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

Birch, K.G.

Guymer, T.H.

Quartly, G.D.

English

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

NERC Open Research Archive

Back to basics:Measuring rainfall at sea:Part 1 - In situ sensorsG. D. Quartly, T. H. Guymer and K. G. BirchSouthampton Oceanography CentreWeatherSeptember 2002    Vol. 57   No. 9  pp. ii & 315-320Back to basics:Measuring rainfall at sea: Part 1 ± In situ sensorsG. D. Quartly, T. H. Guymer and K. G. BirchSouthampton Oceanography CentreRainfall is an important climatic variable.Extremes in rainfall accumulations over land ±either floods or droughts ± have major societalimplications and are obvious. At sea, the effectson human activity are less evident, apart fromthe inconvenience to deck passengers on cruiseliners! However, improved knowledge of therainfall associated with weather systemsapproaching the UK from the Atlantic wouldbe beneficial to weather forecasting, especiallyif assimilated into atmospheric models. Thereis an additional, more subtle, effect involvingthe ocean itself. At sea, the balance betweenprecipitation and evaporation provides a criti-cal feedback in climate change. The presentocean circulation involves both surface anddeep currents (see Fig. 1), with the passage ofwater from the former to the latter occurring inthe Labrador and Greenland Seas whereintense cooling by the winds makes the surfacewaters dense enough to sink to the oceanbottom ± a process known as `deep convec-tion’ . However, where precipitation exceedsevaporation the surface waters become fresher,and thus less dense, making them less suscept-ible to deep convection.Long-term changes in the freshwater bal-ance (evaporation minus precipitation and icemelt) can thus potentially change the locationand extent of such deep mixing, and ultimatelydisturb the `conveyor belt’ illustrated in Fig. 1.Some modelling studies suggest that a changein the freshwater flux could trigger `abrupt cli-mate change’ , leading to a marked reduction inUK temperatures over a decade, as the countryloses the benefit of weather systems heated bytheir passage over warm Atlantic waters (Ellett1993).However, before we panic about long-termchanges in rainfall and impending climatic cat-astrophe, we need to know how much rain fallsat sea, and what is the natural variability onseasonal and interannual time-scales. This isnot so simple a measurement task as it is onland, where there exist a large number of rain-gauges, and the whole of the British Isles andneighbouring waters are covered by a rainfallradar network. In this article we look at the var-ious in situ technologies for recording rain ratesat sea; a succeeding article will cover tech-niques using satellite data.Ship-borne observationsClearly, it would be a great waste of resourcesto deploy ships solely to give point observationsof rainfall; however, much has been achievedsimply by using the `present weather’ reportsreturned by various ships at the synoptic hours.Such observations are subjective, and do notcorrespond directly to actual rain rates. How-ever, Tucker (1961) did develop a realistic-looking climatology of the North Atlanticthrough ascribing specific rain rate values tothe various classes of `present weather’reported by the ten weather ships stationedthere during the late 1950s. Petty (1995)extended the climatology by using the reportsfrom voluntary observing ships on a globalbasis. The accuracy of individual rain observa-tions is probably no better than a factor of twoand, whilst those regions along the principalshipping lanes are fairly well sampled, there arelarge expanses of the ocean for which no obser-vations are reported for months at a time.For many meteorological parameters, data-sets can be extended greatly by the instrumen-tation of voluntary observing ships. Althoughthis has been done for rain, the individual mea-surements have large biases that vary from shipto ship and direction of travel relative to thewind. This is because the ship’s superstructure315Weather Vol. 57 September 2002greatly affects the local airflow, leading to achange in the wind speed of +30% and, moreimportantly, preventing many of the smallerraindrops from entering any collection flask.Rather than contend with using small col-lecting vessels and attempting to correct for thedistortion of the airflow, we may use the ship asa platform for a meteorological radar station.This can allow frequent measurements andareal coverage rather than providing solelypoint measurements. Such systems weredeployed in intensive field campaigns such asthe GARPAtlantic Tropical Experiment and theTropical Ocean Global Atmosphere CombinedOcean Atmosphere Response Experiment(TOGA COARE) (see Fig. 2, inside front cover).As this requires dedicated research ships, theseinstruments are available