Report of Activities on the RVIB N.B. Palmer Cruise 02-04

30 August 2002

 

August 30 should have been the day that the L.M. Gould and the N.B. Palmer met up again and then convoyed to the location for the Gould's process station 3 in the northern sector of the Southern Ocean GLOBEC grid of stations. A major storm with sustained winds in the 45 to 55 knot range (with higher gusts) and a heavy snowfall created white-out conditions that forced both vessels to stop steaming and to heave-to some 40 nm apart. Travel through the pack ice requires good visibility to avoid collision with the myriads of bergy bits, growlers, and icebergs scattered throughout. White-out conditions make it impossible to see ahead much farther than the bow of the vessel, hence the imperative to stop and wait for conditions to improve. The Palmer stopped about 0900 and began to steam again about 2230. During the hove-to period, the ship drifted with the wind-driven pack ice predominately to the southwest with the wind. The distance traveled was 10.5 nm in 13.3 hours for an average drift rate of 0.79 knots.

 

Very little over-the-side work was done on 30 August while the Palmer was hove-to. It was not possible to position the ship and open a hole next to the Baltic room door to deploy the CTD. This was prevented by the need to keep the vessel positioned into the wind and stay tucked into the floe that was selected for protection against the storm. However, it was possible to deploy the two 1-m ring nets off the stern in the upper 5 to 10 m of water in an attempt to collect more krill furcilia for experimental purposes. The deployment was done in the afternoon before the highest winds came upon us and some furcilia were caught, but not in the numbers that were caught the night before. Shortly after the nets were retrieved, the outside decks were closed because conditions had worsened. Shipboard experiments and data processing continued unabated.

 

Over the course of the day air temperature increased from about -4 to +0.6C as the barometric pressure steadily declined from 1008 to 982 mb. Winds were consistently out of the northeast during the day. Wind speed increased dramatically from 20 to 30 kts around midnight of 29/30 August to 30 to 40 kts in the morning, and then to 45 to 55 kts in the late afternoon and evening. Snowfall, which started after midnight, intensified in the morning and, coupled with the wind, caused the white-out conditions.

 

CTD Group report (Eileen Hofmann, Bob Beardsley, Baris Salihoglu, Chris MacKay, Francisco (Chico) Viddi, Sue Beardsley)

August 30 was spent waiting out a storm that produced white-out conditions extensive enough to curtail CTD and related activities. Thus, no CTD casts were made.

 

In the early morning of 30 August, when we began the transit to rendezvous with the Gould, the CTD group began an XBT survey to measure the vertical temperature structure. Probes were to be dropped at 10 nm intervals. However, at the first XBT site the temperature profile acquired from the probe was clearly in error. The structure of the profile, as well as the temperature value, did not correspond to conditions that are expected on this part of the continental shelf.

Additional XBT probes were dropped at this site and all returned incorrect vertical temperature distributions. This suggested that something other than bad probes was causing the problem. The XBT survey was stopped after this first site because of the weather.

 

Fred Stuart, the RPSC electronics technician, spent part of the afternoon trouble-shooting the XBT equipment and data acquisition system. After checking various hardware and software components of the XBT system, the problem was discovered to be a bad connector at the aft bulkhead where the wire is connected to the XBT launcher. This was replaced and checking with the XBT test canister indicated that the system was operational. Deteriorating weather conditions prevented testing with actual XBT launches. XBT surveying will resume once weather conditions clear sufficiently for the Palmer to get underway and the system will be tested at that time. Many thanks to Fred Stuart for identifying the problem and fixing the XBT system.

 

Report on Meteorological Measurements (Bob Beardsley)

The NBP is equipped with a full suite of sensing systems to measure the following atmospheric variables: wind speed (WS) and direction (WD), air temperature (AT), relative humidity (RH), barometric pressure (BP), incident shortwave (SW) and long-wave (LW) radiation, and PAR. These sensors are mounted on the top of the science mast or its base. The following oceanic variables are also measured: sea surface temperature (SST), sea surface salinity (SSS), fluorescence (Fluor), and light transmission (Trans). The SST sensor is mounted in the sea water intake manifold, while SSS, Fluor, and Trans are measured using a thermosalinograph, fluorometer, and transmissometer placed in the Wet Lab.

 

Despite occasional problems with the wind props icing up, the meteorological sensors have worked well during the cruise. For reasons not clear, the thermosaligraph has been reporting SSS significantly fresher than the CTD values and the Fluor and Trans signals are extremely noisy. All three conditions are thought to be due to having ice or melting snow flowing into the sea-water intake.

 

Scalar or vector values of these variables are averaged over one minute intervals and saved to disk in the JGOFS format at the end of each day (GMT). In order to convert the relative wind measurements made on the mast into true wind, the ship's GPS position, speed over ground, and gyro heading are also recorded as part of the JGOFS file.

 

Some MATLAB programs have been written to convert the JGOFS data into edited 1-min and 5-min time series starting with year-day 212. The surface wind stress and heat flux (computed using expressions valid only for open water and not ice) have also been computed and added to the 5-min data set. Each day, the previous day's JGOFS data are acquired and new 1-minute and 5-minute data sets are produced and stored on disk. On board the Palmer, the latest plots are posted outside the Dry Lab.

 

Current Position and Conditions

The storm of 30 August has passed and we are enjoying rather easy steaming through much thinner pack ice than encountered earlier in the cruise. The convoy of the L.M. Gould and the N.B. Palmer are within a few hours of reaching station 4, the location selected as the possible site for the Gould's third process station. Our current position at 2207 on 31 August is -66 16.547′S; -69 31.608′W. The air temperature is a more normal -4.1C and the barometric pressure is 980.3 mb and rising slowly. Winds have abated and are < 5 kts in variable directions. Snow has been falling intermittently, but visibility is OK.

 

Cheers, Peter