Ship Observations Research Articles

Study on Estimation of Original Location of Water Sampled through Inlet Set on Volunteer Observing Ship

Study on Estimation of Original Location of Water Sampled through Inlet Set on Volunteer Observing Ship

Numerical simulation of radar surveillance for the ship target in oceanic clutters

A fast iterative approach of generalized forward-backward method with the spectrum acceleration algorithm (GFBM/SAA) is developed for solving electromagnetic field integral equation and is applied to numerical simulation of radar surveillance of the ship target in oceanic clutters. Randomly rough surface is realized by the Monte Carlo method using the Pierson-Moskowitz spectrum. Numerical results of bistatic and back-scattering from the ship target and oceanic clutters demonstrate the functional dependencies upon the situation of radar surveillance such as the incidence and observation angles, radar altitude, ship RCS and other oceanic conditions.

Ocean–Atmosphere Interaction in the Agulhas Current Region and a South African Extreme Weather Event

Ocean‐atmosphere interaction above warm western boundary currents such as the Gulf Stream, the Kuroshio Current, and the Agulhas Current often leads to very high evaporation rates. In the case of the Agulhas Current, which flows just off the southeast coast of Africa, such high latent heat fluxes may lead to increased low-level advection of moisture onshore and local intensification of storm systems. Observational evidence for the significant latent heat fluxes in the Agulhas Current area was obtained during the Agulhas Current Air Sea Exchange Experiment, which showed that about 5 times as much water vapor is transferred to the atmosphere above the 80‐100-km-wide core of the current than from the neighboring waters. Using NCEP reanalyses, Meteosat, and Tropical Rainfall Measuring Mission (TRMM) satellite data and local station and radiosonde observations, this study investigates the evolution of a severe storm and flood event that occurred over the southern coastal regions of South Africa on 14‐15 December 1998. Heavy rainfall occurred in two widely separated locations, and tornadoes were reported. Moisture flux transects through the storm region and backward trajectories of air parcels suggest that low-level onshore flow of moisture from the Agulhas Current region played a significant role in the storm evolution. However, because the NCEP data on which these moisture fluxes are based are known to significantly underestimate the surface latent heat flux when compared with ship observations, it is suggested that the actual contribution of the Agulhas Current moisture source to the storm may have been even greater than is documented in this paper.

Solar influences on cosmic rays and cloud formation: A reassessment

Svensmark and Friis‐Christensen [1997] proposed a “cosmic ray‐cloud cover” hypothesis that cosmic ray flux, modulated by solar activity, may modify global cloud cover and thus global surface temperature by increasing the number of ions in the atmosphere, leading to enhanced condensation of water vapor and cloud droplet formation. We evaluate this idea by extending their period of study and examining long‐term surface‐based cloud data (from national weather services and the Global Telecommunication System) as well as newer satellite data (International Satellite Cloud Climatology Project (ISCCP) D2, 1983–1993). No meaningful relationship is found between cosmic ray intensity and cloud cover over tropical and extratropical land areas back to the 1950s. The high cosmic ray‐cloud cover correlation in the period 1983–1991 over the Atlantic Ocean, the only large ocean area over which the correlation is statistically significant, is greatly weakened when the extended satellite data set (1983–1993) is used. Cloud cover data from ship observations over the North Atlantic, where measurements are denser, did not show any relationship with solar activity over the period 1953–1995, though a large discrepancy exists between ISCCP D2 data and surface marine observations. Our analysis also suggests that there is not a solid relationship between cosmic ray flux and low cloudiness as proposed by Marsh and Svensmark [2000].

Wind stress forcing of the Oregon coastal ocean during the 1999 upwelling season

The wind stress forcing of the Oregon coastal ocean during June–August 1999 is estimated from a regional mesoscale atmospheric model and from satellite scatterometer observations, supplemented by moored and coastal surface data and by a land‐based wind profiler. Both the mean and variable components of model alongshore wind stress increase by factors of 3–4 from north to south along the Oregon coast. There is evidence of orographic intensification near Cape Blanco, which is supported by previous aircraft and ship observations during August 1995. The systematic southward increase of southward stress will drive enhanced ocean upwelling along the southern Oregon coast and suggests the new hypothesis that systematic variations in local wind stress may contribute to the observed offshore displacement of the coastal upwelling jet in this region. It is inferred from an analysis of the model and surface data that ocean upwelling modifies coastal surface air temperatures by 1–5°C over timescales of 12–24 hours.