only for local experi-ments or validation of satellite algorithms.Of course, all large vessels already haveradar systems on them for navigation purposes.Recently, Lebedev and Tomczak (1999) lookedat the possibility of calibrating them to yieldrainfall rates. One of the problems is that shipradars may be susceptible to instrument drift.This is true even for land-based systems, whichhave to be regularly recalibrated with respect toraingauges. Even assuming that calibration ofthe ship radars is perfect, there are still largeerrors in the retrieval of rain rate. This isbecause the rain rate and radar reflectivity (theparameter measured) are related to the dropsize distribution (DSD) through differentexpressions (Spilhaus 1948). Thus whilst, ingeneral, heavier rain rates have a greater reflec-tivity, a few large drops at the beginning of adownpour may have the same reflectivity as alarge number of small drops near the end of astorm, but the latter will have a much greaterrain rate. For instance, in TOGACOARE the rainrate inferred assuming the system to be convec-tive was only half that calculated if the systemwas deemed to be stratiform. Doviak (1983)details some of the other errors in using asingle reflectivity measure to infer rain rate,and discusses the options of using dual-frequency or dual-polarisation systems.There is a final factor that affects the repre-sentativeness of all rain estimates from ships,and that is the f`air weather bias’ , i.e. the ten-dency of ships’ captains to select the quickestor most expeditious route, avoiding most pre-dicted storms. This bias is a key motivator forthe use of buoy-mounted systems, where acarefully planned network of observation sitescan be established.Fig. 1 Thermohaline `conveyor belt’ (after Broecker 1991, modified D. Webb, personal communication). Surface cur-rents are marked in light grey, with sinking of surface waters occurring in the Labrador and Greenland Seas, followed bythe deep circulation indicated by the dark arrows, with a gradual rise to the surface around Antarctica and throughout theIndian and Pacific Oceans.Weather Vol. 57 September 2002316Buoy-mounted sensorsThe potential advantages of developing buoy-mounted systems are that they will sample allconditions in their region evenly, that they donot affect the rain’s descent through distortionof the airflow, and that they should be relativelycheap to maintain in the large areas of oceanpoorly sampled by ship traffic. Of course, it isimportant that the sensor package has a lowpower requirement and is robust enough towithstand the rigours of long-term deploymentat sea, given the scarcity of shipping for deploy-ment and maintenance. Here we examine anumber of candidate technologies.Many land-based stations use tipping-bucket gauges for recording rainfall. These areof little use in a rough marine environment,where the rocking of the instrument will givefalse readings of rain; however, new technol-ogies are emerging that could prove to do abetter job in a wide variety of conditions.The first of these is an adaptation of the sim-pler technology of the collection flask (see Fig.3(a)). This instrument records rain accumula-tion rather than rain rate, but the latter can befound by differencing the time record. Theheight of the water column is determinedthrough changes in the capacitance across aninsulator, with the device being set to emptyautomatically whenever it fills up. Such instru-ments have been successfully deployed for ayear at a time on the TOGA± TAO (TropicalAtmosphere Ocean) moorings in the equatorialPacific (Nystuen and McPhaden 2001); therethe main problem was modern-day piratesstealing the equipment from the buoys!An alternative application of recording elec-trical capacitance is the inclined capacitanceplate, which has interwoven electrical contacts(Fig. 3(b)) and is used to provide instant-aneous rainfall rates. Raindrops on the plateaffect the capacitance between the two term-inals; gravity and/or a heating circuit removethe drops from the plate. The problems arethat the plate needs to be directed into thewind, and that sea spray can lead to a coatingof salt on the surface. In our trial with a rudi-mentary system, we found it unreliable in thehigh humidity conditions found at sea.Another approach is to use a pressure-sensitive diaphragm to record the momentumof each impacting raindrop. The earliestdesigns used a flat horizontal membrane (e.g.Joss and Waldvogel 1967); more recently,investigators have proposed using a micro-phone with a spherical surface (FoÈ rster 1994)that can thus detect raindrops equally wellfrom all directions (Fig. 3(c)). The terminalvelocity of raindrops increases with size andwind speed; thus some calibration is requiredto infer mass of drop from the impulse it gener-ates. A typical size for such a sensor