Envisat carries a number of sensors able to provide quantitative information on raining clouds: AATSR delivers information on cloud microphysics (particle size, temperature etc.), MWR-2 gives columnar totals for liquid and vapour forms of water, and RA-2 yields rain rate and wind speed. This paper examines the complementarity of these sensors, with a focussed study on significant rain events in the N. Atlantic, covering both coherent large storms and fronts with smaller scale structure. The difference in liquid water estimates from the infra-red and passive systems appears to be related to the temperature and sizes of drops being detected.

Satellites play a major role in the determination of the rainfall at sea. Researchers at Southampton Oceanography Centre (SOC) have been involved in two projects addressing this task. First they have been instrumental in developing techniques to retrieve rain rate information from the 10+ years of dual-frequency altimeter data. The TOPEX radar measures rainfall via the attenuation it causes, producing a climatology that is independent of those derived from passive microwave (PM) and infrared (IR) sensors. Because TOPEX is an active microwave sensor, it can have a much smaller footprint than PM sensors. Therefore it can be used to estimate the size of rain cells, showing that the ITCZ and mid-latitude storm tracks are characterized by larger rain systems than elsewhere. TOPEX’s simultaneous recording of wind and wave data reveal that, for mid-latitude systems, rain is most likely in association with developing seas. All satellite-based datasets require validation, and SOC's work on the development and testing of acoustic rain gauges is the second aspect of this paper. By listening at a range of frequencies, an underwater hydrophone may distinguish the spectra of wind, rain, shipping etc., and estimate the wind speed or rain rate according to the magnitude of the signals. All our campaigns have shown a good acoustic response to changes in wind speed. However the quantitative inversion for recent trials has given values that are too high, possibly because of significant acoustic reflection from the sea bottom. The changes in spectral slope often agree with other observations of rain, although validation experiments in coastal regions are hampered by the extraneous sources present. Acoustic rain gauges would eventually see service not only for routine satellite validation, but also for real-time monitoring of locations of interest.

Multi-frequency measurements of underwater noise offer the possibility of estimating environmental parameters to useful accuracy and, specifically, of separately providing wind speed and rainfall data. The technique and the experimental set-up are described and results presented for several locations under different conditions. Comparisons of rainfall are made with in situ measurements and with rain radar data. The acoustic technique appears to be sensitive to relatively low precipitation rates but there are significant quantitative differences which we relate to the particular characteristics of each system. Ways in which the buoy systems might be deployed to complement satellite measurements are also discussed.

An Acoustic Rain Gauge (ARG) analyses the underwater sound levels across a wide frequency range, classifies the observed spectrum according to likely source and then determines the local wind speed or rain rate as appropriate. This paper covers a trial on the Scotian Shelf off Canada, comparing the geophysical information derived from the acoustic signals with those obtained from other sources.

In the 20 years since Seasat demonstrated that several ocean parameters could be measured to useful accuracy from a single platform satellite oceanography has advanced significantly. Not only has it been possible to measure new parameters but the ability to undertake global monitoring for a decade or so has given new insights to some phenomena which could not have been obtained from in situ data alone.

This paper compares the observed ambient sound levels at two very different sites, relating both to independent estimates of wind speed and rain rate. The spectra for wind-only conditions at the two sites show great differences, especially at low wind speed. The spectra associated with rain were sufficiently different from the wind-only spectra (either in terms of spectral slope or the intensity at 14.5 kHz) to support the development of a generic rather than site-specific rain detection algorithm.

As part of the Observations and Modelling of Eddy–scale Geostrophic and Ageostrophic circulation project, a cruise to the Almeria–Oran front in the Eastern Alboràn (Western Mediterranean) was carried out from November 1996 to January 1997. During the cruise, a fine–scale survey, designed to be oriented along European Remote–sensing Satellite ground tracks, was repeated several times. Hydrographic and current profile data were collected continuously using an undulating, towed conductivity–temperature–depth sensor and an acoustic Doppler current profiler. The in situ data have been processed to give profiles of the absolute surface current at several locations across the front. Estimates of the absolute current profile have been made from repeated tracks in order to understand some of the sources of error. These ‘one–time’ calculations of absolute profiles have been merged with several years worth of altimeter data to monitor the flow across the Almeria–Oran front. At times the front appears to move to the south, apparently when the eastern Alboràn gyre collapses, as has been observed in previous studies.

