Showing papers by "Robert Evans published in 2009"

The GODAE High-Resolution Sea Surface Temperature Pilot Project

Abstract: Sea surface temperature (SST) measurements are required by operational ocean and atmospheric forecasting systems to constrain modeled upper ocean circulation and thermal structure. The Global Ocean Data Assimilation Experiment (GODAE) High Resolution SST Pilot Project (GHRSST-PP) was initiated to address these needs by coordinating the provision of accurate, high-resolution, SST products for the global domain. The pilot project is now complete, but activities continue within the Group for High Resolution SST (GHRSST). The pilot project focused on harmonizing diverse satellite and in situ data streams that were indexed, processed, quality controlled, analyzed, and documented within a Regional/Global Task Sharing (R/GTS) framework implemented in an internationally distributed manner. Data with meaningful error estimates developed within GHRSST are provided by services within R/GTS. Currently, several terabytes of data are processed at international centers daily, creating more than 25 gigabytes of product. Ensemble SST analyses together with anomaly SST outputs are generated each day, providing confidence in SST analyses via diagnostic outputs. Diagnostic data sets are generated and Web interfaces are provided to monitor the quality of observation and analysis products. GHRSST research and development projects continue to tackle problems of instrument calibration, algorithm development, diurnal variability, skin temperature deviation, and validation/verification of GHRSST products. GHRSST also works closely with applications and users, providing a forum for discussion and feedback between SST users and producers on a regular basis. All data within the GHRSST R/GTS framework are freely available. This paper reviews the progress of GHRSST-PP, highlighting achievements that have been fundamental to the success of the pilot project.

Satellite observations of the impact of dust in a low‐nutrient, low‐chlorophyll region: Fertilization or artifact?

Abstract: [1] The coupling between dust aerosols and phytoplankton concentrations in the Mediterranean Sea, a low-nutrient, low-chlorophyll region, is examined at different timescales using Sea-viewing Wide Field-of-view Sensor observations (1998–2002). Aerosol optical thickness (AOT) and chlorophyll (MCHL) were used as proxies for dust aerosol and phytoplankton biomass, respectively. The AOT data was qualified using quasi-true color images in order to ascertain the presence of dust. Strong positive correlations were found between AOT and MCHL on weekly but not on seasonal timescales. However, weekly analyses cannot distinguish between real phytoplankton response and artifacts due to residual dust in the atmosphere or water. Daily time series of AOT and MCHL, for single-dust events, and their temporal cross-correlation function were analyzed. Apparent AOT-driven MCHL increases principally occurred within 0–2 days and most cross correlations were significant at zero lag. In contrast, significant negative or positive correlations at lag greater than 2 days were very few, indicating no compelling evidence that dust enhancement of phytoplankton growth is significant and that the response at near-zero lags is an artifact of the satellite data processing. Our analysis demonstrates that the dust fertilization does not play a significant role in the sustainment of the phytoplankton dynamics in the Mediterranean Sea.

Solvent mediated interactions between model colloids and interfaces: a microscopic approach.

Abstract: We determine the solvent mediated contribution to the effective potentials for model colloidal or nanoparticles dispersed in a binary solvent that exhibits fluid-fluid phase separation. The interactions between the solvent particles are taken to be purely repulsive point Yukawa pair potentials. Using a simple density functional theory we calculate the density profiles of both solvent species in the presence of the “colloids,” which are treated as external potentials, and determine the solvent mediated (SM) potentials. Specifically, we calculate SM potentials between (i) two colloids, (ii) a colloid and a planar fluid-fluidinterface, and (iii) a colloid and a planar wall with an adsorbed wetting film. We consider three different types of colloidal particles: Colloid A that prefers the bulk solvent phase rich in species 2, colloid C that prefers the solvent phase rich in species 1, and “neutral” colloid B that has no strong preference for either phase, i.e., the free energies to insert the colloid into either of the coexisting bulk phases are almost equal. When a colloid that has a preference for one of the two solvent phases is inserted into the disfavored phase at state points close to coexistence a thick adsorbed “wetting” film of the preferred phase may form around the colloids. The presence of the adsorbed film has a profound influence on the form of the SM potentials. In case (i) reducing the separation between the two colloids of type A leads to a bridging transition whereby the two adsorbed films connect abruptly and form a single fluid bridge. The SM potential is strongly attractive in the bridged configuration. A similar phenomenon occurs in case (iii) whereby the thick adsorbed film on colloid A and that at the planar wall, which prefers the same phase as colloid A, connect as the separation between the colloid and the wall is reduced. In both cases the bridging transition is accompanied, in this mean-field treatment, by a discontinuity of the SM force. On the other hand, for the same wall, and a colloid of type C, the SM potential is strongly repulsive at small separations. For case (ii), inserting a single colloidal particle near the planar fluid-fluidinterface of the solvent, the density profiles of the solvent show that the interface distortion depends strongly on the nature of the colloid-solvent interactions. When the interface disconnects from the colloid there is, once again, a discontinuity in the SM force.