is *5cmacross, which gives a reasonable sensitivity inlight to heavy rain. Because of the small surfacearea it is not reliable at very low rain rates, andthere are significant errors in very heavy rainswhere the sensor cannot distinguish theimpacts of individual drops.A different idea is to count the raindropsthrough their effect on a light beam (Wang etal. 1978). Raindrops falling between the lightsource and the receiver will diminish the signalfor a tiny fraction of a second (Fig. 3(d)).Larger drops will have a greater effect, andthus there is scope to both count and size thedrops. In practice, a filter is designed to pickout the variability on a certain time-scale, cor-responding to the flight time of a raindrop inthe beam. Typically, there is some electricalnoise in the source and receiver circuits, givinga little variability in perfectly dry conditions.To make a clearly detectable change Nystuen etal. (1996) found the rain rate had to exceed0.4mmh± 1 (a light drizzle). The lenses have tobe heated or else condensation on them canlead to spurious detections of rain. Althoughthe sensor must be mounted well above thesurface of the sea, occasional spray will lead tothe deposition of salt crystals on the lenses thusaffecting performance.A very different approach is to use a hydro-phone (an underwater microphone) to detectthe sound generated by rain. This is not placednear to the surface to detect individual rain-drops, but 10m or more down, so that it isresponding to the average rainfall over a largecatchment area (Fig. 3(e)). This large areabeing monitored gives the system a good sensi-tivity to light rain; the problems are in distin-guishing the sound of rain from other sources,and in yielding quantitative estimates of the317Weather Vol. 57 September 2002rain rate. Rain generates sound through a vari-ety of mechanisms ± the initial drop impact,ringing of generated air bubbles, and (for thelarger drops) through various splash products.It is usually possible to discern the distinctivespectra of rain in the presence of both windand wave activity. Nystuen and Selsor (1997)have used such sensors on expendable drifters;if an underwater acoustic sensor is to bedeployed on a moored buoy, the mooring linemust not generate noise itself and must bearranged so that the hydrophone’s umbilicalcable does not get wrapped around it.Various of these technologies have alreadyprogressed from testing in the controlled con-ditions of the laboratory to evaluation in thereal world. The important differences are theeffects of wind and waves. Above-surface sys-tems need to be mounted high enough not toconfuse spray from breaking waves with rain-fall. This in turn requires large buoys, so as toprovide stable platforms that will not rock inthe slightest wind. All the buoy systems dis-cussed have relatively low power requirements(to enable long deployments), and contain nomoving parts (which could wear out throughuse, or be caused to seize up by encrusted saltgrains).In a monitoring exercise near Miami, wherethe average duration of rain from a passingstorm is *26 minutes, Nystuen (1998)deduced that an adequate monthly climatologycould be recorded by an instrument only oper-ating for one minute in every ten, with a conse-quent saving in power consumption and datatransmission. For weather-forecasting applica-tions, better temporal sampling would berequired of individual storms. At SouthamptonOceanography Centre we have been evaluatingthe reliability and accuracy of the acousticFig. 3 Schematic of various technologies suitable for deployments on buoys: (a) collection flask recording capacitanceacross insulator between the electrode and the water column; the siphon empties the flask once it has filled; (b) inclinedplate, such that trickling rain reduces the capacitance between the two terminals; (c) spherical pressure sensor, provides anomni-directional response; (d) light transmissometer, detecting fluctuations in light intensity due to intervening raindrops;and (e) acoustic raingauge, which senses rain over a large area of the water surfaceWeather Vol. 57 September 2002318system in collaboration with the Met Office. Intests in a sheltered loch (Quartly et al. 2001),the acoustic system gave closer agreement witha nearby tipping-bucket gauge than did themeteorological radar coverage (Fig. 4); we haveyet to try it in more exposed conditions.SummaryThe task of providing reliable in situ measure-ments of rainfall is both important and challeng-ing. Such a task is not readily done on ships, asthey avoid the worst weather and the ship’ ssuperstructure affects local measurements. Anumber of in situ technologies have beendescribed that are suitable for deployment onbuoys, and which have low power requirementsand no moving parts that could seize up. Ofthese, the systems using the