Rutgers University
  • Atlantic Shores Buoy off Atlantic City – 90% ahead of eye cooling

    Posted on July 12th, 2020 Scott Glenn No comments

    Fay continues to capture our attention with its ahead of eye cooling.  Travis and I are now look at Fay using the Atlantic Shores buoy deployed in the Wind Energy Area off Atlantic City, NJ.  This buoy is closer to landfall and south of the NDBC buoy in last night’s post. Atlantic Shores makes its buoy data available through the MARACOOS OceansMap, and you can download it as ERDDAP files.  Here we look at it through the ERDDAP interface.

    First plot is the air pressure.  The minimum occurs at about 20 GMT on July 10.  We will use this as the time of eye passage.

    We now look at the ocean temperature from the sensor at 1 m depth below the surface.  The water temperature cools from 26.5C before the storm to about 22C at eye passage at 20 GMT on July 10.  Total of 4.5C cooling ahead of eye. The minimum temperature looks to be about 21.5C, so an additional 0.5C of cooling after eye passage.  So for the total of 5C cooling caused by the passage of the storm, 90% occurs ahed of eye, and 10% after eye passage.  Another good case for ahead of eye cooling and rapid co-evolution of the ocean and atmosphere.

     

    So how does this feedback on the atmosphere?  Lets look at the air temperature from the Atlantic Shores buoy.  Because of the rapid ocean cooling, the ocean is colder than air temperatures.  The heat flux is from the hurricane into the ocean, which acts to weaken the storm.  If our forecast had kept the ocean temperatures warm – a static ocean rather than a co-evolving ocean – the ocean would be warmer than the air temperatures, changing the sign of the heat flux and strengthening the storm.

    This clearly demonstrates the value of IOOS collaborations between the Regional Associations, industry, and academic partners.

  • Fay from a NDBC buoy – 2/3 ahead of eye cooling

    Posted on July 11th, 2020 Scott Glenn No comments

    Tropical Cyclone Fay shot up the Mid Atlantic Bight yesterday, making landfall around Atlantic City.  Like an early season Irene (2011).

    Irene is the famous storm where the MARACOOS ocean observatory, and its associated regional ocean and atmospheric forecast models, was used to identify the Essential Ocean Processes responsible for rapid ahead-of-eye cooling of the ocean, and the subsequent Rapid Weakening (RW) of the storm (see the paper linked in yesterday’s blog). The paper looked at over 130 atmospheric forecast sensitivity runs to discover that hurricane-forced ahead-of-eye ocean cooling was the critical missing physical process required to properly match the observed Rapid Weakening (RW) of Irene as it tracked through the Mid Atlantic.  It’s a process we call rapid co-evolution of the atmosphere and ocean during the intense forced phased of a hurricane.

    Looks like the best NDBC buoy to check for ahead of eye cooling is 44025, up in the Bight Apex.

     

    First we look at the atmospheric pressure.  The minimum pressure is July 11, about 01 GMT.

    Now we look at the ocean temperature.  Starts about 78F before the storm.  At 01 GMT on July 11, the water temperature has cooled to about 72F, so 6 degrees F ahead of eye center cooling.  The minimum temperature on 7/11 is about 69F, so 3F cooling after eye passage.

    Result is 2/3 of the cooling ahead of eye, 1/3 of the cooling after eye passage.  And this is for a buoy north of the landfall location.  I would expect more cooling opportunity south of landfall.  We’ll need to check the models for that.

    How does this compare to the other Irene-class hurricanes that track across the Mid Atlantic?  That same Nature Comms paper quoted yesterday has the table of all 11 Irene-class hurricane occurring between Charlie (1986) and Arthur (2014).  The average percentage of ahead of eye cooling for these 11 hurricanes, based on the truly awesome 40 year NDBC buoy record, is 73%. And Fay is already at 66% ahead of eye cooling on our first look.

    Physics is awesome.  Thank you Adm. Lautenbacher.

  • The Operational System – Thank you Fay

    Posted on July 10th, 2020 Scott Glenn No comments

    HWRF is one of the world’s best hurricane forecast models.  Our job is to improve the ocean model component of HWRF to make it even better.

    Now we just can’t look at the operational HWRF whenever and wherever we like.  The operational HWRF is kicked-off by the presence of a hurricane and it follows the hurricane.  So we need a hurricane, or at least a tropical storm, to track through the Mid Atlantic to get a look at the ocean that the operational HWRF is using in this region.

    Thank you Fay.

