, influence of Oquirrh and Wasatch range on precipitation, small-scale precipitation structure in and around Cottonwoods (Location: South Jordan Trax Station)

DOW at the South Jordan Trax Station with frontal/orographic cloud over Wasatch
IOP7- Mountain and lake-effect precipitation (Location: Baccus Highway near 7000 South)

Mother Nature's stinginess forced us to take what we could get and do a couple of all-night operations.  IOP1 and IOP2 covered the same storm.  We just moved the DOW from Huntsville to Antelope Island as the storm slid south, changing the IOP number.  IOP6 and IOP7 were also the same storm and we just moved the radar from the South Jordan Trax Station to the Baccus Highway as the winds veered and orographic and lake-effect precipitation evolved.  Knowledge of meteorology, terrain, and potential site characteristics are a real key to making such efforts successful.  Not to mention some motivated graduate students willing to work graveyard shifts.  During such operations, we rotate crews and bring in a fresh driver for moving the DOW in the morning.

Special thanks goes to our sponsor, the National Science Foundation, and the operators of the DOW, the Center for Severe Weather Research, for making the visit possible.  The Center for Severe Weather Research extended the DOW visit a few days to let us capture our most recent storm and for that we are grateful.  I'm fairly certain that storm will make it into at least one master's thesis and maybe more.

Yesterday’s Storm Chasing Bounty

Yesterday's frontal passage was a bummer for skiers, providing little in the way of the white stuff in the Cottonwood Canyons, but provided us with plenty of excitement thanks to the Doppler on Wheels.

We deployed that morning to a rattlesnake speedway in the Utah desert where a dark cloud rose from the desert floor and we headed straight into the storm.  If you have no idea what that means, watch the video below. 

More accurately, we set up along the side of the causeway to Stansbury Island, just north of the Tooele Valley.  When I drove out to meet the team, the surface front had already pushed into the northern Tooele Valley, with low level "fractus clouds" seen in the photo below at levels just abouve the ground, near its leading nose.  At this time, the front was quite shallow.  

The rattlesnake speedway in the Utah desert was actually the Stansbury Island Causeway, ideal for surveying the frontal structure over the Tooele Valley and precipitation processes over the Oquirrh Mountains.  We could also can over the Stansbury Mountains (background below). 

The shallow nature of the front was very apparent in the radar data we collected.  A Doppler radar is capable of measuring how fast scatterers in the atmosphere, in this case snow and rain, are moving toward or away from the radar.  This allows us to use radar scans, known as PPIs, which are oriented at a slight angle to the horizon, to infer changes in the wind across the area and in the vertical.  In the plot below, cool colors represent flow toward the radar, warm away, with the radar in the middle of the image.  There is a clear indication of flow from the northwest near the radar and south-southwest at ranges more removed from the radar site.  Since the radar scan is tilted at a slight angle to the horizon, this is an indication of strong vertical wind shear in the frontal zone not far above the Earth's surface.  

We can also configure the DOW to scan in vertical slices, known as RHIs.  The RHI below is oriented to the east and scans over the southern Great Salt Lake, eventually hitting the lower slope of the Oquirrh Mountains near Point of the Mountain where they rise above the south lake shore.  Doppler velocities in this image are primarily away from the radar, consistent with northwesterly flow, except near the ground just to the west of Point of the Mountain.  This reflects the splitting of flow around the north end of the Oquirrh Mountains, with the flow there having perhaps a slight NNE component, which results in a weak flow component toward the radar (green).  

The DOW is also a polarimetric radar, which means it sends out and receives radar energy in two planes, one horizontal and one vertical.  The shape of the raindrops or snowflakes can be inferred using this information.  One product we use to do this is known as "differential reflectivity," with reflectivity the amount of radar energy is scattered by back to the radar.  If the horizontal and vertical radar energy is similar, the differential reflectivity is zero, and the precipitation is likely circular.  If on the other hand, the horizontal radar energy is larger, then the precipitation is wider than it is high, and the differential reflectivity is positive.  Dendritic snow, those wonderful flakes with six arms that produce blower powder, often produces high differential reflectivity, because the flakes tend to fall "flat." 

The RHI below shows two layers of high differential reflectivity (indicated by yellows).  One is near the ground and reflects the melting layer in which snowflakes are sticking together and falling relatively flat.  The other is farther aloft and likely reflects a layer in which dendrites are growing and falling.  The temperatures in this layer were likely between -12ºC and -18ºC, which favors the formation of dendrites.  