Pair correlation function decay in models of simple fluids that contain dispersion interactions

Abstract: We investigate the intermediate-and longest-range decay of the total pair correlation function h(r) in model fluids where the inter-particle potential decays as −r−6, as is appropriate to real fluids in which dispersion forces govern the attraction between particles. It is well-known that such interactions give rise to a term in q3 in the expansion of , the Fourier transform of the direct correlation function. Here we show that the presence of the r−6 tail changes significantly the analytic structure of from that found in models where the inter-particle potential is short ranged. In particular the pure imaginary pole at q = iα0, which generates monotonic-exponential decay of rh(r) in the short-ranged case, is replaced by a complex (pseudo-exponential) pole at q = iα0+α1 whose real part α1 is negative and generally very small in magnitude. Near the critical point α1~−α02 and we show how classical Ornstein–Zernike behaviour of the pair correlation function is recovered on approaching the mean-field critical point. Explicit calculations, based on the random phase approximation, enable us to demonstrate the accuracy of asymptotic formulae for h(r) in all regions of the phase diagram and to determine a pseudo-Fisher–Widom (pFW) line. On the high density side of this line, intermediate-range decay of rh(r) is exponentially damped-oscillatory and the ultimate long-range decay is power-law, proportional to r−6, whereas on the low density side this damped-oscillatory decay is sub-dominant to both monotonic-exponential and power-law decay. Earlier analyses did not identify the pseudo-exponential pole and therefore the existence of the pFW line. Our results enable us to write down the generic wetting potential for a 'real' fluid exhibiting both short-ranged and dispersion interactions. The monotonic-exponential decay of correlations associated with the pseudo-exponential pole introduces additional terms into the wetting potential that are important in determining the existence and order of wetting transitions.

On the heat budget of the Arabian Sea

Abstract: This study analyzes the heat budget of the Arabian Sea using satellite-derived sea-surface temperature (SST) from 1985 to 1995 along with other data sets. For a better understanding of air–sea interaction, canonical average monthly fields representing the spatial and temporal structure of the various components of the heat balance of the Arabian Sea are constructed from up to 30 years of monthly atmospheric and oceanic data. The SST over the Arabian Sea is not uniform and continually evolves with time. Cooling occurs over most of the basin during November through January and May through July, with the greatest cooling in June and July. Warming occurs over most of the basin during the remainder of the year, with the greatest warming occurring in March and September. Results indicate that the sign of the net heat flux is strongly dependent on the location and month. The effects of net heat flux and penetrative solar radiation strongly influence the change in SST during February and are less important during August and September. Horizontal advection acts to cool the sea surface during the northeast monsoon months. During the southwest monsoon horizontal advection of surface waters warms the SST over approximately the southern half of the basin, while the advection of upwelled water from the Somalia and Oman coasts substantially cools the northern basin. The central Arabian Sea during the southwest monsoon is the only area where the change in SST is balanced by the entrainment and turbulent diffusion at the base of the mixed layer. Agreement between the temporal change in the satellite-derived SST and the change calculated from the conservation of heat equation is surprisingly good given the errors in the measured variables and the bulk formula parameters. Throughout the year, monthly results over half of the basin agree within 3°. Considering that the SST changes between 8° and 12° over the year, this means that our results explain from 62% to 75% of the change in SST over 56% of the Arabian Sea. Two major processes contribute to the discrepancy in the change in SST calculated according to the heat budget equation and the change in SST derived from satellite observations. The first is the effect of the horizontal advection term. The position of the major eddies and currents during the southwest monsoon greatly affects the change in SST due to the large gradient in temperature between the cold upwelled waters along the Somali coast to the warm waters in the interior of the basin. The second major process is the thermocline effect. In areas of shallow mixed-layer depth, high insolation and wind speeds of either less than 3 m/s or greater than 15 m/s, the bulk formulae parameterization of the surface heat fluxes is inappropriate.

The GODAE High-Resolution Sea Surface Temperature Pilot Project [In: Special Issue on the Revolution of Global Ocean Forecasting - GODAE: 10 Years of Achievement]

Abstract: Citation for published version: Donlon, CJ, Casey, KS, Robinson, IS, Gentemann, CL, Reynolds, RW, Barton, I, Arino, O, Stark, J, Rayner, N, LeBorgne, P, Poulter, D, Vazquez-Cuervo, J, Armstrong, E, Beggs, H, Llewellyn-Jones, D, Minnett, PJ, Merchant, C & Evans, R 2009, 'The GODAE High-Resolution Sea Surface Temperature Pilot Project', Oceanography, vol. 22, no. 3, pp. 34-45. https://doi.org/10.5670/oceanog.2009.64