    Below is temperature transect across the Mid Atlantic shelf along the Tuckerton Endurance Line.  It is along the same cross-shelf transect as the previous blog post from yesterday (below), but it uses a different color scale since it had to be generated on one of the operational computers.  We’ll download the data and fix that next week.  But here is a quick look just as Fay was beginning its northward track up the Mid Atlantic coast.  Right away you can see some differences between this ocean model and the Navy, NOAA and European global models, plus the MARACOOS regional model, posted in the previous blog.

     

    First issue is the water depth.  Moving offshore from 0 on the X-axis, the actual bottom should start at about 20 m depth, and run to about 100 m depth about 125 km offshore.  Here the water depth is deeper than 100 m almost along the entire transect.

    Second issue is the temperature structure.  We are missing the Essential Ocean Features of the Mid Atlantic shelf – the thin warm surface layer, the intense thermocline between 10 m and 15 m, and the widespread Cold Pool beneath. Our NOAA-sponsored science has proven that these Essential Ocean Features are critical for better intensity forecasts in our Mid Atlantic research models (https://www.nature.com/articles/ncomms10887).  And if this ocean does give us a better intensity forecast in the operational model, it means something else is wrong and is compensating for the less accurate ocean.  This is why we need intermediate metrics along the entire forecast value chain.  Did we improve the ocean model? Did we improve the air-sea interactions? Did we improve the intensity forecast?  The intermediate metrics help us identify where we are doing well, and where we need to focus more work.

    There is an easy to read viewpoint article on this by Kerry Emanuel, MIT’s expert on hurricanes. https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2019AV000129#.XvXR-OhrkjI.twitter

    Six paragraphs from the end, the one that starts “Sometimes the quest for better simulations…”, is especially relevant.

    Thank you Fay.

  • Tropical Storm Fay in the Mid Atlantic

    Posted on July 9th, 2020 Scott Glenn No comments

    Tropical Storm Fay is shown here leaving the Gulf Stream where it will rapidly be heading north across the Mid Atlantic shelf.  NHC forecasters called for Fay to intensify over the Gulf Stream then weaken as it passed over the Mid Atlantic shelf. Surface waters on the shelf are anomalously warm this year, but they much cooler than the Gulf Stream as seen here.  Still the biggest source of cold water in the region – the Mid Atlantic Cold Pool –  is unseen from space.  It spends much of its summer below a 10-20m thick layer of warm, fresh surface water.  To see this Cold Pool, you have to go out and touch it with a CTD. Thats why we send out the shallow water gliders in the summer. Simultaneously WHOI is sending out the deep gliders to monitor the evolving Gulf Stream.  This way there are gliders in the Gulf Stream making sure the models have the right characteristics where the hurricanes are expected to intensify, and gliders on the shelf where the storms have been shown to rapidly weaken due to ahead-of-eye mixing and cooling.  These rapid intensity changes, both intensification and weakening, are one the the challenges of modern hurricane forecasting.

     

     

    With Fay about to cross over the Mid Atlantic shelf, lets take look at the structure of the Cold Pool and the warm, fresh surface layer on a cross shelf line known as the Endurance Line, a glider line originating from Tuckerton, NJ and extending across the shelf, a line first occupied by gliders in 2003.

     

    First model – the Navy Global Ocean Forecast System (GOFS). Top panel is temperature, bottom panel is salinity. In the temperature section Maria has drawn in the 12C isotherm as a black line. You can see the Cold Pool on the shelf between about 20 km to 100 km offshore. And it has a thin warm surface layer about 10m thick.  A global model that has a Cold Pool on the continental shelf – pretty awesome how far we have come since hurricane Irene in 2011.  Salinity will need to be watched. The surface fresh water barrier layers in the Mid Atlantic usually mimic the temperature field.  If the temperature Mixed Layer Depth (MLD) is 10 m deep, the salinity MLD is often also about 10 m deep.  Here there is no barrier layer, simply a cross-shelf gradient.

    Global model #2 – NOAA Real Time Ocean Forecast System (RTOFS).  This uses the data assimilative ocean produced by GOFS but forces it with NOAA winds.  The shelf looks pretty similar for temperature, but offshore the shelf break, the cold water has moved farther offshore.

    Global Model #3, the European Copernicus system.  It has a Cold Pool, still an amazing accomplishment for a global model, but the structure on the shelf is very different. The Cold Pool is much farther offshore, and the thermocline is weaker and thicker.  More downwelling pushing the cold bottom water offshore?