An interesting aspect of the storm was a near-complete lack of any enhancement of precipitation over the mountains.  It was a frontally forced event, rather than a mountain forced event.  In fact, it clearly precipitated more over the Tooele Valley than over the Oquirrh Mountains.  Only in the late stages of the event did the front finally decide to move over the Oquirrhs.  An example is the RHI of horizontal radar reflectivity below, taken looking east-south-east across the northern Oquirrhs.  The yellows are ground clutter from the radar energy bouncing off the Earth's surface, with the sloping area representing the western slope of the Oquirrh Mountains.  Above the ground clutter, the transition from light to dark blue reflects increasing precipitation toward the mountains, but this doesn't reflect an orographic effect, but is the frontal band as it moved into the Oquirrh Mountains.  

It's such a pity that the storm didn't produce much in the Wasatch.  However, thanks to the mobile capabilities of the DOW, we were able to go to the storm and get a wonderful dataset.  

DOW Arrives at the U!

The Center for Severe Weather Research Doppler on Wheels arrived on the University of Utah campus early last night for this month's Outreach and Radar Education in Orography (OREO) field campaign.

The visit of the DOW, made possible by the National Science Foundation and Center for Severe Weather Research, will give University of Utah students a hands-on education in radar operations and interpretation, mountain and lake-effect precipitation processes, and the use of mobile observing platforms for field research.

We will exhibit the DOW at the Natural History Museum of Utah this Saturday, November 4, as part of their "Behind the Scenes" Weekend.  This is the best opportunity for the general public to check out this unique weather instrument.

My students have been working the past couple of weeks on plans to deploy the DOW for field research in northern Utah.  They plan to focus on five areas:

  1. The spillover of orographic (i.e., mountain enhanced) precipitation into the lee of a mountain barrier.  This work will concentrate on the northern Wasatch near Huntsville and the southern Wasatch near Heber.  
  2. Multirange effects.  This work will examine how upstream ranges affect precipitation on downstream topography, such as the Oquirrh Mountains affecting the Wasatch.  A number of possibilities exist depending on the flow dynamics.  
  3. Lake effect.  Always of interest, but one never knows if this fickle phenomenon will show its face.  We'll see if Mother Nature cooperates.
  4. Front-Mountain interactions.  What happens on small scales when a front plows into a mountain?  The students hope to find out.
  5. Polarimetric adventures.  The DOW is a polarimetric radar, which means that it transmits and receives radar horizontally and vertically polarized radar signals (this capability now also exists in National Weather Service radars).  Such information can be used in a number of ways, including to characterize the types of particles in precipitating clouds and to improve radar estimates of precipitation rate. 
This week focuses on training some of the students in DOW operations.  It's not quite as easy as piloting the Starship Enterprise and one also needs to learn how to configure the scanning strategies of the radar in an effective way to address key scientific objectives.  Our currently benign, warm weather is actually perfect for such training.  

After Saturday's exhibit, we will be using the DOW for a variety of teaching endeavors, both near campus and at sites across northern Utah to address the areas above.  Much depends on the weather and the teaching requirements.  Stay tuned to this blog for updates.

Storm Chasing Doppler on Wheels Radar Coming to Utah!

A storm-chasing Doppler on Wheels (DOW) radar will be coming to the University of Utah in November for the Outreach and Radar Education in Orography (OREO) field program.  

The visit of the DOW, operated by the Center for Severe Weather Research and supported by the National Science Foundation, will give University of Utah students a hands-on education in radar operations and interpretation, mountain and lake-effect precipitation processes, and the use of mobile observing platforms for field research. 

The DOW last visited campus in fall of 2011.  We had a field day, observing everything from intense fronts pushing through the Salt Lake Valley to lake-effect bands pushing into the Wasatch Range. 

During the visit, atmospheric sciences graduate students will plan and lead several DOW field deployments and educational activities for majors and non majors.  We are also planning a major public display of the DOW.

More information on these activities will be forthcoming in the next couple of weeks.  Stay tuned!

This Radar Loop Is for the Birds

Meteorological radars are designed to detect precipitation, but any objects of sufficient size and number concentration will give you a return.

This morning's radar loop, for examples, shows returns that are almost certainty from birds leaving their roosts along the wetlands surrounding the Great Salt Lake.  Note in particular the plume-like development and dispersion of echoes from near the southeast shore of Farmington Bay and the Bear River Migratory Bird Refuge.

To the west, you can see some echoes created by precipitation.  Some April showers are on tap for today.  

Weird Stuff on Radar

Perhaps I'm just suffering from jet lag, but I could use an assist from a radar guru out there to help decipher this morning's radar loop.

