Stephan Bakan

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...

Comparison of HOAPS, QuikSCAT, and Buoy Wind Speed in the Eastern North Atlantic and the North Sea

A systematic investigation and comparison of near-surface marine wind speed obtained from in situ and satellite observations, a reanalysis, and a reanalysis-driven regional climate model (RCM) are presented for the eastern North Atlantic and the North Sea. Wind-speed retrievals from QuikSCAT Level 2B 12.5 km and HOAPS-3-S are analyzed. The root-mean-square error (rmse) between QuikSCAT and buoy 10-m equivalent neutral wind (EQNW) is 1.50 (1.87) m · s-1 using a colocation criteria of 0.1° and 0.06° (0.3° and 0.2°) in longitudinal and latitudinal distances from buoy locations and within 10 (20) min, demonstrating that QuikSCATs mission requirement of providing wind speed with an rmse of 2 m · s-1 is met for the investigated area. The influence of three different stability and anemometer height correction algorithms for buoy wind speed on the buoy/QuikSCAT error is assessed: EQNW gives the best agreement with QuikSCAT data; however, differences are smaller than the buoy measurement error. The rmse between HOAPS and buoy wind converted to 10 m by the logarithmic wind profile is 2.27 (2.36) m · s-1 using a colocation of 0.1° × 0.06° (0.3° × 0.2°) and within 10 (20) min. QuikSCAT shows good agreement with buoy wind for speeds up to 20 m · s-1. HOAPS shows an underestimation of high wind speeds beyond 15-20 m · s-1 probably due to a saturation of the return signal. The rmse between buoy wind speed and the National Centers of Environmental Prediction/National Center for Atmospheric Research Reanalysis (NRA R1) and the spectrally nudged RCM REMO (SN-REMO) are 2.2 and 2.5 m · s-1, respectively.

HOAPS - A new satellite-derived freshwater flux climatology

HOAPS (Hamburg Ocean Atmosphere Parameters and Fluxes from Satellite Data) is a global climatology of sea surface parameters and surface energy and freshwater fluxes derived from satellite radiances for the time period July 1987 to December 1998. Data from polar orbiting radiometers, the Special Sensor Microwave/Imager (SSM/I) and the Advanced Very High Resolution Radiometer (AVHRR), have been used to get global fields of surface meteorological and oceanographic parameters but also latent heat flux, evaporation, precipitation and net freshwater flux over ice free ocean areas for various averaging periods and grid sizes. All retrieval methods have been validated with in situ data on a global scale to the extent possible. In this paper some of the major outcomes are exemplarily shown together with the results of comparison studies with the in situ climatology provided by the Southampton Oceanography Centre (SOC).

Improvements of satellite-derived cyclonic rainfall over the North Atlantic

Abstract Case studies of rainfall, derived from Special Sensor Microwave Imager (SSM/I) satellite data during the passage of individual cyclones over the North Atlantic, are presented to enhance the knowledge of rainfall processes associated with frontal systems. A multisatellite method is applied for complete coverage of the North Atlantic twice a day. Different SSM/I precipitation algorithms have been tested for individual cyclones and compared to the Global Precipitation Climatology Project (GPCP) datasets. An independent rainfall pattern and intensity validation method is presented using voluntary observing ship (VOS) datasets and Advanced Very High Resolution Radiometer (AVHRR) images. Intense cyclones occur frequently in the wintertime period, with cold fronts propagating far south over the North Atlantic. Following upstream, large cloud clusters are frequently embedded in the cellular structured cold air of the backside regions, which produce heavy convective rainfall events, especially in the regi...

Microphysical and radiative properties of stratocumulus clouds: the EUCREX mission 206 case study

In this conclusion paper, remote sensing retrievals of cloud optical thickness performed during the EUCREX mission 206 are analyzed. The comparison with estimates derived from in situ measurements demonstrates that the adiabatic model of cloud microphysics is more realistic than the vertically uniform plane parallel model (VUPPM) for parameterization of optical thickness. The analysis of the frequency distributions of optical thickness in the cloud layer then shows that the adiabatic model provides a good prediction when the cloud layer is thick and homogeneous, while it overestimates significantly the optical thickness when the layer is thin and broken. Finally, it is shown that the effective optical thickness over the whole sampled cloud is smaller than the adiabatic prediction based on the mean geometrical thickness of the cloud layer. The high sensitivity of the optical thickness on cloud geometrical thickness suggests that the effect of aerosol and droplet concentration on precipitation efficiency, and therefore on cloud extent and lifetime, is likely to be more significant than the Twomey effect.

Correction of detector nonlinearity in Fourier transform spectroscopy with a low-temperature blackbody

The nonlinearity of a mercury cadmium telluride photoconductive detector, an integral part of a modified commercial interferometer used for airborne research, has been analyzed and evaluated against a number of correction schemes. A high-quality blackbody with accurate temperature control has been used as a stable and well-characterized radiation source. The detector nonlinearity was established as a function of scene temperature between 194 and 263 K. Second- and third-order corrections to the measured interferogram have been tested by analyzing the measured signal both within and outside the spectral response region of the detector. A combined correction scheme is proposed that best represents the real nonlinear response of the detector.

