Ship Observations Research Articles

Spatiotemporal Variability of West African Rainfall. Part II: Associated Surface and Airmass Characteristics

In the second part of this study, the variability of surface and airmass characteristics associated with West African rainfall fields is investigated. Ship observations over the tropical Atlantic and rawinsonde records over West Africa are used for the months of July, August, and September from 1948 to 1978. A composite analysis based on the sign of monthly rainfall departures north of 10°N indicates that the Sahelian drought may result from a more southern than normal position of the intertropical convergence zone (ITCZ) over the tropical Atlantic. A composite analysis into four monthly rainfall types is performed by using rainfall information north and south of 10°N. It shows two types of Sahelian drought. The first is linked to an enhanced surface atmospheric circulation over the northern tropical Atlantic, higher sea surface temperatures in the Gulf of Guinea, a convergent moisture flux anomaly south of 10°N along the western African coast, and a lower (larger) water content in the monsoon flux north (south) of 10°N. It may be associated with a southward location of the ITCZ and of the Hadley cells. The second is related to positive sea level pressure and surface easterly circulation anomalies over the northern tropical Atlantic and weaker convection inside the ITCZ over West Africa. It may be associated with an enhancement and/or an eastward and equatorward displacement of the subsiding branch of the Atlantic Walker cell.

Comparison of satellite and ship observations for total cloud amount

The data series of monthly clouldiness over global ocean from COADS was compared with that of from satellite Nimbus-7 during April 1979 to March 1985. The correspondence between them is good. Both the two methods of observation can provide useful information of the distribution of cloudiness and the two data sets can be mutually complementary.

Latent Heat Flux over the North Atlantic Ocean—A Case Study

Nimbus-7 SMMR data and ship observations are combined to compute the latent heat flux using the bulk aerodynamic method. Sea surface temperature (SST) and the surface humidity are determined with the microwave data. The surface wind field is derived from an analysis of ship observations of wind speed and surface pressure by means of a boundary-layer model by Bumke and Hasse. The microwave-derived SSTs are calibrated against those calculated from Advanced Very High-Resolution Radiometer (AVHRR) data. To get reliable results in the northern parts of the North Atlantic, only ascending (daytime) orbits of Nimbus-7 were used. Daytime data show a larger bias due to solar heating of the instrument but lack the complicating effects of differential cooling when the satellite enters the earth's shadow at the beginning of the descending orbits. The evaporation fields are derived over the North Atlantic for individual overpasses of the satellite during July 1983, with a spatial resolution of 1° × 1°. High temporal and spatial gradients are observed, which are consistent with the prevailing synoptic situations. In the area south of Greenland and east of Canada, where the Labrador Current is located, latent heat flux (LE) is negative even in the monthly mean. The reliability of the negative values is demonstrated by a case study. They coincide well with ship observations of fog events. The flux of latent heat can be determined with an acceptable accuracy of 25–40 W m−2 for individual values if the bias of the SMMR data can be reliably removed.

Estimation of new production in the ocean by compound remote sensing

Oceanic new production can be estimated from remotely sensed data on ocean colour and temperature. This approach, which depends on parameterizations developed from ship observations, as well as on satellite data, yields more representative estimates of the large-scale average new production than those calculated from ship data alone.

Derivation of ocean—Atmosphere heat fluxes in a tropical environment using satellite and surface data

AbstractOcean—atmosphere heat fluxes measured at John Brewer Reef (80 km north‐north‐east of Townsville, Australia) were compared with regional heat fluxes estimated from non‐direct surface data and information from a number of satellite sensors. Data from the visible channel of the Japanese Geostationary Meteorological Satellite (CMS) were collected for the location, as well as daily atmospheric soundings from NOAA‐9. Sea‐surface temperatures were obtained from a blend of ship observations and NOAA‐9 data. A relationship was developed that estimated solar radiation from GMS visible reflectance data. Incoming longwave radiation was estimated with the LOWTRAN 6 radiative transfer model applied to profiles of temperature and humidity derived from NOAA‐9 soundings. The bulk aerodynamic formulation was used to form regional estimates of the turbulent heat fluxes, combining wind speed from the Darwin Tropical Wind Analysis, blended sea‐surface temperatures, and near‐surface specific humidity related to satellite‐derived precipitable water vapour. A variety of approaches for estimating the near‐surface specific humidity were examined. The root mean square differences (measured—estimated) between the weekly averaged values of the net solar radiation flux, the net longwave radiation flux, the total turbulent heat flux, and the total ocean—atmosphere heat flux were estimated as 12, 10, 45 and 50 Wm−2 respectively. Time series of the weekly heat fluxes are presented for the period June 1985 to April 1986.