capacitance flask,light beam and hydrophone appear the mostpromising.Nystuen and McPhaden (2001) and Nys-tuen et al (1996) have tested all three in tropicallocations. The optical system suffers from adrift in calibration, as the lenses become pro-gressively caked with salt crystals. The collec-tion flask does not provide instantaneousmeasurements such as peak rain rate, but ratheran integral quantity ± the total accumulation ofrain. It is believed to be accurate, but suffersfrom a little data loss during the siphoningoperation. The acoustic system provides an esti-mate over a much larger area than the others(see Fig. 3(e)), which is a bonus as a large arealaverage is preferable to a point measurement,both for satellite validation and local studies offreshwater input. However, at present, morework is required to minimise the effect of extra-neous sounds (whether from the mooring line,distant shipping or marine life).The development of systems to work in allwaters is far from trivial. The higher-latitudeoceans provide a rougher environment, poss-ibly countenancing against those that involveabove-surface instrumentation. Also, in thecolder climes an increased proportion of theprecipitation falls as hail or snow, which affectsthe performance of all three devices: the differ-ent particle size and velocity of snow will causea marked change in the occultation time of thelight beam; snow impacts on the water surfacegenerate a very different acoustic spectrum torain; and the collection flask will require aheater, so that the capacitance measured is of awater column and so that siphoning will oper-ate. None of these technologies have yet beenthoroughly tested in all climatic regimes.In our next article we look at the variousspace-borne technologies. Although these givea better global coverage, in situ measurementswill remain important both for validation andfor providing frequent temporal sampling. Thedevelopment of reliable low-cost systems willgreatly aid in the understanding of rain systemsoriginating over the Atlantic, thus contributingto our ability to predict rainfall.AcknowledgementsWe are grateful to Paul Kucera for provision ofthe TOGA COARE data, and to Jeff Nystuen andPeter Taylor for comments and advice.Fig. 4 Comparison of 15-minute average rain rates from acoustic system with conventional tipping-bucket gauge andmeteorological radar. (Note the radar values are instantaneous observations every 15 minutes for a 5 km65km cell.)Data are from Loch Etive (Argyllshire) on 24 May 2000.319Weather Vol. 57 September 2002ReferencesBroecker, W. S. (1991) The great ocean conveyor.Oceanography, 4, pp. 79± 89Doviak, R. J. (1983) A survey of radar rain measure-ment techniques. J. Clim. Appl. Meteorol., 22,pp.832± 849Ellett, D. J. (1993) The north-east Atlantic: A fan-assisted storage heater?Weather, 48, pp. 118± 126FoÈ rster, J. (1994) Rain measurement on buoys usinghydrophones. IEEE J. Oceanic Eng., 19, pp. 23± 29Joss, J. and Waldvogel, A. (1967) Ein spektrographfuÈ r Niederschlagstropfen mit automatischer Aus-wertung. Pure Appl. Geophys. , 68, pp. 240± 246Lebedev, I. and Tomczak, M. (1999) Rainfall mea-surements with navigational radar. J. Geophys.Res., 104, pp. 13697± 13708Nystuen, J. A. (1998) Temporal sampling require-ments for automatic rain gauges. J. Atmos. OceanicTechnol., 15, pp. 1253± 1260Nystuen, J. A. and McPhaden, M. J. (2001) Thebeginnings of operational marine weather observa-tions using underwater ambient sound. In: Pro-ceedings of Acoustical Oceanography, 6± 10 April2001, Southampton, Institute of Acoustics,London, pp.135± 141Nystuen, J. A. and Selsor, H. D. (1997) Weatherclassification using passive acoustic drifters. J.Atmos. Oceanic Technol., 14, pp. 656± 666Nystuen, J. A., Proni, J. R., Black, P. G. andWilkerson, J. C. (1996) A comparison of auto-matic rain gauges. J. Atmos. Oceanic Technol., 13,pp.62± 73Petty, G. W. (1995) Frequencies and characteristicsof global oceanic precipitation from shipboardpresent-weather reports. Bull. Am. Meteorol. Soc.,76, pp.1593± 1616Quartly, G. D., Gregory, J. W., Guymer, T. H.,Birch, K. G., Jones, D.W. and Keogh, S. J. (2001)How reliable are acoustic rain sensors? In: Proceed-ings of Acoustical Oceanography, 6± 10 April 2001,Southampton, Institute of Acoustics, London,pp.142± 148Spilhaus, A. F. (1948) Drop size, intensity, and radarecho of rain. J. Meteorol., 5, pp. 161± 164Tucker, G. B. (1961) Precipitation over the NorthAtlantic Ocean. Q. J. R. Meteorol. Soc., 87,pp.147± 158Wang, T. L., Earnshaw, K. B. and Lawrence, R. S.(1978) Simplified optical path-averaged raingauge. Appl. Optics, 17, pp.384± 390Correspondence to: Dr G. D. Quartly, SouthamptonOceanography Centre, Empress Dock, Southamp-ton, Hampshire SO14 3ZH. e-mail: gdq@soton.ac.uk# Royal Meteorological Society, 2002.Weather Vol. 57 September 2002320

