ALBERT

Description: Turbulent fluxes of momentum and sensible heat were estimated from sonic anemometer measurements gathered over the Labrador Sea during a winter cruise of the R/V Knorr. The inertial dissipation method was used to calculate turbulent fluxes of momentum. The resulting drag coefficients agree well with earlier findings. Sensible heat fluxes were computed using both cross-correlation and inertial dissipation techniques. There is good agreement between results from both methods, although there is more scatter in the correlation fluxes than the dissipation fluxes. The inertial dissipation method gives reasonable results even under conditions of high wind speeds and low air temperatures, which combined with the relatively warm sea surface temperatures lead to sensible heat fluxes of several hundred watts per square meter. Sensible heat fluxes obtained from the sonic anemometer measurements agree well with bulk turbulent fluxes according to the formulation of Isemer and Hasse.

Description: In the autumn of 1996 the field component of an experiment designed to observe water mass transformation began in the Labrador Sea. Intense observations of ocean convection were taken in the following two winters. The purpose of the experiment was, by a combination of meteorological and oceanographic field observations, laboratory studies, theory, and modeling, to improve understanding of the convective process in the ocean and its representation in models. The dataset that has been gathered far exceeds previous efforts to observe the convective process anywhere in the ocean, both in its scope and range of techniques deployed. Combined with a comprehensive set of meteorological and air-sea flux measurements, it is giving unprecedented insights into the dynamics and thermodynamics of a closely coupled, semienclosed system known to have direct influence on the processes that control global climate.

Description: Comparisons are made between a time series of meteorological surface layer observational data taken on board the R/V Knorr, and model analysis data from the European Centre for Medium-Range Weather Forecasting (ECMWF) and the National Centers for Environmental Prediction (NCEP). The observational data were gathered during a winter cruise of the R/V Knorr, from 6 February to 13 March 1997, as part of the Labrador Sea Deep Convection Experiment. The surface layer observations generally compare well with both model representations of the wintertime atmosphere. The biases that exist are mainly related to discrepancies in the sea surface temperature or the relative humidity of the analyses. The surface layer observations are used to generate bulk estimates of the surface momentum flux, and the surface sensible and latent heat fluxes. These are then compared with the model-generated turbulent surface fluxes. The ECMWF surface sensible and latent heat flux time series compare reasonably well, with overestimates of only 13% and 10%, respectively. In contrast, the NCEP model overestimates the bulk fluxes by 51% and 27%, respectively. The differences between the bulk estimates and those of the two models are due to different surface heat flux algorithms. It is shown that the roughness length formula used in the NCEP reanalysis project is inappropriate for moderate to high wind speeds. Its failings are acute for situations of large air–sea temperature difference and high wind speed, that is, for areas of high sensible heat fluxes such as the Labrador Sea, the Norwegian Sea, the Gulf Stream, and the Kuroshio. The new operational NCEP bulk algorithm is found to be more appropriate for such areas. It is concluded that surface turbulent flux fields from the ECMWF are within the bounds of observational uncertainty and therefore suitable for driving ocean models. This is in contrast to the surface flux fields from the NCEP reanalysis project, where the application of a more suitable algorithm to the model surface-layer meteorological data is recommended

Description: European Centre for Medium-Range Weather Forecasts (ECMWF) ERA-Interim reanalysis total precipitation estimates are validated against ten years of in situ precipitation measurements onboard of ships over the Baltic Sea. A statistical analysis for binary forecasts and mean rain rates derived from all data show a good agreement with observations. However, a closer look reveals an underestimation of ERA-Interim total precipitation in spring and an overestimation in autumn, obviously related to stability. Deriving stability and evaporation by a bulk flux scheme it could be shown, in fact, that ERA-Interim underestimates precipitation for conditions with low evaporation and strongly overestimates it for conditions with high evaporation. Since ERA-Interim surface fields become too dry with increasing evaporation compared to independent synoptic ship observations, uncertainties in the ECMWF convection scheme may possibly cause these biases in seasonal precipitation.