     

    Fourth model – regional Doppio.  This definitely looks like a downwelling example, where the core of the Cold Pool is moved offshore and the nearshore isotherms in the bottom layer bend downward.  And the salinity also has the usual barrier layer structure, the there is a surface pool of low salinity water and the temperature and salinity MLDs are both about the same.

    In the Caribbean, the low salinity surface layer is usually shallower than the temperature mixed layer depth, so any surface induced mixing has to break through both the halocline and the thermocline, so it is important to get both temperature and salinity upper ocean structures right.  In the Mid Atlantic, the halocline and thermocline are often co-located, resulting in a stronger pycnocline, so it is important to get both the temperature and salinity upper ocean structure right.

     

     

     

  • Complimentary aspects of Argo and Gliders

    Posted on July 1st, 2020 Scott Glenn No comments

    A question from the IOOS-OAR collaboration workshop yesterday concerned how Argo and Gliders are complimentary.

    Below is a plot of the locations of the Argo profiles during the 2019 hurricane season compared to the tracks of the gliders during the same time period.

    The Argo floats provide the broad spatial coverage over large areas of the ocean.  The gliders are concentrated in regions that have less or no Argo coverage, like the Gulf Stream, the Caribbean, and definitely on the continental shelves.   Each Argo float typically provides a profile once every 10 days. Gliders provide multiple profiles a day, and can be directed across strong currents.  Over 100,000 glider profiles were provided by the community’s glider fleet during the 2019 hurricane season.

     

  • Three Oceans Under Cristobal

    Posted on June 16th, 2020 Scott Glenn No comments

    We have multiple ocean models in the coupled atmosphere-ocean hurricane forecast workflow:

    1. The Navy Global Ocean Forecast System (GOFS3.1) – “It starts with us” to quote the Naval Oceanographic Command.  This is were the data assimilation takes place.
    2. The NOAA global Real Time Ocean Forecast System (RTOFS) – derived from GOFS3.1 where the NOAA global winds are layered in.
    3. Regional Hybrid Coordinate Ocean Model (HYCOM )- derived from RTOFS where the high resolution processes are layered in to properly respond to the intense hurricane forecasting.
    4. Regional Princeton Ocean Model (POM) – currently initialized with climatology modified by feature models for the Loop Current and Eddies. Note that the climatology modified by feature models is scheduled to be retired July 1, 2020 and be replaced by an ocean derived from one of the RTOFS (items 2 or 5).
    5. The NOAA global Real Time Ocean Forecast Model with Data Assimilation (RTOFS-DA) – this is the data assimilative version of RTOFS.  The Data Assimilation (DA) scheme is derived from the Navy Coupled Ocean Data Assimilation (NCODA) scheme, but some adjustments.  The biggest difference with NCODA is how altimetry is assimilated.  NCODA assimilates the altimetric sea surface heights by transforming them into temperature and salinity profiles using the Improved Synthetic Ocean Profiles (ISOP) and assimilating the synthetic profiles as “super obs”.  RTOFS-DA assimilates the sea surface height (causing a barotropic pressure change throughout the water column), then adjusts the isopycnal layer thicknesses (to adjust the baroclinic pressure) to produce no total pressure change at the bottom.  The result is that RTOFS-DA and NCODA (used in GOFS) have two different ways to adjust the subsurface temperature and salinity fields.  This is important because Argo floats provide us the large scale thermal and salinity structure of the ocean, the altimeter data layers in the Mighty Mesoscale, and the underwater gliders fly in to tidy up the fuzzy spots.  Three critical steps in our efforts to provide the hurricane models with the best representation of the ocean possible.  RTOFS-DA changes how we do the critical step of layering in the Mesoscale, making this an important piece of our summer research.

    Now the important step, an ongoing collaboration between Maria at Rutgers and Hyun-Sook and Zulema at NWS. They are an awesome team dedicated to improving the ocean side of the hurricane forecast models. None of this would be possible if these three scientists were not actively collaborating and talking to each other multiple times a week.

    Maria has prepared two views of each of the different model oceans that were available for Tropical Storm Cristobal.  Each model is represented as a column in the figures.  The top row is a map of the surface temperature in the Gulf of Mexico immediately before the passage of Cristobal.  The bottom row is the vertical temperature section of the upper 200 m from south to north along 90W, the approximate track Cristobal was about to follow across the Gulf.

     

    The first plot below compares GOFS3.1 (model 1-  where data assimilation takes place) with RTOFS (model 2- where nosaa winds are layered in).  As expected, the two models look very similar in both the map view and the vertical section.  The important feature is warm anticyclonic Loop Current Eddy centered at 25N, 9oW.