A study of the use of scatterometer data in the European Centre for Medium‐Range Weather Forecasts operational analysis‐forecast model: 1. Quality assurance and validation

This paper is Part 1 of a study concerned with the feasibility of using wind scatterometer data in a real‐time data assimilation system. The potential that such a system offers for quality assurance and validation of scatterometer data is illustrated using Seasat A satellite scatterometer (SASS) data. The results of passive assimilations of scatterometer data (where the SASS data are passed to the assimilation system but not used), and of active assimilations of scatterometer data (where the data are used), are presented. It is shown that assimilation allows validation and quality assurance of the scatterometer data, through comparisons with collocated ship data and through comparison with the wind fields generated by the assimilation. Intercomparison can be carried out quickly and efficiently at an operational weather centre where large amounts of data are collected, so allowing comprehensive quality assurance and validation of wind scatterometer data to be carried out in near‐real‐time. A number of comparisons are made which illustrate a speed‐dependent bias between SASS and ship observations and between SASS and forecast model, as well as directional irregularities in the SASS data.

海洋大気への土壌粒子および汚染物質の輸送移動とその濃度分布 南極観測船「しらせ」の航路上における海洋大気調査

1988年11月に日本を出航した南極観測船「しらせ」の航路上において海洋大気中の浮遊粉じんを採取しyその分析結果から土壌粒子および汚染物質の海洋大気への輸送移動とその濃度分布について検討を行った。その結果,海洋大気中の土壌起源元素は,東アジアに近い西部太平洋では比較的高濃度となる傾向が認められ,陸地から海洋大気へと土壌粒子が輸送されていることがわかった。一方,陸地がほとんど存在しないインド洋から南極近海では,土壌起源元素の大気濃度はAl,Feで数 ng/m3, Siで10ng/m3と極めて低く,バックグラウンド濃度と言える。また,日本近海,東南アジア周辺の海域では・石漉燃焼などによる人為的起源の硫黄の影響がかなり大きいことがわかった。

An Example of the Importance of Ship Observations

Abstract The impact of one ship observation on the National Meteorological Center's (NMC) North Atlantic surface pressure analysis, forecast models, and various forecasts is discussed. In addition, the resources available to surface pressure analysts at NMC in determining the validity of ship observations are described. Based, primarily, on one ship observation, the analyzed central pressure of a low pressure system was deepened by 39 mb in 6 h. This dramatic change in central pressure impacted the NMC's North Atlantic High Seas forecasts, High-Level Aviation turbulence forecasts and the validity of the results of the NMC Medium Range Forecast (MRF) Model. It is suggested that the dramatic 6-h change in the analyzed central pressure of the low pressure system was most likely due to an initial underestimate of the depth of the low prior to the receipt of the specific ship observation and an overestimation of the central pressure as a result of the ship observation.

Large-Scale Atmospheric Circulation Features of Warm and Cold Episodes in the Tropical Pacific

Ship observations of sea surface temperature (SST), sea level pressure and surface wind, and satellite measurements of outgoing longwave radiation (OLR) (an indicator of deep tropical convection) are used to describe the large-scale atmospheric circulation over the tropical Pacific during composite warm and cold episodes. Results are based on linear regression analysis between the circulation parameters and an index of SST in the tropical Pacific during the period 1946–85 (1974–89 for OLR). Warm episodes along the Peru coast (i.e., El Niño events) and basin-wide warmings associated with the Southern Oscillation are examined separately. Charts of the total as well as anomalous fields of SST, sea level pressure, surface wind and OLR for both warm and cold episodes are presented. SST and surface wind anomalies associated with warm episodes are consistent with the results of Rasmusson and Carpenter (1982). El Niño events are characterized by strong positive SST anomalies along the coasts of Ecuador and Peru and along the equator eastward of 130°W, and by an equatorward expansion and intensification of the Inter Tropical Convergence Zone (ITCZ) over the eastern Pacific. Basin-wide warm episodes exhibit positive SST anomalies along the equator eastward of 170°E, a southward expansion and intensification of the ITCZ, and an eastward shift and strengthening of the Indonesian convective zone. The movements of the precipitation zones are in good agreement with anomalous large scale surface wind convergence, Meridional wind anomalies dominate the anomalous surface convergence throughout the tropical Pacific. Surface winds are consistent with the sea level pressure distribution, with down-gradient flow near the equator, and with Ekman balance in the subtropics. A center of below normal sea level pressure over the equatorial eastern Pacific, distinct from the negative pressure anomalies over the subtropical southeast Pacific, is observed during basin-wide warm episodes. This equatorial feature is highly correlated with local SST and appears to be a boundary layer phenomenon. There is a net increase in deep convection over the tropical Pacific during warm episodes. Enhanced convection in the ITCZ during warm years is not accompanied by a net increase in surface wind convergence. A comparison between precipitation and surface wind convergence suggests that moisture convergence extends through a deeper layer in the equatorial western Pacific than in the ITCZ over the eastern Pacific. The contrasting distributions of surface relative humidity, total cloudiness and air-sea temperature difference over the eastern tropical Pacific during basin-wide warm and cold episodes are described in the context of boundary layer processes.

