Hartmut Grassl

Warm pool heat budget and shortwave cloud forcing: A missing physics?

Ship observations and ocean models indicate that heat export from the mixed layer of the western Pacific warm pool is small (<20 watts per square meter). This value was used to deduce the effect of clouds on the net solar radiation at the sea surface. The inferred magnitude of this shortwave cloud forcing was large (≈ – 100 watts per square meter) and exceeded its observed value at the top of the atmosphere by a factor of about 1.5. This result implies that clouds (at least over the warm pool) reduce net solar radiation at the sea surface not only by reflecting a significant amount back to space, but also by trapping a large amount in the cloudy atmosphere, an inference that is at variance with most model results. The excess cloud absorption, if confirmed, has many climatic implications, including a significant reduction in the required tropics to extrattropics heat transport in the oceans.

Humans transforming the Global Water System

Fresh water figures prominently in the machinery of the Earth system and is key to understanding the full scope of global change. Greenhouse warming with a potentially accelerated hydrologic cycle is already a well-articulated science issue, with strong policy implications. A broad array of other anthropogenic factors—widespread land cover change, engineering of river channels, irrigation and other consumptive losses, aquatic habitat disappearance, and pollution—also influences the water system in direct and important ways. A rich history of site-specific research demonstrates the clear impact of such factors on local environments. Evidence now shows that humans are rapidly intervening in the basic character of the water cycle over much broader domains. The collective significance of these many transformations on both the Earth system and human society remains fundamentally unknown [Framing Committee of the GWSP, 2004].

The dependence of the measured cool skin of the ocean on wind stress and total heat flux

The temperature drop δT between the ocean surface and the 5-cm depth was recorded during GATE, Phase III. With measured values of the total heat flux Q and an assumption about the thickness of the viscous boundary layer of the ocean, the wind-speed dependence of the factor of proportionality between δT and Q is determined. This factor depends on the deviations of the thickness of the conductive layer from the thickness of the viscous layer and possibly partially on the wind stress. A further assumption about the thickness of the conductive layer leads to a wind-speed dependence of the ratio between total wind stress and its wave supporting part of it. This ratio increases from a value 1.5 at 1 m s−1 to 9 at 10 m s−1, which is in agreement with existing estimates.

Gas/particle partitioning and global distribution of polycyclic aromatic hydrocarbons - a modelling approach.

The global atmospheric distribution and long-range transport (LRT) potential of three polycyclic aromatic hydrocarbons (PAH) - anthracene, fluoranthene and benzo[a]pyrene - are studied. The model used is a global aerosol-chemistry-transport-model, which is based on an atmospheric general circulation model. The model includes an in-built dynamic aerosol model coupled to two-dimensional surface compartments. Several parameterisations of gas/particle partitioning and different assumptions of degradation in the aerosol particulate phase were tested. PAHs are mostly distributed in the source regions but reach the Arctic and the Antarctic. The Canadian Arctic is predicted to be significantly less affected by mid-latitude PAH emissions than the European Arctic. Re-volatilisation is significant for semivolatile PAHs. Accumulation of semivolatile PAHs in polar regions, however, is not indicated. The model study suggests that gas/particle partitioning in air drastically influences the atmospheric cycling, the total environmental fate (e.g. compartmental distributions) and the LRT potential of the substances studied. A parameterisation which calculates the gas/particle partitioning assuming absorption into organic matter and adsorption to black carbon (soot) agrees best with the observations at remote sites. The study provides evidence that the degradation in the particulate phase must be slower than that in the gas-phase. The predicted concentrations of the semivolatile PAHs anthracene and fluoranthene in near-ground air at remote sites in mid and high northern latitudes are in line with measured concentrations, if adsorption of the substances to soot combined with absorption in particulate organic matter is assumed to determine gas/particle partitioning, but cannot be explained by adsorption alone (Junge-Pankow parameterisation of gas/particle partitioning). The results suggest that PAHs absorbed in the organic matrix of particulate matter is shielded from the gas-phase.

Evaluation of HOAPS-3 Ocean Surface Freshwater Flux Components

Abstract Today, latent heat flux and precipitation over the global ocean surface can be determined from microwave satellite data as a basis for estimating the related fields of the ocean surface freshwater flux. The Hamburg Ocean Atmosphere Parameters and Fluxes from Satellite Data (HOAPS) is the only generally available satellite-based dataset with consistently derived global fields of both evaporation and precipitation and hence of freshwater flux for the period 1987–2005. This paper presents a comparison of the evaporation E, precipitation P, and the resulting freshwater flux E − P in HOAPS with recently available reference datasets from reanalysis and other satellite observation projects as well as in situ ship measurements. In addition, the humidity and wind speed input parameters for the evaporation are examined to identify sources for differences between the datasets. Results show that the general climatological patterns are reproduced by all datasets. Global mean time series often agree within abo...

