Atlantic surface freshwater fluxes in the HOAPS satellite climatology

R. Hollmann and S. Bakan

During the last 15 years several climatologies of surface evaporation and precipitation over the global ocean were derived from satellite observations. An example is the Hamburg Ocean Atmosphere Parameters and Fluxes from Satellite (HOAPS) climatology, based on observations from satellite microwave radiometers.

Previous inter-comparison efforts demonstrated the good quality of the flux fields and a high level of maturity. However, problems remain to quantify uncertainties in the HOAPS evaporation and precipitation fields. This information is required before the fields can be assimilated in ocean models.

The focus of this project is to analyze the observed surface freshwater fluxes through improved estimates of evaporation and precipitation and their individual error characteristics in the HOAPS climatology and its ground validation in climate-related hotspots of the Atlantic Ocean. To enable that in a consistent manner we will establish an error characterization of the HOAPS evaporation data by triple collocations with ship and buoy measurements and between individual satellites and improve the error characterization of the HOAPS precipitation by analyzing available shipboard disdrometer data using point to area statistics.

After these improvements, an analysis of the spatio-temporal variability of the surface fresh water balance E-P over the Atlantic Ocean is planned, especially with respect to the Hadley circulation and the hotspot regions of interest to related WPs. Also the atmospheric water transport will be analysed in order to find the source or target regions of local fresh water imbalances. And finally, a consistent inter-comparison of the upcoming global ocean surface salinity fields from SMOS with freshwater fluxes from the HOAPS climatology is planned.

Main goals of this project are:

  • Analysis of long-term stability and quality of inter-sensor calibration and impact on derived final products;

  • Implementation of a full error propagation procedure from microwave radiance measurement to final products;

  • Validation of HOAPS precipitation rate accuracy with systematic shipboard disdrometer measurements by point-to-area methods;

  • Improvements of the HOAPS precipitation retrieval including error estimation;

  • Use of HOAPS data for assimilation and testing of long-term stability.

Freshwater budgets in the Atlantic Ocean from observations obtained in TPs 1.2 and 1.3 will be used to evaluate the satellite derived freshwater budget and it’s components. The satellite derived surface fluxes from HOAPS will be used in the synthesis project of TP 3.1 to better determine the large scale fields of E-P. The use of several E-P climatologies, including those available from assimilation experiments in TP 3.1 will allow a stringent test of the HOAPS quality, but will also use the HOAPS data-set together with all other information to infer the exchange of net freshwater with the atmosphere and to infer transports in the ocean.

Figure: Climatological mean field (left) and zonal mean annual cycle (right) of HOAPS evaporation (top), precipitation (middle) and freshwater flux (bottom) for the years 1988 - 2008.