Description: Latent heat fluxes (LHF) play an essential role in the global energy budget and are thus important for understanding the climate system. Satellite-based remote sensing permits a large-scale determination of LHF, which, amongst others, are based on near-surface specific humidity qa. However, the qa random retrieval error (Etot) remains unknown. Here, a novel approach is presented to quantify the error contributions to pixel-level qa of the Hamburg Ocean Atmosphere Parameters and Fluxes from Satellite (HOAPS, version 3.2) dataset. The methodology makes use of multiple triple collocation (MTC) analysis between 1995-2008 over the global ice-free oceans. Apart from satellite records, these datasets include selected ship records extracted from the Seewetteramt Hamburg (SWA) archive and the International Comprehensive Ocean-Atmosphere Data Set (ICOADS), serving as the in-situ ground reference. The MTC approach permits the derivation of Etot as the sum of model uncertainty EM and sensor noise EN, while random uncertainties due to in-situ measurement errors (Eins) and collocation (EC) are isolated concurrently. Results show an Etot average of 1.1 ± 0.3 g kg-1, whereas the mean EC (Eins) is in the order of 0.5 ± 0.1 g kg-1 (0.5 ± 0.3 g kg-1). Regional analyses indicate a maximum of Etot exceeding 1.5 g kg-1 within humidity regimes of 12-17 g kg-1, associated with the single-parameter, multilinear qa retrieval applied in HOAPS. Multi-dimensional bias analysis reveals that global maxima are located off the Arabian Peninsula.

Description: Sensible and latent heat fluxes were estimated from turbulence measurements gathered during several Atlantic transects of the R/V Polarstern. The inertial dissipation method was used to analyze the data. Resulting bulk transfer coefficients were then applied to the data from the ship’s meteorological system to get continuous time series of the heat fluxes. Combined to the measured downward solar and longwave radiation fluxes allows for an estimate of the total energy budget at the air-sea interface. Comparing these parameterized energy fluxes to ones based on the COARE 3.0 bulk flux algorithm show very strong agreement.

Description: The satellite-derived HOAPS (Hamburg Ocean Atmosphere Parameters and Fluxes from Satellite Data) precipitation estimates have been validated against in-situ precipitation measurements from optical disdrometers, available from OceanRAIN (Ocean Rainfall And Ice-phase precipitation measurement Network) over the open-ocean by applying a statistical analysis for binary estimates. In addition to using directly collocated pairs of data, collocated data were merged within a certain temporal and spatial threshold into single events, according to the observation times. Although binary statistics do not show perfect agreement, simulations of areal estimates from the observations themselves indicate a reasonable performance of HOAPS to detect rain. However, there are deficits at low and mid-latitudes. Weaknesses also occur when analyzing the mean precipitation rates; HOAPS underperforms in the area of the intertropical convergence zone, where OceanRAIN observations show the highest mean precipitation rates. Histograms indicate that this is due to an underestimation of the frequency of moderate to high precipitation rates by HOAPS, which cannot be explained by areal averaging.

Description: The Baltic Sea Experiment (BALTEX) is one of the five continental–scale experiments of the Global Energy and Water Cycle Experiment (GEWEX). More than 50 research groups from 14 European countries are participating in this project to measure and model the energy and water cycle over the large drainage basin of the Baltic Sea in northern Europe. BALTEX aims to provide a better understanding of the processes of the climate system and to improve and to validate the water cycle in regional numerical models for weather forecasting and climate studies. A major effort is undertaken to couple interactively the atmosphere with the vegetated continental surfaces and the Baltic Sea including its sea ice. The intensive observational and modeling phase BRIDGE, which is a contribution to the Coordinated Enhanced Observing Period of GEWEX, will provide enhanced datasets for the period October 1999–February 2002 to validate numerical models and satellite products. Major achievements have been obtained in an improved understanding of related exchange processes. For the first time an interactive atmosphere–ocean–land surface model for the Baltic Sea was tested. This paper reports on major activities and some results.

Description: The satellite derived HOAPS (Hamburg Ocean Atmosphere Parameters and Fluxes from Satellite data) and ECMWF (European Centre for Medium-Range Weather Forecasts) ERA-Interim reanalysis data sets have been validated against in-situ precipitation measurements from ship rain gauges and optical disdrometers over the open-ocean by applying a statistical analysis for binary forecasts. For this purpose collocated pairs of data were merged within a certain temporal and spatial threshold into single events, according to the satellites' overpass, the observation and the forecast times. HOAPS detects the frequency of precipitation well, while ERA-Interim strongly overestimates it, especially in the tropics and subtropics. Although precipitation rates are difficult to compare because along-track point measurements are collocated with areal estimates and the numbers of available data are limited, we find that HOAPS underestimates precipitation rates, while ERA-Interim's Atlantic-wide average precipitation rate is close to measurements. However, regionally averaged over latitudinal belts, there are deviations between the observed mean precipitation rates and ERA-Interim. The most obvious ERA-Interim feature is an overestimation of precipitation in the area of the intertropical convergence zone and the southern sub-tropics over the Atlantic Ocean. For a limited number of snow measurements by optical disdrometers it can be concluded that both HOAPS and ERA-Interim are suitable to detect the occurrence of solid precipitation.

Description: Rain rates measured onboard ships, merchant ships and research vessels, by ship rain gauges. Data are gained over the Baltic Sea area, measurement intervals are 8 min.