Temporal and spatial variability of chlorophyll-a in the western subarctic Pacific determined from satellite and ship observations from 1997 to 1999

Abstract The objectives of this study are to describe and understand the processes controlling the temporal and spatial variability of chlorophyll-a (chl-a) in the subarctic northwestern Pacific Ocean from 1997 to 1999. Remotely sensed data from multi sensors, including ocean color (OCTS and SeaWiFS), sea-surface temperature (SST, AVHRR), and wind (SSM/I) datasets were utilized for the purpose of this study. Ocean-color imagery clearly showed seasonal and interannual variability in the spatial abundance and distribution of chl-a in the study area. Chl-a concentrations were generally low (0.53±0.24 mg m−3) for most part of the year, except for a few peaks (1.0–2.0 mg m−3) observed in the spring and fall bloom seasons (May, June, September, and October). Chl-a concentrations (>10 mg m−3) were consistently high along the Kuril Islands and in the coastal waters around the Kamchatka Peninsula, and in 1998 they were clearly higher than in 1999. The Western Subarctic Gyre (WSG) was characterized by positive sea-surface temperature anomalies (SSTA) during the summer to fall of 1998. These anomalies appear to be a high-latitude response to 1997/1998 ENSO event. High concentrations of chl-a appeared in WSG in September–November only in 1998. We suggest that the high chl-a around the WSG from summer to fall in 1998 was facilitated by (1) negative SSTA in winter, in spite of the negative wind anomaly, which provided larger amount of nutrients to the sea surface, (2) positive wind anomaly from April to June, which might cause light limitation of the phytoplankton growth, due to deepening of the surface-mixed layer in summer, and (3) positive SSTA, even with the slightly positive wind anomaly, which was accompanied with the water-column stabilization and hence recovered light limitation of the phytoplankton. Warmer SST also might have enhanced the phytoplankton growth. During summer to fall in 1999, the phytoplankton biomass between 42°N and 43°N along 165°E was greater than in other years. The enhanced chl-a concentration coincided with a distinct frontal temperature gradient located between 40°N and 45°N in September 1998 and 1999. The peak chl-a concentration in September 1999 (>2.0 mg m−3) within the temperature front was much larger than in 1998 (about 0.7 mg m−3). The position of this front in September 1999 (41°N) was south of its position in 1998 (43°N). Convergence of chl-a is linked to the temperature gradient and southward expansion of well-mixed subarctic water that is cold, low in salinity, and nutrient-rich. Our results showed significant interannual biological variability, and illustrate how remote sensing with multi sensors can aid in monitoring relationships between local ecosystems and global climate change.

Numerical Simulations with MM5 3DVAR Initialization

The recently developed MM5 three-dimensional variational (3DVAR) assimilation system at NCAR has been used to investigate influences of ingested sounding and ship observation data on numerical simulations of severe weather in the vicinity of Taiwan. Three weather events were simulated in this study, which include a Mei-Yu front in June 1998, Supertyphoon Bilis in August 2000 and Typhoon Nari in September 2001. For the Mei-Yu front, the simulated low-pressure system northeast of Taiwan is stronger. when the 3DVAR is performed during initialization. The simulated patterns of heavy rainfall just off the southern tip of Taiwan are also closer to observations for the run with 3DV AR. For the second case of Typhoon Bilis, both runs with and without 3DVAR show a northward bias in track upstream of eastern Taiwan. However, due to less northward track deflection, the simulated heavy rainfall in Taiwan for the 3DV AR run is in better agreement with observations. For the no-3DVAR run, the simulated Typhoon Nari consistently moves southwestward toward Taiwan but then makes an incorrect landfall at northwestern Taiwan. With 3DVAR, the track simulation is improved with a landfall position at northeastern Taiwan. During landfall, the associated could convection is enhanced as the intense vortex core is in confrontation with the leading edge of the Central Mountain Range (CMR) and its movement is slowed down along the northwestern coast. The combination of both compression and stagnation of the embedded convective system may explain the extremely intense rainfall in northern Taiwan. The feature of observed intense rainfall over the southwestern slope base of the C／MR is also captured, but the intensity is considerably underpredicted due to lagging and weakening of the vortex core at later -times for both runs with and without 30V AR. The track simulation at later times has also been improved in the 3DVAR run, but the associated geometric distributions of 72h accumulated rainfall amounts in general are similar to those without 3DVAR. In the typhoon experiment with a bogus vortex, the 3DVAR rum preserves the initial vortex, quite well. However, the rainfall and track simulations are not improved by the ingestion of the bogus vortex for both runs with and without 3DVAR.