Back to basics: measuring rainfall at sea. Part 1 - in situ sensors

Southampton (e-Prints Soton)

Fig. 1.  Thermohaline "conveyor belt" (after Broecker 1991, modified D. Webb, pers. comm.).  Surfacecurrents are marked in light grey, with sinking of surface waters occurring in the Labrador andGreenland Seas, followed by the deep circulation indicated by the dark arrows, with a gradual rise tothe surface around Antarctica and throughout the Indian and Pacific Oceans.154 155 156 157  3  2  1Longitude (degs E)Lattude (degs S)Fig. 2.  Example of shipborne radar data from TOGA COARE (Tropical Ocean Global AtmosphereCombined Ocean Atmosphere Response Experiment).  There were 2 ships, with each radar coveringa disk of radius 120 km.  [ The heavier rain rates are in red. ]154 155 156 157  3  2  1Longitude (degs E)Lattude (degs S)Fig. 2.  Example of shipborne radar data from TOGA COARE (Tropical Ocean Global AtmosphereCombined Ocean Atmosphere Response Experiment).  There were 2 ships, with each radar coveringa disk of radius 120 km.  [ The darker greys represent heavier rain rates. ]

A comparison of automatic rain gauges.

Drop size, intensity, and radar echo of rain.

Ein spektrograph für Niederschlagstropfen mit automatischer Auswertung,

Frequencies and characteristics of global oceanic precipitation from shipboard present-weather reports.

How reliable are acoustic rain sensors?

Precipitation over the North Atlantic Ocean.

Rain measurement on buoys using hydrophones.

Rainfall measurements with navigational radar.

Simplified optical path-averaged rain gauge.

Temporal sampling requirements for automatic rain gauges.

The beginnings of operational marine weather observations using underwater ambient sound.

Back to basics: measuring rainfall at sea. Part 1 - in situ sensors

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