WeatherVolume 57, Issue 10 p. 363-366 Back to Basics Back to basics: Measuring rainfall at sea: Part 2 – Space-borne sensors G. D. Quartly, Corresponding Author gdq@soc.soton.ac.uk Southampton Oceanography CentreSouthampton Oceanography Centre, Empress Dock, Southampton, Hampshire SO14 3ZH.Search for more papers by this authorT. H. Guymer, Southampton Oceanography CentreSearch for more papers by this authorM. A. Srokosz, Southampton Oceanography CentreSearch for more papers by this author G. D. Quartly, Corresponding Author gdq@soc.soton.ac.uk Southampton Oceanography CentreSouthampton Oceanography Centre, Empress Dock, Southampton, Hampshire SO14 3ZH.Search for more papers by this authorT. H. Guymer, Southampton Oceanography CentreSearch for more papers by this authorM. A. Srokosz, Southampton Oceanography CentreSearch for more papers by this author First published: 29 December 2006 https://doi.org/10.1256/wea.198.01Citations: 3AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onEmailFacebookTwitterLinked InRedditWechat Citing Literature Volume57, Issue10October 2002Pages 363-366 RelatedInformation

WeatherVolume 57, Issue 9 p. 315-320 Back to basics Back to basics: Measuring rainfall at sea: Part 1 – In situ sensors G. D. Quartly, Corresponding Author G. D. Quartly gdq@soton.ac.uk Southampton Oceanography CentreSouthampton Oceanography Centre, Empress Dock, Southampton, Hampshire SO14 3ZH.Search for more papers by this authorT. H. Guymer, T. H. Guymer Southampton Oceanography CentreSearch for more papers by this authorK. G. Birch, K. G. Birch Southampton Oceanography CentreSearch for more papers by this author G. D. Quartly, Corresponding Author G. D. Quartly gdq@soton.ac.uk Southampton Oceanography CentreSouthampton Oceanography Centre, Empress Dock, Southampton, Hampshire SO14 3ZH.Search for more papers by this authorT. H. Guymer, T. H. Guymer Southampton Oceanography CentreSearch for more papers by this authorK. G. Birch, K. G. Birch Southampton Oceanography CentreSearch for more papers by this author First published: 29 December 2006 https://doi.org/10.1256/00431650260283488Citations: 13AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat Citing Literature Volume57, Issue9September 2002Pages 315-320 RelatedInformation

1) Introduction Rainfall is an important climatic variable. Extremes in rainfall accumulations over land — either floods or droughts — have major societal implications and are obvious. At sea the effects on human activity are less evident, apart from the inconvenience to deck passengers on cruise liners! However improved knowledge of the rainfall associated with weather systems approaching the UK from the Atlantic would be beneficial to weather forecasting, especially if assimilated into atmospheric models There is an additional, more subtle, effect involving the ocean itself. At sea, the balance between precipitation and evaporation provides a critical feedback in climate change. The present ocean circulation involves both surface and deep currents (see Fig. 1), with the passage of water from the former to the latter occurring in the Labrador and Greenland Seas, where intense cooling by the winds makes the surface waters dense enough to sink to the ocean bottom, a process known as "deep convection". However, where precipitation exceeds evaporation the surface waters become fresher, and thus less dense, making them less susceptible to deep convection. Long-term changes in the freshwater balance (evaporation minus precipitation and ice melt) can thus potentially change the location and extent of such deep mixing, and ultimately disturb the "conveyor belt" illustrated in Fig. 1. Some modelling studies suggest that a change in the freshwater flux could trigger "abrupt climate change", leading to a marked reduction in UK temperatures over a decade, as the country loses the benefit of weather systems heated by their passage over warm Atlantic waters (Ellett 1993). However, before we panic about long-term changes in rainfall and impending climatic catastrophe, we need to know how much rain falls at sea, and what is the natural variability on seasonal and interannual timescales. This is not so simple a measurement task as it is on land, where there exist a large number of rain gauges, and the whole of the British Isles and neighbouring waters are covered by a rain radar network. In this article we look at the various in situ technologies for recording rain rates at sea; a succeeding article will cover techniques using satellite data.

This study presents a basic analysis of ERS-1 altimeter data, from its 3-day repeat track periods taken over a section of the Gulf Stream. Along track variations of radar backscatter (sigma (o)) have been compared with positions of the Gulf Stream derived from the composite maps of sea surface temperature from ship and satellite-sensor observations, produced by NOAA. Variations in sigma (o), indicating potential interaction with the Gulf Stream, are found at both high and low wind speed conditions. At high wind speeds, comparison with model wind data indicates that peaks in altimeter-derived wind speeds along the path of the North Wall of the Gulf Stream may be related to real increases in wind speed. A possible reason may be a tendency for winter storm tracks to follow the same path as sections of the Gulf Stream. At low wind speeds, minima in altimeter-derived winds are associated with sections of the path of the Gulf Stream but there were no comparable occurrences of low wind speeds in model wind fields, with implied consequences for winds derived from satellite sensor data. Although, for low wind speeds, the cause of these 'anomalously high' backscatter regions is unknown, such associated relatively 'calmer' waters have been previously noted as the result either of a tendency for surface slicks to be confined to the shears associated with local current systems or by wave-current interaction mechanisms causing a local reduction in the sea surface roughness.