Solvolysis of 1‐tert‐Butyl‐1‐(4‐phenoxyphenyl) Methyl Bromide and Chloride. Further Example for Possible Contribution of Non‐canonical Resonance Structure in the Transition State

AbstractThe solvolysis of 1‐tert‐butyl‐1‐(4‐phenoxy)methyl bromide (5b) and chloride (5c) in a variety of solvents were carried out. Limiting SN 1 mechanisms were suggested from the observation of linear relation ships using single‐parameter Winstein‐Grunwald type correlation between logarithm of rate constants with YxBnBr and YxBnCl scales, respectively. This result indicated positive charge delocalization over both arylrings and provided further evidence for possible contribution of non‐canonical resonance structure in the cationic transition state.

Quantifying Uncertainties in NCEP Reanalyses Using High-Quality Research Vessel Observations

Abstract The uncertainties in the NCEP–NCAR reanalysis (NCEPR) products are not well known. Using a newly developed, high-resolution, quality controlled, surface meteorology dataset from research vessels participating in the World Ocean Circulation Experiment (WOCE), regional and global uncertainties are quantified for the NCEPR air–sea fluxes and the component fields used to create those fluxes. For the period 1990–95, WOCE vessel and gridded NCEPR fields are matched in time and space. All in situ data are subject to data quality review to remove suspect data. Adjustment of ship observations to the reference height of the NCEPR variables, and calculation of air–sea fluxes from the in situ data are accomplished using bulk formulas that take atmospheric stability, height of the measurements, and other adjustments into consideration. The advantages of using this new set of WOCE ship observations include the ability to compare 6-h integrated fluxes (much of the ship data originate from automated observing sy...

FASTEX IOP 18: A very deep tropopause fold. I: Synoptic description and modelling

AbstractThe life cycle of a very deep tropopause fold (820 hPa) is documented with aircraft and ship observations during the Intensive Observing Period 18 of the Fronts and Atlantic Storm‐Track EXperiment (FASTEX). The initial setting involves a coherent tropopause disturbance and an associated Arctic tropopause fold. The confluence episode that results from the phasing up of the tropopause disturbance and a southern ridge, ends in the formation of an intense jet streak, the dynamics of which are associated with the development of a polar tropopause fold. A diagnostic analysis suggests that the final dramatic stratospheric intrusion is the consequence of the vertical superposition of the Arctic and polar tropopause folds. The Mesoscale Non‐Hydrostatic (Meso‐NH) model is used to discuss this hypothesis. Mixing of the passive stratospheric tracer within the marine boundary layer is investigated with sensitivity tests which unplug, in turn, the model physical parametrizations. Finally, upper‐level forcings associated with the development of the tropopause fold are investigated in detail in a companion paper.

Intercomparison of Global Precipitation Products: The Third Precipitation Intercomparison Project (PIP–3)

A set of global, monthly rainfall products has been intercompared to understand the quality and utility of the estimates. The products include 25 observational (satellite-based), four model and two climatological products. The results of the intercomparison indicate a very large range (factor of two or three) of values when all products are considered. The range of values is reduced considerably when the set of observational products is limited to those considered quasi-standard. The model products do significantly poorer in the tropics, but are competitive with satellite-based fields in mid-latitudes over land. Over ocean, products are compared to frequency of precipitation from ship observations. The evaluation of the observational products point to merged data products (including rain gauge information) as providing the overall best results.