Analysis of Thematic Mapper data for studying the suspended matter distribution in the coastal area of the German Bight (North Sea)

Abstract Thematic Mapper data were analyzed with respect to its capability for mapping the complex structure and dynamics of suspeded matter distribution in the coastal area of the German Bight (North Sea). Three independent pieces of information were found by factor analysis of all seven TM channels: suspended matter concentration, atmospheric scattering, and sea surface temperature. For the required atmospheric correction the signal to noise ratios of Channels 5 and 7 have to be improved by averaging over 25×25 pixels, which allows us also to monitor aerosol optical depth and aerosol type over cloudfree water surfaces. Near surface suspended matter concentrations may be detected with an accuracy of factor

Factor analysis of multispectral radiances over coastal and open ocean water based on radiative transfer calculations

Factor analysis is applied to multispectral (seventeen wavelengths) radiances simulated by a radiative transfer model (matrix-operator method) in and above coastal and open ocean waters. The calculated radiances were compared with measured radiances before applying factor analysis. They agree well for different sun elevations and even for turbid coastal waters. The factor analysis technique allows us to extract the characteristic signatures of phytoplankton, suspended matter, and yellow substance. The fluorescence of chlorophyll at lambda = 685 nm is found to be a clear signal for phytoplankton, also in the presence of other suspensions and yellow substance. A comparison of different algorithms for the extraction of the fluorescence peak favors the addition of chlorophyll absorption at lambda = 670 nm. The blue-green ratio is found to be useless for chlorophyll detection in coastal waters. Suspended matter and yellow substance can also clearly be seen in the factor loading for all multispectral radiances analyzed. However, suspended matter is reflected more strongly than yellow substance.

Cloud detection and classification over oceans at night with NOAA-7

Abstract Clouds above the ocean are detected with three thermal-infrared channels of the AVHRR on board NOAA-7. A classification into low, middle and high clouds is possible. As only spectral and not spatial information is used, it is possible to analyse each pixel individually. The classification has been developed with a radiative transfer model for clouds adapted to the spectral domain of the NOAA-7 infrared channels. The ruling parameter is the ratio between scattering and extinction. The spectral optical depth has slightly less influence, while different anisotrophy of scattering for individual types of clouds is not important. Two parameters are best suited to classifying clouds, namely, brightness temperature in channel 5(12·0 μm) and the difference of brightness temperature between channel 3 (3·7μm) and channel 4 (10·8 μm). The algorithm to classify clouds with thresholds referring to the standard atmosphere ‘autumn 45° north’ is applied to actual satellite data.

An advanced algorithm for operational retrieval of water quality from satellite data in the visible

We present a new operational algorithm for the retrieval of water quality from optical remote sensing data for both clear and turbid waters. It contains an array of neural networks providing input for the Levenberg–Marquardt multivariate optimization procedure as the final retrieval tool. With a given accuracy threshold, the developed algorithm is sufficiently robust to data with noise up to 15% for certain hydro‐optical conditions. To avoid inadequate retrieval results, the algorithm identifies and eventually discards the pixels with inadequate atmospheric correction and/or water optical properties incompatible with the applied hydro‐optical model. This procedure also identifies coccolith expressions. Examples of practical applications of the developed algorithm are given.

Change in cloud-top temperatures over Europe

Long-term measurements from Advanced Very High Resolution Radiometer (AVHRR) onboard the National Oceanic and Atmospheric Administration satellites were evaluated to assess variability in cloud-top temperatures over central and eastern Europe that saw radical infrastructural changes after the fall of the East Bloc in 1989 that has affected the pollution levels and hence cloud albedo. Four years in the late 1980s (1985-1988) and in the late 1990s (1997-2000) were chosen, as these are distinctively marked as episodes of very high and lower air pollution (sulphates and particulate matter). During the late 1980s, low- and medium-level clouds were colder by more than 2 K and convective clouds even by 4 K. Cloud-tops over and around polluted regions are higher, and their temperatures showed stronger variability, suggesting an indirect aerosol effect in the thermal spectral range as well.