     

    Next we move from the global RTOFS (model 2) to regional HYCOM (model 3).  Regional HYCOM is the ocean in the “Hurricanes in a Multi-scale Ocean-coupled Non-hydrostatic (HMON)”.  The US is required to operate two official hurricane models and HMON is one of them.  In the figure below we see that the regional HYCOM that HMON is coupled to looks very much like ROTFS which looks very much like GOFS.  This is the way the workflow is designed to work, – get the best available assimilation, get the best available winds, and get the ocean into the high resolution models we need – setting up the best initial condition for the ocean before the hurricane even enters the region.  HMON has the best ocean the Navy and NOAA – working together – can provide.

     

     

    Now we look at the second of the two hurricane models, the Hurricane Weather Research and Forecast  (HWRF) model.  HWRF is one of the highest performing hurricane models in the global ensemble. In the Atlantic Basin, the ocean model that HWRF is coupled to is the Princeton Ocean Model (POM). The Princeton Ocean Model is not initialized using the GOFS to RTOFS to HYCOM workflow described above.  In the Atlantic Basin, it is initialized using climatology modified by feature models for the Loop Current and Eddies. Comparing the maps below, we see that the Loop Current is in the wrong location based on the satellite altimetry, and the Loop Current Eddy is not present at all.  The POM model under HWRF appears to be initialized by climatology only.

    This is a significant dilemma.  One of our required hurricane models, HMON, is coupled to the best ocean Navy and NOAA can provide.  And our other hurricane model, HWRF, one of the global leaders in hurricane forecasting, is coupled to a climatological ocean that does not include the Essential Ocean Features impacting hurricane intensity.  We look forward to the next update of operational HWRF scheduled for July 1, 2020.

    Enter RTOFS-DA (model 5 above). Can we reduce the distance between the data assimilative model and the regional scales we need to resolve the Essential Ocean Processes. RTOFS-DA is currently running at NWS in an experimental mode. In the figure below, we have the operational ROTFS in the left column and the experimental RTOFS-DA in the right column.  The Loop Current is a bit different, but not disturbingly different like it is in the climatology.  And both models have the warm anticyclonic Loop Current Eddy near 25N, 90W.  But there are differences in the eddy structure.  In the deeper part of the temperature section, say between 50 m and 200 m, the Loop Current Eddy is stronger in ROTFS-DA.  So at depth, one has a stronger eddy, one has a weaker eddy, but both have an eddy, so we are doing good.  But what about the upper 50 m.  Here we see differences in the structure of the eddy in the water warmer than 26C (the black line in the bottom row of plots).  The 26C isotherm domes up in the middle of the eddy like is does in a cold core ring with cyclonic circulation.  The impact on the surface map is we don’t see the swirling motion of the alternating bands of warm and cold water wrapping around the eddy due to the clockwise surface circulation that we see in RTOFS.  No this is an important difference for hurricane forecasting since it is the water above 26C that contributes to the calculated Ocean Heat Content (OHC).  And maps of the OHC are used by the forecasters at the National Hurricane Center to alert them to potential regions where hurricanes may intensify.  The operational RTOFS has a maximum in the Ocean Heat Content in the center of the Loop Current Eddy.  And the Experimental RTOFS-DA has a local minimum in the Ocean Heat Content in the center of the eddy.  These are two very different answers, and provide the NHC forecasters with very different guidance. So now we have work to do.   Which one is right and why?

  • OceansMap view of Cristobal

    Posted on June 6th, 2020 Scott Glenn No comments

    Here are two quick views of Cristobal from OceansMap.

    First we look at the  current position and track of Cristobal over the Navy GOFS 3.1 model forecast for today’s Sea Surface Temperature.  Cristobal is currently on the southern side of the Anticyclonic warm Loop Current Eddy. It will take most of today (Saturday) to cross over the eddy.  Official NHC forecast has the intensity at 40 knots. The forecast is for a slight increase in intensity by about 11 mph as it crosses the Gulf with landfall Sunday.

     

    But Cristobal is a very broad storm as seen in the satellite cloud top water vapor product below, with the strongest convection to the north and east. The official NHC warnings extend well to the east of the expected landfall region.

    To get an idea of the scale of the impacts of this broad storm, we can check the global NOAA Wavewatch III model.  The modeled waves indicate that this is a full-Gulf event.