Cloud climatology of the Indian Ocean based on ship observations

Surface cloud data based on synoptic observations made by Voluntary Observing Ships (VOS) during the period 1951-98 were used to prepare the seasonal and annual cloud climatology of the Indian Ocean. The analysis has been carried out by separating the long-term trends, decadal and inter-annual components from the monthly cloud anomaly time series at each 5° × 5° grids.
 
 Maximum zone of total and low cloud cover shifts from equator to northern parts of India during the monsoon season. During the monsoon season (June-September), maximum total cloud cover exceeding 70% and low cloud cover exceeding 50% are observed over north Bay of Bengal. Maximum standard deviation of total and low cloud cover is observed near the equator and in the southern hemisphere. Both total and low cloud cover over Arabian Sea and the equatorial Indian Ocean are observed to decrease during the ENSO events. However, cloud cover over Bay of Bengal is not modulated by the ENSO events. On inter-decadal scale, low cloud cover shifted from a "low regime" to a "high regime" after 1980 which may be associated with the corresponding inter-decadal changes of sea surface temperatures over north Indian Ocean observed during the late 1970s.

Cloud coverage and height during FIRE ACE derived from AVHRR data

Cloud cover and height are derived from NOAA‐12 and NOAA‐14 advanced very high resolution radiometer (AVHRR) data taken over the Arctic Ocean for an 8° latitude by 30° longitude domain centered on the Surface Heat Budget of the Arctic Ocean (SHEBA) ship Des Groseilliers. Multispectral thresholds were determined subjectively and applied to each image, providing excellent temporal coverage during the May‐July 1998 First ISCCP Regional Experiment Arctic Clouds Experiment (FIRE ACE). Mean cloud amounts were near 70% for the entire period but varied regionally from 55 to 85%. On the basis of a limited climatology of ship observations, these values appear to be typical for this part of the Arctic, suggesting that most of FIRE ACE was conducted in representative cloud conditions. A diurnal cycle of mean cloud amount was found for the domain during June and July having a range of 10% with a middle‐to‐late morning maximum. The AVHRR‐derived cloud amounts are in good agreement with visual and radar measurements taken from the Des Groseilliers, except for a few subvisual and low cloud cases. Average AVHRR‐derived cloudiness differ from the mean values obtained at the surface by −1 to +3%; this represents a significant improvement over previous satellite retrievals. The satellite‐derived cloud heights are very accurate for most of the low cloud cases. Higher cloud altitudes are less certain because cloud optical depths were not available to adjust the temperature observed for the optically thin high clouds, and the radiating temperature of many of the high clouds is representative of some altitude deep in the cloud rather than the highest altitude level of condensate. The development of a more accurate automated algorithm for detecting polar clouds at AVHRR wavelengths will require inclusion of variable thresholds to account for the angular dependence of the surface reflectance as well as the seasonally changing albedos of the ice pack. The use of a 1.6‐μm channel on the AVHRR, or other complement of instruments, will greatly enhance the capabilities for detecting clouds over poles during summer.

Comments on “A Statistical Determination of the Random Observational Errors Present in Voluntary Observing Ships’ Meteorological Reports”

Abstract The accuracy achievable for surface ship pressure reports and the sources of error in them are discussed. In The Met Office numerical weather prediction (NWP) system the error is estimated as 1 hPa, whereas Kent et al. calculated a figure of about 2.3 hPa. There are a number of reasons for this discrepancy, but the main one is that quality control using short-range forecasts is much better at detecting gross errors in the reports than are climatological checks. Statistics from the Met Office NWP system are presented for comparison of pressure and other atmospheric variables. Other variables show much less discrepancy than pressure because, relative to observation error, they have a smaller climatological range. Both sets of statistics show larger air temperature errors at high latitudes/low temperatures.

