Headed south - Update #6

We've got less than a week to go at sea, so we're going all out for this last leg.  Keep reading to hear about our new sampling location, our quest for rain, and a bit about tracking weather out at sea.

Day 9

Now that we've finished up in our first region of study, the eddy to the east of Delaware, we've moved to a new location.  While we were sampling earlier, we noticed that an eddy further to the south was getting a fair amount of rain.  The weather report for after Bertha shows that this region has a good chance to get more rain, either over the weekend, or into next week.

Sea Surface Height map showing our starting point, our first eddy, and our new location to the south.

Early in the afternoon we arrived at our new station.  The first task was to do a CTD profile to see what the water column was like in this eddy.  We noticed a deeper surface mixed layer, possibly an after effect of the recent stormy weather.  Once we completed the first CTD cast, we decided where to deploy the three drifters.

Deploying the drifter

This eddy isn't quite as well defined as the last eddy we were in.  There is some concern that the drifters may not stay at the center of the eddy.  They could travel to the outside of the eddy and possibly get caught in the Gulf Stream.  It would be quite hard to catch them if that happened!  So we've been keeping a close eye on them just to be sure.

The rest of the evening was spent on our normal suite of surveying.  We deployed the trace metal CTD, the towfish, and then did another normal CTD cast.  There was a slight chance of a pop-up rain shower overnight, so we wanted to be sure we measured the "before-rain" conditions well.  Then we turned on the aerosol sampler and headed into the wind.

Recovering the trace metal CTD after a cast

Day 10

As is our typical routine now, we stopped aerosol sampling in the very early morning and headed back to our drifters to do more water sampling.  We did our sunrise CTD cast, followed by a trace metal CTD cast.  The PAR cast was done around noon, as usual, and then we put the towfish in the water and steamed in circles around the drifters.  We collected extra water samples using the towfish to start another round of bioassay experiments.

Collecting samples from the CTD

Now that we are a good ways into our cruise, we are getting more efficient at using our sampling equipment.  This means that, while we are spending just as much time working in the lab, the equipment on board, such as the CTD, isn't in use for as long.  Our chief scientist, feeling enthusiastic and always wanting more data, decided to start deploying the CTD more often.  The object is not to collect more water samples, but to get more frequent temperature and salinity profiles of the water column.  Instead of our normal one or two CTD casts, today we ended up doing four!

Taking a short break from science to fish - no luck today.

Later, at our evening science meeting, we had to make a critical decision on what to do now.  Rain was originally forecast to start tonight, but the system has moved further to the south than originally anticipated.  We expect rain in our current region sometime on Wednesday, but that's a long ways off.  So, do we just wait around here and take more profiles and samples until it rains?  Or do we travel south, knowing that we need to be back here for sure by Tuesday morning, but having some time to chase and collect the rain we know is down there?  After much discussion, we decided to travel south.  We already have good measurements of the first eddy, before, during, and after rain.  And we've taken initial samples in this new location and can be back in time for the next set of rain.  It is unlikely that we will miss anything, and we may get more rain samples, which we don't have much of so far.

Weather

The science update for today is on weather and forecasting out at sea.  Our on board meteorologist wrote a very nice bit on the weather side of chasing rain.  I'll let him take it from here:

My name is Dan Tomaso, and I am a meteorologist and current PhD student in the Department of Meteorology at Penn State.  I work as a research assistant for Dr. Raymond Najjar, and I have joined the DANCE cruise as the main forecaster for the project.  I am also a part-time broadcast meteorologist for abc27 WHTM television in Harrisburg, PA where I work with my fellow meteorologists to create and broadcast forecasts for Central Pennsylvania.

Let me first preface this blog post by saying that weather forecasting over water is a totally different beast compared to forecasting over land.  In the United States we are blessed with far-reaching weather networks and data.  I am accustomed to waking up each morning and looking over current weather for many local sites along with different forecast products that I trust.  Over the open ocean you do not have the benefit of numerous weather observations to keep the forecast data honest.  Forecasting for this journey has been quite the challenge, but we are working with the data we have and making the most of it.

When I create my forecasts out at sea I rely mainly on satellite images of clouds and radar from land.  For most of the cruise we have been out of the range of weather radars along the United States east coast.  However, our southern location east of North Carolina is close enough to the Morehead City, NC radar site that it does provide some useful information.  Knowledge of the cloud cover helps me determine if rain heading out to sea will be heavy or if thunderstorms are forming.  On visible satellite imagery, or satellite images that rely on the visible spectrum of light to take pictures of clouds from outer space, I can see textures in cloud tops as well as color differences.  Brighter white coloration and the bubbly texture of clouds usually indicate thunderstorm activity, so I know to ready the crew for heavy rain or lightning.  Radar images over land give me a history the rain/storms that form.  I can see if more rain is developing or weakening and how heavy we can expect the rain to be.  The three rain events we have intercepted thus far have originated from shallow stratus clouds and the rain has not been heavy.  I could tell those conditions were present based on satellite and radar data, however looking along the horizon has also been very helpful in spotting rain shafts ahead of us.  On very clear days you can visibly tell how heavy the rain is too by simply looking ahead at the horizon.

Satellite photo showing clouds.  Note the bright white clouds with texture in the red box - those are thunderstorms.  There is also a line of storms developing over western Georgia.

Beyond the reach of radar, model data becomes important.  In the last five or so years operational high-resolution weather guidance has become more mainstream as this set of weather models shows some skill or forecast ability in predicting what on-going precipitation may do in the future.  We have made use of the North American Non-Hydrostatic Mesoscale Model and the High Resolution Rapid Refresh models to provide clues as to how rain will develop and move over the water.  This data allowed us to make decisions on where to direct the boat and has helped us intercept three rain events so far.  Hopefully there will be more to come in the next week ahead!

Example model forecast of rain for the southern region.

As for current forecast information we are steaming to an area that is favorable for rain to develop.  There is a stationary front or zone of baroclinicity that has parked itself over North and South Carolina.  Along this boundary a coastal low pressure looks to form by late in the day Sunday heading into Monday.  This may be the shot of heavy rain we have been looking for.  The chances for heavy rain last each day through late Tuesday before a large cold front pushes the moisture well out to sea beyond our reach.  It should be an interesting few days following the DANCE cruise team and whatever rain we can intercept!

You can connect with Dan through Facebook or Twitter to hear more about weather.

More posts in this series:
Upcoming Cruise
Cruise Delays
Upcoming Cruise, Part II
Update #1
Update #2
Update #3
Update #4
Update #5
Update #7 
Update #8 
Update #9