  • Two Different Oceans

    Posted on June 5th, 2020 Scott Glenn No comments

    Below is a side by side comparison of the two oceans we have been looking at for the last 24 hours.  The left hand column is the Navy’s data assimilative Global Ocean Forecast System (GOFS) 3.1.  Right column is the regional-scale Princeton Ocean Model (POM) initialized using Loop Current and Eddy features models to adjust climatology.  The top row is a map of the Sea Surface Temperature (SST).  The black line running south to north along 90W is the approximate forecast path of TS Cristobal.  Bottom row is the vertical temperature section along the 90W line between the surface and 200 m depth.

  • HWRF-POM initialized with climatology and feature models

    Posted on June 5th, 2020 Scott Glenn No comments

    The last few blogs talked about what we see in the Navy GOFS 3.1, the data assimilative step on the ocean side of the coupled atmosphere-ocean hurricane models.  My blog post from May 15, 2020 describes the multi-step process that gets us to the operational models.  GOFS3.1 is the data assimilation step.  This information flows into Global RTOFS to layer on the NOAA global windfields.   But remember, these both are global models, their job is through data assimilation and the good windfields to get the Essential Ocean Features in the right locations.  We then initialize a high resolution regional ocean model capable of resolving the Essential Ocean Processes that impact hurricane intensity during the rapid co-evolution phase of the atmosphere and ocean subject to intense hurricane forcing.

    Another path to the regional scale ocean model underneath the hurricane is the feature model initialization process. Here climatology is modified by typical structural models for key Essential Ocean Features.  In the Gulf of Mexico, a forecaster provides 3 points to describe the location of the Loop Current, and the a model for a typical Loop Current is inserted.  For the warm eddies, climatology is modified by inserting bowl shaped structures to represent the warm core.

    Here we investigate the results of that feature model initialization procedure used in the most recent HWRF-POM coupled forecast for Cristobal. The first image below is the Sea Surface temperature for  the POM ocean model coupled to the HWRF atmospheric model.  We see the Loop Current is present, but it looks to be shifted west of the Loop Current seen in the data assimilative GOFS or in the CoastWatch altimetry product.  It definitely is not up against the West Florida Shelf as it exits the Gulf.  Along the 90W line we have been talking about, the water is relatively warm on much of the Yucatan Shelf, with some upwelling occurring on the shelf.  But as you head north along 90W and leave the Yucatan Shelf, you encounter some of the coldest water in the Gulf.  The large warm eddy observed in GOFS and the CoastWatch altimetric product centered near 25N, 90W is absent from POM.

     

    So lets layer on the surface currents to search for the structures in the current field.  The GOFS experience taught us that the SST may be confusing due to surface processes, and that the sea surface height and the surface currents are better for identifying the eddies.  Below is a plot of the POM ocean model SST and Surface currents. The Loop Current is certainly present, but the anticyclonic circulation we have been talking about since last night is absent.

    Ok, so lets just move straight to the subsurface section running from south to north along 90W.  There is no bowl shaped warm core ring in the temperature section running from the Yucatan Shelf to the Louisiana Shelf.  The 26C isotherm is pretty shallow, hovering around 30 m deep.  Also wondering where the continental shelves on the north and south are.  Looks like more to investigate.

     

     

  • An Argo Float from the critical Warm Eddy

    Posted on June 5th, 2020 Scott Glenn No comments

    Here we look back on the Argo floats locations for the last 20 days.  Looking for one closer to the eddy center.  So we chose one near  25N, 90W.

     

    For the temperature comparison, we see the near surface region is similar in magnitude but with multiple mixed layers in temperature in the Argo data. Profiles at the depths below 600 m are also similar, but the main warm core of the eddy between 100 m and 600 m is different.  Argo data is warmer than the model.  Perhaps this is a true temperature difference, or perhaps it is simply a difference in location.  That the real world Argo profile may be closer to the center of the eddy than the same location in the model.  To help answer this question, it is often good to look at the profiles relative to an eddy center based coordinate system.   We can do that in the model.  More difficult in the data when you only have a single float profile and you can’t direct the float into the eddy center.

     

    The same discussion applies to salinity.  The core of the anticyclonic eddy is warm and salty.  The Argo float is saltier than the model.  Maybe the Argo float is closer to the eddy center?  The Argo salinity profile also has a deeper mixed layer than the model.  Is this a difference in surface mixing?  or maybe it is part of the banding we see in the circulation around the eddy.   Easier to check in the model.  More difficult when you are dealing with a single profile instead of a cross-section through an eddy.