Dynamic sea surface topography, gravity, and improved orbit accuracies from the direct evaluation of Seasat altimeter data

A gravitational model incorporating Seasat altimetry, surface gravimetry, and satellite tracking data has been determined in terms of global spherical harmonics complete to degree and order 50. This model, PGS‐3337, uses altimeter data as a dynamic observation of the satellite's height above the sea surface. A solution for the ocean's dynamic topography is recovered simultaneously with the orbit parameters, gravity, and ocean tidal terms. The recovered dynamic topography reveals the global long wavelength circulation of the oceans with a resolution of 2000 km and is very similar to the mean upper ocean dynamic height derived from historical ship observations. The PGS‐3337 geoid has an uncertainty of 60 cm rms globally but only 25 cm rms over the ocean because of the altimeter measurements. Seasat orbits determined in this solution have an estimated accuracy for the radial position of 20 cm rms. The difference between the altimeter observed sea height and the geoid plus dynamic topography model is 30 cm rms. Contained in these residuals are the sea height variability, as well as errors from the geoid, orbits, tidal models, and altimeter range measurement. This performance level is 2 to 3 times better than that achieved with previous Goddard gravitational models.

A comparison of ship and coastal zone color scanner mapped distribution of phytoplankton in the southeastern Bering Sea

We examined 21 Coastal Zone Color Scanner (CZCS) images of the southeastern Bering Sea to map the near‐surface distribution of phytoplankton during 1979 and 1980 and compared this information with the mesoscale (100–1000 km) distribution of phytoplankton inferred from ship sampling. Pigment patches coherent over length scales of about 1000 km along isobaths were detected in the satellite data. The patches straddled boundaries of hydrographic domains that were previously defined from the ship observations. Smaller patches (order of 100 km diameter and less) were common. The imagery indicated that open‐water phytoplankton blooms occur first in late April in coastal waters, peak in early May over the middle shelf, and decay rapidly afterwards, reaching concentration minima in June in both regions. These patters show that earlier ship observations are valid for most of the eastern Bering shelf. Images from August and September 1979 revealed the occurrence of fall blooms, previously unrecorded in this area perhaps because of scarce ship observations after summer. Interannual variation in the distribution of biomass was evident, with April–May 1979 showing less pigment near the surface than April–May 1980. These changes were in part related to aliased sampling by the CZCS. Also, the wind was downwelling favorable in late April 1979 but upwelling favorable during late April 1980. CZCS values underestimated the ship data by a factor of at least 2 during spring 1979. During spring of 1980 the CZCS and ship‐based data were similar. The apparent errors remain unexplained, but it appears that in particular time aliasing of the evolution of the blooms by the CZCS leads to serious problems in comparisons with ship time series. It also seems that region‐specific pigment algorithms are required for proper quantitative interpretation of remote sensing data from the Bering Sea.

Decadal‐scale changes of the circulation in the tropical atlantic sector associated with Sahel drought

AbstractTrends in the general circulation setting during 1948–1983 accompanying the progressive aggravation of drought in sub‐Saharan Africa are investigated from ship observations over the tropical Atlantic and surface station records in western Africa.Decadal‐scale circulation changes in the Atlantic sector include a pressure rise over the tropical North Atlantic; southward displacement of the near‐equatorial wind confluence and associated maximum cloud belt; acceleration of the north‐east trades and possibly a strengthening of the South Atlantic trades, along with increasing cloudiness in the equatorial zone; increasing cloudiness over the tropical North Atlantic and Central American seas; and cooling of surface waters in a band across the tropical North Atlantic contrasting with warming in the South Atlantic. These changes occurred in all seasons, but were most pronounced at the height of boreal summer.Records at land stations in western Africa show that near the coasts the temperature and pressure trends are consistent with the adjacent ocean areas. In the interior of sub‐Saharan Africa, warming trends prevail in boreal summer, presumably as a direct consequence of reduced inflow of cool maritime monsoon air; the concomitant downward pressure trends at continental stations in sub‐Saharan Africa may reflect the hydrostatic effect of increasing temperature. In contrast to boreal summer, slight cooling trends and pressure rises prevail in the interior during winter.In the course of the past four decades, the near‐equatorial wind confluence over the eastern Atlantic migrated southward by 200 km in July–August and 150 km for the rainy season as a whole. Given the steep meridional precipitation gradients across the Sahel, these circulation shifts are large enough to account for the observed downward trend in Sahel rainfall.

