Introduction

OceanRAIN-1.0

The Ocean Rainfall And Ice-phase precipitation measurement Network (OceanRAIN) provides in-situ along-track shipboard data of precipitation, evaporation and the resulting freshwater flux in 1-min resolution over the global oceans. All routinely measured atmospheric and oceanographic state variables along with those required to derive the turbulent heat fluxes are included.

The precipitation parameter is based on measurements from the optical disdrometer ODM470 that is specifically designed for all-weather shipboard operation. The rain, snow and mixed-phase precipitation occurrence, intensity and accumulation are derived from particle size distributions (PSD). Additionally, microphysical parameters and radar-related parameters are provided.

OceanRAIN Version 1.0 contains 73 parameters plus PSD data in 128 size bins. The time period from 06/2010 to 04/2017 comprises more than 6.83 million minutes of data from eight ships with precipitation observed in about 10% of the time. The research vessels sail the global oceans during all seasons, avoiding the fair-weather bias and thus covering the entire spectrum of weather events.

OceanRAIN provides in-situ water cycle surface reference data for satellite product validation and retrieval calibration of the GPM (Global Precipitation Measurement) era, to analyze the point-to-area representativeness of precipitation and to improve our understanding of water cycle processes over the global oceans. Moreover, the data can be applied to evaluate re-analysis and climate model data.

Precipitation is one of the key air–sea flux parameters and a fundamental component of the Earth's hydrological and energy cycle. Thus, precipitation is among the essential climate variables for understanding and modelling the climate system. Furthermore the global water cycle is largely driven over the oceans. Moreover, precipitation is one of the most intermitting and inhomogeneous meteorological parameters with higest spatio-temporal variability and occurs as rain, snow and mixed-phase.

Consequently, measuring precipitation is notoriously difficult and results in large uncertainties. This holds true for both in-situ measurements as well as satellite remote sensing through passive and active microwave radiometer and radar data. Satellite-based climatologies mainly differ to the usage of different retrieval methods and hence require rigorous surface validation.

Surface validation in turn was largely hampered because of the hitherto lack of available instrumentation. Gauges are prone to large wind-induced errors and cannot measure snowfall. Therefore, optical disdrometers are regarded as the reference although most existing types were not constructed for oceanic environments. The OceanRAIN optical disdrometer ODM470 is the first of its type that was especially designed for shipboard operation and to perform under all-weather conditions.

Systematic high quality oceanic in-situ precipitation measurements are requested on an international science level and are essential for improved understanding and validation of hydrological processes in satellite, re-analysis and model data. OceanRAIN, the shipboard “Ocean Rainfall And Ice-phase precipitation measurement Network” for surface validation is, to date, the only systematic long-term disdrometer-based oceanic shipboard precipitation data collection effort to establish a comprehensive statistical basis of precipitation for all climate related hotspots over the global oceans. OceanRAIN utilizes automated disdrometer systems (ODM470) capable of measuring precipitation occurrence, intensity and accumulation and discriminates for rain, snow and mixed-phase precipitation through minute-based particle size distributions. The ODM470 was especially designed for shipboard operation under high and frequently varying wind speeds and rough sea states.

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