As shown in the loop below, things this morning start innocently enough with wide spread clear-air returns.  As the morning progresses, however, some blobs of high returns persist, in some cases generate over the Great Salt Lake, and move downstream with the large-scale flow.

What are these blobs?  I doubt they are meteorological as there is nothing to be seen on satellite.  Dust or birds?  I've looked at the some of the additional fields provided by the polarimetric radar, including correlation coefficient, and nothing obvious jumped out at me, although I confess my experience examining bugs and insects with radars is limited.

Any thoughts?

Happy 20th Anniversary KMTX!

Weather prediction over northern Utah reached a major milestone this week with the 20th anniversary of the KMTX WSR-88D radar on Promontory Point (right image in the @NEXRADROC tweet below).
WSR-88D stands for Weather Surveillance Radar, 1988, Doppler, a name only the National Organization for the Advancement of Acronyms [(a.k.a., National Oceanic and Atmospheric Administration (NOAA)] could love.  The non-technical name is NEXRAD, for NEXt-generation RADar.

The development, installation, and subsequent operation of the 160-radar NEXRAD network is a great success story, paying dividends to the American taxpayer many times over.  Mike Smith, author of the book Warnings: The True Story of How Science Tamed the Weather, calls the NEXRAD network "one of the best investments the federal government has ever made when viewed on a cost/benefit basis."

The NEXRAD network replaced the pre-existing network of WSR-57 and WSR-74 radars, adding increased resolution and the ability to measure Doppler velocity, the magnitude of the flow towards or away from the radar, which is critical for warnings related to severe convective storms and tornadoes.  The NWS recently added polarimetric capabilities to the NEXRAD network, which allows each radar to send and receive horizontal and vertical radar waves and better discriminate between rain, hail, snow, and other precipitation types.

Although beneficial across the country, the NEXRAD network was a revolutionary advance across most of the western United States and northern Utah, which were completely uncovered by the WSR-57 and WSR-74 radars.

Source: National Academies Press (1995).
In fact, prior to the installation of the KMTX radar, forecasters at the Salt Lake City National Weather Service Office relied on hand-drawn analyses from a FAA radar — hardly a recipe for success during rapidly evolving severe and hazardous weather events.

Today, thanks to KMTX, you can view radar loops in real-time on your smart phone while skiing in the Wasatch.  That's what I call progress!  Chances are you are using radar apps developed by private sector companies, which have leveraged the NEXRAD network to tap into all sorts of emerging markets in weather sensitive areas.  None of this was possible when I started graduate school.  Kudos to all in the weather enterprise who have contributed to these advances.

Unusually Strong Clear-Air Radar Returns

Some of the strongest and most widespread clear-air returns that I've seen from the Salt Lake City KMTX NEXRAD radar were evident in radar loops this morning.  Note in the loop below the widespread coverage of echoes with reflectivities of 5–20 dBZ prior to sunrise, followed by their rapid decay after about 1130 UTC (0530 MDT).  

Clear-air returns are not produced by precipitation, but instead by other meteorological or non-meteorological phenomenon including insects, birds, aerosols and something known as Bragg Scatter, which is produced when there are large variations in atmospheric density on scales comparable to or smaller than the wavelength of the radar.  They are frequently found in the eastern United States.  Just check out all the blotches of clear air returns at 0400 UTC this morning in the composite radar analysis below.

I've got a busy day ahead, so I leave it to some of the radar wonks out there to comment on the factors contributing to some of this morning's strong returns, especially since we see them less commonly than the eastern U.S.

More Radar Potpourri and an Unbelievable Photo

So much cool stuff today.  Here's more from radar and a photo that will BLOW YOUR MIND.

It's quite unusual for a front to blow through the Wasatch like there's nothing there, but that's largely what happened this morning.  The radar image below, for 9:51 AM MST, shows a well formed frontal precipitation band approaching extending from the eastern Uintas southwestward across the Wasatch Range and Utah Valley.  Terrain blockage of the beam prevents analysis of the frontal band further north.

Now, check out the radar loop that ends at the time above and watch the frontal band move through the Wasatch Range and eventually become a distinct feature in the lee.

The other cool thing is the very intense area of precipitation along the front just to the west (or arguably over the western slopes) of the Wastach.  You can see it in the top image as an area of yellow (i.e., radar reflectivity > 35 dBZ) along the front.

And, this from @skitheu showing what I suspect is the front pushing into the mouth of Little Cottonwood.  OMG!

Source; @skitheu