Comparison of ship and aerial surveys of sardine schools

Field tests were made to compare estimates of sardine schools resulting from simultaneous ship and aerial observations. The aeriallship observations were performed around an area where large catches of sardine have been taken. In the surveyed area, the sardine schools can be sighted from the air and counted easily, making direct counting very advantageous. The schools are distinguished by their dark color against the lighter background of the ocean. A comparison of density estimated with the two types of survey are discussed here. Aerial counts of schools were 16.5% of those from the ship. This may be caused by the differences in visibility afforded by the two methods. Direct detection of schools and counting may be used more widely in the future.

Climatology comparison and long‐term variations of sea surface temperature over the tropical Atlantic Ocean

The primary aim of the present study is to compare two sea surface temperature (SST) climatologies produced from merchant ship observations over the tropical Atlantic Ocean. The first climatology uses measurements taken between 1911 and 1972. The second uses 1964–1984 data. The second climatology is generally warmer by a few tenths of degrees. Part of the difference is due to changes in the temperature measurement techniques with time. The difference also shows, especially along shipping routes, some spatial discrepancies which are not due to instrument changes. It is demonstrated the difference is mostly caused by inhomogeneity in the spatial and temporal density of the available observations used to construct the first climatology. Using a simple technique to eliminate spurious bias, the analysis allows isolation of the long‐term trend of SST anomalies in the tropical Atlantic Ocean during this century. South of the thermal equator, a warming trend occurs principally during boreal summers and extends to the present with an increasing range when calculated over the past 26‐year period. In the north, such a warming is more questionable between the 1940s and 1970s. It is more evident during the past 26‐year period but with a temperature range less than half of that for the southern basin. Thus large abnormal patterns have a tendency to straddle the thermal equator, including a north‐south climate dipole. Three spatially limited areas with a cooling trend are suggested. The reliability of SST anomalies along the tropical section of the Europe‐South America shipping track in recent years is also discussed.

Convection and rainfall over Mexico and their modulation by the Southern Oscillation

AbstractThe role of the Southern Oscillation (SO) in the interannual variability of convection and rainfall over Mexico is studied by correlation analysis of long‐term precipitation records, a satellite‐derived set of highly reflective clouds (HRC), upper‐air soundings, surface ship observations, and an SO index defined as high for anomalously high low pressure at Tahiti/Darwin.Highly reflective clouds and rainfall have a similar annual cycle and interannual variability, except in certain regions and seasons with prevalence of stratiform cloudiness. During the boreal winter (November‐April) dry season, the low SO phase (or El NiñMo) is, in much of the country, characterized by increased precipitation associated with the enhanced influence of mid‐latitude westerlies. However, in the Isthmus of Tehuantepec, most exposed to Norte invasions, precipitation is larger in the high SO phase. During the boreal summer (May‐October) rainy season, precipitation is more abundant in the high SO phase, when the northward retraction of the North Atlantic High, weaker trades, and a northward displaced near‐equatorial trough are conducive to enhanced ascending motion over the area.

Surface Winds from Tropical Pacific Islands—Climatological Statistics

Abstract Multidecadal time series of surface wind observations from tropical Pacific islands have been examined in order to investigate the space and time scales of variability. Climatological monthly means and variances are compared with comparable means and variances derived from ship observations; usually the means agree to within ∼1 m s−1 in speed and ∼10 degrees in direction. Annual and semiannual cycles differ in detail. Island zonal wind variances are often significantly larger, by up to 10 m2 s−2 near the equator between September and December; because of the spatial coherence of the island results, these discrepancies are believed to result from the poor high-frequency sampling typical of ship data. A substantial near-equatorial zonal wind variance maximum is shown to be related to ENSO period variability; excluding ENSO time periods leaves a relatively spatially uniform variance of ∼5 m2 s−2 over a broad region. The frequency distribution of variance, derived from daily-averaged data, exhibits c...