QuikSCAT's sea winds facilitates early identification of tropical depressions in 1999 hurricane season

Far from land and surface ship observations, most tropical depressions are identified by examining images from geostationary satellites for the presence of rotation of the convective cloud masses. During the 1999 hurricane season, surface wind vectors obtained by the SeaWinds scatterometer on the QuikSCAT satellite for the tropical Atlantic and Caribbean Sea were examined to test the hypothesis that developing tropical depressions (TDs) could be observed with this satellite sensor, before identification by the traditional means. QuikSCAT was able to detect the presence of closed circulation in the surface winds before the systems were designated as depressions. The satellite's unprecedented large swath width of 1800 km allows twice a day observation of most of the tropical oceans. SeaWinds data can, therefore, provide valuable guidance that are an important addition to the tools available to the tropical cyclone forecasting community.

Air Mass Transformation Processes over the Southwestern Region of the Ice-covered Sea of Okhotsk during Cold Air Outbreaks.

Radio sounding and in-situ observations at land-based (26 January-11 February 1998) and ship stations (4-11 February 1998) were performed over the southwestern Sea of Okhotsk covered with sea ice to estimate turbulent heat flux and investigate characteristics of thermodynamic air mass transformation processes during cold-air outbreaks. Surface heat flux estimations were carried out using two indirect methods; an atmospheric heat budget analysis using the radio sounding data at three stations and a bulk method utilizing meteorological and sea ice thickness data obtained by the ship observations. The estimated total upward turbulent heat flux over the analyzed area was approximately 100 Wm-2 even within the intense cold-air outbreak period. The height of the mixing layer associated with this small amount of the heat flux is only about 1 km, which implies that sea ice acts as an insulating material between the ocean and atmosphere and thus significantly reduces the turbulent heat fluxes. Nevertheless, the upward turbulent heat flux from the ice surface was found to be non-negligible during cold-air outbreaks and it is suggested that the sea ice grows through the sea or ice surface cooling.

Sea-ice concentration retrieval in the Antarctic based on the SSM/ I 85.5 GHz polarization

AbstractUsing data from the 85 GHz channels of the Special Sensor Microwave/ Imager (SSM/I) allows a resolution improvement by at least a factor of four compared to the other channels. Consequently higher-resolution sea-ice concentration data can be obtained which in turn can be used to improve the results of numerical weather-prediction (NWP) and global circulation models. The proposed new sea-ice concentration retrieval algorithm (SEA LION algorithm) uses the polarization at 85 GHz (p). Emission from atmospheric water and scattering at the wind-roughened sea surface (weather effect) decrease p and cause an overestimate of the sea-ice concentration. We quantify the weather effect with a radiative transfer model and atmospheric data obtained from NWP models and the other SSM/I channels, and correct p for this effect. Tie points of open water and sea ice are determined for each month separately from daily gridded 85 GHz SSM/I brightness temperatures. Sea-ice concentrations are calculated with the new algorithm for the entire Southern Ocean for each day of the period 1992−98 with a spatial resolution of 12.5 × 12.5 km2. Comparisons of these ice concentrations with Operational Linescan System visible images reveal convincing results concerning the monitoring of coastal polynyas and the break-up of the pack ice in spring. SEA LION sea-ice extents and areas, and comparisons between SEA LION sea-ice concentrations and ship observations, agree with those obtained by the NASA Team and the Bootstrap algorithms:

Ocean feature detection using altimetry backscatter: The Gulf Stream

This study presents a basic analysis of ERS-1 altimeter data, from its 3-day repeat track periods taken over a section of the Gulf Stream. Along track variations of radar backscatter (?0) have been compared with positions of the Gulf Stream derived from the composite maps of sea surface temperature from ship and satellite-sensor observations, produced by NOAA. Variations in ?0 , indicating potential interaction with the Gulf Stream, are found at both high and low wind speed conditions. At high wind speeds, comparison with model wind data indicates that peaks in altimeter-derived wind speeds along the path of the North Wall of the Gulf Stream may be related to real increases in wind speed. A possible reason may be a tendency for winter storm tracks to follow the same path as sections of the Gulf Stream. At low wind speeds, minima in altimeterderived winds are associated with sections of the path of the Gulf Stream but there were no comparable occurrences of low wind speeds in model wind fields, with implied consequences for winds derived from satellite sensor data. Although, for low wind speeds, the cause of these 'anomalously high' backscatter regions is unknown, such associated relatively 'calmer' waters have been previously noted as the result either of a tendency for surface slicks to be confined to the shears associated with local current systems or by wave-current interaction mechanisms causing a local reduction in the sea surface roughness.