Missing North Atlantic cyclonic precipitation in ECMWF numerical weather prediction and ERA-40 data detected through the satellite climatology HOAPS II

Intense precipitation associated with wintertime North Atlantic cyclones occurs not only in connection with frontal zones but also, and often mainly, embedded in strong cold air outbreaks to the west of mature cold fronts. Coherent structures of cloud clusters organized in mesoscale postfrontal low-pressure systems are frequently found in satellite data. Such postfrontal lows (PFL) can develop into severe weather events within few hours and can even reach Europe causing intense convective rainfall and gale force winds. Despite predicting the major storm systems numerical weather prediction (NWP) additionally needs to account for PFLs due to their frequent occurrence connected with high impact weather. But while the major cyclone systems are mostly well predicted, the forecast of PFLs remains poor. Using North Atlantic weather observations from the 1997 Fronts and Atlantic Storm Track Experiment (FASTEX) along with the standard voluntary observing ship (VOS) data led to a high quality validation data set for this usually data sparse region. For individual case studies of FASTEX cyclones with mesoscale PFLs investigations were carried out using the well calibrated precipitation estimates from HOAPS (Hamburg Ocean Atmosphere Parameters and fluxes from satellite data) compared to the NWP model output of the ECMWF (European Centre for Medium-Range Weather Forecasts). Preceding studies showed that the HOAPS precipitation structure and intensities are in good agreement with the VOS observations for all observed precipitation types within the cyclones, including PFLs. To assure that the results found in the 1997 data are still valid in the more recent ECMWF model system, a PFL rainfall comparison is carried out using HOAPS and ERA-40 (ECMWF Re-Analysis) data for the winter of 2001 and 2002. The results indicate that the ECMWF model is mostly well reproducing precipitation structures and intensities associated with frontal systems as observed in the VOS and HOAPS data, whereas PFL precipitation is mostly missing. Further investigations within the regions of PFL point out that the VOS observed surface pressure is systematically lower than reproduced in the models. This leads to the conclusion that the missing PFL precipitation in ECMWF may be primarily due to the absence of the corresponding mesoscale low-pressure system.

Interferometric measurements of sea surface temperature and emissivity

A new multispectral method to derive sea surface emissivity and temperature by using interferometer measurements of the near surface upwelling radiation in the infrared window region is presented. As reflected sky radiation adds substantial spectral variability to the otherwise spectrally smooth surface radiation, an appropriate estimate of surface emissivity allows the measured upwelling radiation to be corrected for the reflected sky component. The remaining radiation, together with the estimated surface emissivity, yields an estimate of the sea surface temperature. Measurements from an ocean pier in the Baltic Sea in October 1995 indicate an accuracy of about 0.1 K for the sea surface temperature thus derived. A strong sea surface skin effect of about 0.6 K is found in that particular case.

Passive remote sensing of the atmospheric water vapour content above land surfaces

Abstract The global distribution of the atmospheric water vapour content plays an important role in the weather forecast and climate research. Nowadays there exist various methods dealing with remote sensing of the atmospheric water vapour content. Unfortunately, most of them are restricted to ocean areas, since, in general, the emission of land surfaces is not known well enough. Therefore, a new method is developed which allows the detection of the atmospheric total water vapour content from aircraft or satellite with the aid of backscattered solar radiation in the near infrared above land surfaces. The Matrix-Operator-Method has been used to simulate backscattered solar radiances, including various atmospheric profiles of temperature, pressure, water vapour, and aerosols of various types, several sun zenith angles, and different types of land surfaces. From these calculations it can be concluded, that the detection of water vapour content in cloudless atmospheres is possible with an error of In addition to the theoretical results first comparisons with aircraft measurements of the backscattered solar radiances are shown. These measurements have been carried out with the aid of OVID (Optical Visible and near Infrared Detector), a new multichannel array spectrometer, in 1993.

Simulation of ship-track versus satellite-sensor differences in oceanic precipitation using an island-based radar

The point-to-area problem strongly complicates the validation of satellite-based precipitation estimates, using surface-based point measurements. We simulate the limited spatial representation of light-to-moderate oceanic precipitation rates along ship tracks with respect to areal passive microwave satellite estimates using data from a subtropical island-based radar. The radar data serves to estimate the discrepancy between point-like and areal precipitation measurements. From the spatial discrepancy, two statistical adjustments are derived so that along-track precipitation ship data better represent areal precipitation estimates from satellite sensors. The first statistical adjustment uses the average duration of a precipitation event as seen along a ship track, and the second adjustment uses the median-normalized along-track precipitation rate. Both statistical adjustments combined reduce the root mean squared error by 0.24 mm h − 1 (55%) compared to the unadjusted average track of 60 radar pixels in length corresponding to a typical ship speed of 24–34 km h − 1 depending on track orientation. Beyond along-track averaging, the statistical adjustments represent an important step towards a more accurate validation of precipitation derived from passive microwave satellite sensors using point-like along-track surface precipitation reference data.