This paper presents the results of a combined empirical orthogonal function (EOF) analysis of Advanced Very High Resolution Radiometer (AVHRR) sea surface temperature (SST) data and sea-viewing wide field-of-view sensor (SeaWiFS) chlorophyll concentration data over the Alboran Sea (Western Mediterranean), covering a period of 1 year (November 1997–October 1998). The aim of this study is to go beyond the limited temporal extent of available in situ measurements by inferring the temporal and spatial variability of the Alboran Gyre system from long temporal series of satellite observations, in order to gain insight on the interactions between the circulation and the biological activity in the system. In this context, EOF decomposition permits concise and synoptic representation of the effects of physical and biological phenomena traced by SST and chlorophyll concentration. Thus, it is possible to focus the analysis on the most significant phenomena and to understand better the complex interactions between physics and biology at the mesoscale. The results of the EOF analysis of AVHRR-SST and SeaWiFS-chlorophyll concentration data are presented and discussed in detail. These improve and complement the knowledge acquired during the in situ observational campaigns of the MAST-III Observations and Modelling of Eddy scale Geostrophic and Ageostrophic motion (OMEGA) Project.

1) Introduction In our first paper (Quartly et al. 2001) we discussed the importance of measuring rainfall at sea, and the different in situ techniques available. Here we examine the various satellite-based remote-sensing techniques, since only such systems can provide near-global measurements of rain on a frequent basis. The various techniques span the electromagnetic spectrum from microwave to infra-red and visible, and also include active as well as passive sensing systems. None of the sensors measure individual raindrops, but rather the bulk properties of the rain or the storm system bearing it. Below we discuss each technique in turn.

As part of the EU funded OMEGA project a cruise to the Eastern Alboran (Western Mediterranean) was carried out on board RRS Discovery during December 1996 and January 1997. During the cruise, a fine scale survey, designed to be oriented along ERS ground tracks, was repeated several times. Several additional survey legs were carried out, part of which lay along additional ERS or TOPEX/POSEIDON altimeter ground tracks. Hydrographic and current profile data were collected continuously along these survey tracks using an undulating, towed CTD and an Acoustic Doppler Current Profiler referenced to a Global Positioning System. The in situ data have been merged with the along track altimeter data to give profiles of the absolute surface current at several locations across the Almeria-Oran front, which limits the easternmost gyre in the Alboran gyre system. Where a track was repeated, several estimates of the absolute current profile have been made in order to try and understand some of the possible sources of error in the estimates. After the “one-time” calculation of the absolute profiles, several years of repeated altimeter data have been used to monitor the flow across the Almeria-Oran front. At times, the front appears to move to the south, apparently when the eastern Alboran gyre collapses, as observed in previous in situ and remote sensed studies. It is also possible to see times when the front moves northward and intensifies.

Dual-frequency altimeters provide us with simultaneous measurements of wind speed, wave height and rain rate. We examine the spatial patterns in these three variables in the Indian Ocean, using 1995 as a representative year. We also look at their intercorrelations, noting how the sea state in the Arabian Sea varies according to the phase of the monsoon, and also that the rain in the equatorial region tends to be associated with swell conditions. Studying data during the recent El Niño indicates that, in the Indian Ocean, March- April 1998 (i.e. the end of the El Niño) showed a slight increase in winds and waves compared to 3 years earlier, but that the rainfall was considerably more frequent.

The recent El Niño has been by most measures one of the most extreme, and there have been several papers on its thermal signature and associated wind field. There has also been wide coverage of the changes in terrestrial precipitation, with torrential rains in California and devastating fires in Borneo in response to the prolonged drought. Here we complete the picture by examining oceanic precipitation data derived from novel processing of dual‐frequency altimetry. An increased area of precipitation, with more frequent and slightly more intense rainfall, is found to mirror the expansion of the western warm pool.

TOPEX is a nadir-pointing dual-frequency radar altimeter that has been in orbit for more than 6 years. Empirical methods, based on the close correlation between the C band and K-u band backscatter, yield an estimate of the rain rate. The principal rain bands of the Intertropical Convergence Zone (ITCZ) are shown to be bounded by "marine deserts" receiving only one hundredth of the rainfall of the ITCZ. Analysis of the entire data set reveals variations in the frequency of oceanic precipitation over a range of timescales. For most regions the diurnal cycle of precipitation has a minimum likelihood at evening local time, with maximum being early morning for the tropics and northern oceans, but late afternoon for the Southern Ocean. On the seasonal cycle, the Pacific ITCZ reaches its Southernmost limit during March-April, and is then nearly always accompanied by a secondary but weaker band an equal distance to the south of the equator. Finally, on the interannual timescale the 1997-1998 Fl Nino is seen to cause a broadening of the Pacific ITCZ and its translation to the south and east, with increased global levels of rainfall rather than just a geographical redistribution. High-resolution sampling shows that the length scale of rain events varies not only latitudinally, but also longitudinally according to whether they lie in regions of genesis or decay of storms. Simultaneous wind and wave information from TOPEX indicates that the rainfall in midlatitudes occurs preferentially with high winds, but in the tropics is associated with swell. The long-term near-global single-instrument data set from TOPEX is thus complementary to those from other sensors.