Georges Bank Winds: 1975–1997

Twenty-three years (1975–1997) of anemometer records from the four NOAA buoys located near Georges Bank are examined. While the individual buoy records have occasional gaps due to instrument breakdowns and the buoys deployments were limited to certain years, the combination of the four buoys provides a nearly continuous series of observed wind. After correcting for different anemometer heights, sea-surface wind stress is calculated by the neutral stability method of Large and Pond J. Phys. Ocean 11 (1981) 324. Weekly mean stress is plotted for the entire period. Significant coherence (0.72–0.92) was found between sites with very little phase or gain for the 2–10 day storm-band period. An offshore increase of ∼0.006 Pa/100 km is detected in the mean stress for the Winter/Fall seasons. The complex correlation coefficients (calculated as a single measure of coherence across all frequency bands) ranges from 0.68 to 0.90. Other sources of wind data are discussed including the Fleet Numerical Meteorology and Oceanography Center's (FNMOC) estimates from pressure observations, the Comprehensive Ocean Atmosphere Data Set (COADS) from ship observations, and NOAA's National Center for Environmental Prediction (NCEP) recent model/data assimilation. For purposes of representing storm-band frequency, the FNMOC winds account for more than 80% of the variability in the buoy record and they provide a continuous surface wind estimate for the Georges Bank region back to 1967. However, the intensified wind stress as a function of distance offshore, as detected by both the buoys and COADS , is not detected in the FMNOC records. Also, a counterclockwise rotation of 19 max° is needed (in addition to the standard 15°) to correct the FMNOC winds for the atmospheric boundary layer. Recent NCEP products provide a finer scale spatial coverage of the wind field and are a potential source of boundary forcing for coastal ocean circulation models.

Physical and ecological processes in the marginal ice zone of the northern Barents Sea during the summer melt period

The main physical and ecological processes associated with the summer melt period in the marginal ice zone (MIZ) were investigated in a multidisciplinary research programme (ICE-BAR), which was carried out in the northern Barents Sea during June–August 1995–1996. This study provided simultaneous observations of a wide range of physical and chemical factors of importance for the melting processes of sea ice, from its southernmost margins at about 77.5°N to the consolidated Arctic pack ice at 81.5°N. This paper includes a description of the oceanographic processes, ice-density packing and structures in cores, optical properties of water masses and the ice, characteristics of the incident spectral radiation and chlorophyll — leading to primary production. Large seasonal and inter-annual variations in ice cover in the MIZ were evident from satellite images as well as ship observations. Even if the annual variation in ice extent may be large, the inter-annual variations may be even larger. The minimum observed ice extent in March, for example, can be smaller than the maximum observed ice extent in September. Oceanographic phenomena such as the semi-permanent lee polynyas found west and south-west of Kvitøya and Franz Josef Land and the bay of open water, the “Whalers Bay”, north of the Spitsbergen are structures which can change with time intervals of hours to decades. For example, the polynya south of Franz Josef Land was clearly evident in 1995 but was only seen for a short period in 1996. The observed variability in physical conditions directly affects the primary production in the MIZ. From early spring, solar radiation penetrates both leads and the ice itself, initiating algal production under the ice. Light measurements showed that the melt ponds act as windows, permitting the transmission of incoming solar radiation through to the underlying sea ice, thus, accelerating the melting process and enhancing the under-ice primary production. In June 1995, the N–S transect went through a pre-bloom area well inside the ice-covered part of the Barents Sea to a post-bloom phase in the open waters south of the ice edge. The biological conditions in the later season (August) of 1996 were considerably more variable. The longer N–S transect in August 1996 passed through areas with variable ice and oceanographic conditions, and different developmental stages of phytoplankton blooms were encountered. The previously adapted picture of a plankton bloom following the retreating ice edge northwards was not seen.