A Rapid Cyclogenesis Event during GALE IOP 9

A cyclone that developed explosively during Intensive Observation Period (IOP) 9 of the Genesis of Atlantic Lows Experiment (GALE) is studied. Detailed surface analysis is conducted based on operationally available data, late reporting ship observations and GALE special observations to determine the surface storm track and deepening rate. GALE dropsonde and rawinsonde data are used to supplement the normal upper-level database, and are analyzed by a research version of the Navy Operational Regional Analysis and Prediction System (NORAPS) using optimal interpolation analysis. These analyses reveal critical subsynoptic features important in the development. Two surface lows are present during the early coastal development period. The western center is coupled to a mobile 500 mb short-wave trough while the eastern center develops in a strong baroclinic zone offshore. The objective analyses also show a strengthening of a jet streak east of the mobile short wave. The divergent quadrant of this jet streak induces upward vertical motion over the eastern of the two coastal low systems. The rapid development of the eastern center occurs due to the superposition of the upper-level forcing (jet streak) over the low-level perturbation with strong thermal adymion. Dropwindsonde data document the low static stability in the region. NORAPS operational and GALE data forecasts from 1200 UTC 24 February erroneously deepen the western center and result in track errors of 300 to 600 km. The GALE forecast from 0000 UTC 25 February deepens the correct center and makes the best track forecast. All forecasts fail to predict the full extent of the rapid development of this cyclone.

A Rapid Cyclogenesis Event during GALE IOP 9

A cyclone that developed explosively during Intensive Observation Period (IOP) 9 of the Genesis of Atlantic Lows Experiment (GALE) is studied. Detailed surface analysis is conducted based on operationally available data, late reporting ship observations and GALE special observations to determine the surface storm track and deepening rate. GALE dropsonde and rawinsonde data are used to supplement the normal upper-level database, and are analyzed by a research version of the Navy Operational Regional Analysis and Prediction System (NORAPS) using optimal interpolation analysis. These analyses reveal critical subsynoptic features important in the development. Two surface lows are present during the early coastal development period. The western center is coupled to a mobile 500 mb short-wave trough while the eastern center develops in a strong baroclinic zone offshore. The objective analyses also show a strengthening of a jet streak east of the mobile short wave. The divergent quadrant of this jet streak induces upward vertical motion over the eastern of the two coastal low systems. The rapid development of the eastern center occurs due to the superposition of the upper-level forcing (jet streak) over the low-level perturbation with strong thermal adymion. Dropwindsonde data document the low static stability in the region. NORAPS operational and GALE data forecasts from 1200 UTC 24 February erroneously deepen the western center and result in track errors of 300 to 600 km. The GALE forecast from 0000 UTC 25 February deepens the correct center and makes the best track forecast. All forecasts fail to predict the full extent of the rapid development of this cyclone.

Annual Cycle and Long-Term Trends of Circulation and Climate Variability over the Tropical Oceans

Abstract Trends in the large-scale circulation and seasonal characteristics of climate variability are investigated based on ship observations of sea level pressure, zonal and meridional wind components, sea surface temperature, and cloudiness and land station records, during 1948–83. Cluster analysis captures such features as the surface pressure dipoles associated with the Southern Oscillation, the subtropical region of influence of the North Atlantic Oscillation, preferred areas of surges in the North Atlantic trades and monsoon over the Indian Ocean, equatorial warm water regions with El Nino-type behavior in the eastern Pacific and Atlantic, and typical centers of interannual cloudiness variations associated with trade wind variations and the ITCZ. Long-term trends are examined in terms of internal consistency between elements, between ship and land records, and in regard to the possible effect of shifts in observational practices. Most prominent are the following trends over the 1948–83 period: (i) ...

Circulation Regimes of Rainfall Anomalies in the African-South Asian Monsoon Belt

Abstract This study explores the spatial differentiation of climate anomalies and associated circulation mechanisms across the African-South Asian monsoon belt through empirical analyses mainly for the period 1948–83. Observations include surface ship observations in the tropical Atlantic, eastern Pacific, and Indian oceans, and various hydrometeorological and circulation index series, representing the water discharge of the Senegal River (SENEGAL), the rainfall in the West African Sahel (SAHEL), the discharge of rivers in the Nile basin (ROSEIRES, ATBARA), India monsoon rainfall (NIR), and the Southern Oscillation (SO). The field significance of correlation patterns is ascertained through Monte Carlo experiments. There is a strong correlation of hydrometeorological conditions from Senegal to the Sahel, a decrease from each of these domains eastward to the Nile catchment, and even more so to India. Conversely, correlations are remarkably high between the water discharge of the Nile basin (ROSEIRES, ATBARA...