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

Birch, Keith G.

Guymer, Trevor H.

Jones, Claire E.

Quartly, Graham D.

Srokosz, Meric A.

English

Southampton (e-Prints Soton)

MEASURING RAINFALL FROM ABOVE AND BELOW THE SEA SURFACEG. D. Quartly, T. H. Guymer, M.A. Srokosz, K.G. Birch & C.E. JonesSouthampton Oceanography CentreEmpress Dock, Southampton, UKABSTRACTSatellites play a major role in the determination of the rainfall at sea.  Researchers at SouthamptonOceanography Centre (SOC) have been involved in two projects addressing this task.  First they havebeen instrumental in developing techniques to retrieve rain rate information from the 10+ years ofdual-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 infra-red (IR) sensors.  Because TOPEX is an active microwave sensor, it can have a much smaller footprintthan PM sensors.  Therefore it can be used to estimate the size of rain cells, showing that the ITCZ andmid-latitude storm tracks are characterized by larger rain systems than elsewhere.  TOPEX’ssimultaneous recording of wind and wave data reveal that, for mid-latitude systems, rain is most likelyin association with developing seas.All satellite-based datasets require validation, and SOC's work on the development and testing ofacoustic rain gauges is the second aspect of this paper.  By listening at a range of frequencies, anunderwater hydrophone may distinguish the spectra of wind, rain, shipping etc., and estimate the windspeed or rain rate according to the magnitude of the signals.  All our campaigns have shown a goodacoustic response to changes in wind speed.  However the quantitative inversion for recent trials hasgiven 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 validationexperiments in coastal regions are hampered by the extraneous sources present.  Acoustic rain gaugeswould eventually see service not only for routine satellite validation, but also for real-time monitoringof locations of interest.1.  ALTIMETRIC ANALYSIS1.1  BACKGROUND TO TOPEX'S RAIN RECORDSTOPEX, launched in 1992, is the first satellite altimeter to operate at two radar frequencies (Fu et al.,1994).  Comparison of the oceanic backscatter strengths at Ku- and C-band has enabled sub-satelliterainfall to be detected (Quartly et al., 1996).  The differential performance at the two frequencies ismainly caused by the attenuation (absorption) of the Ku-band signal, leading to the development of arain rate algorithm (Quartly et al., 1999).  The formula used is:-∆σ0  =  2 H a Rb (1)where ∆σ0 is the observed attenuation, H is the height of the column of raindrops (which the radiopulses pass through twice), R is the rain rate, and a,b are coefficients obtained from Goldhirsh andWalsh (1982).  To ensure a definite observation of rain, a detection threshold (∆σ0 > 0.5 dB) was used.The retrieved geographical patterns agree with those of other climatologies in terms of showing aseasonal migration of the ITCZ, monsoonal behaviour in the northwest Indian Ocean and the changesin the mid-latitude storm tracks in the N. Atlantic and N. Pacific.  Figure 1 shows the interannualbehaviour in the tropical Pacific.  The start of 1998 is marked by a broader, more southerly ITCZ, witha greater frequency of rain in that region too.   The geographical changes in rainfall coincide with theenlarged region of the western warm pool (Quartly et al., 2000a).Figure 1 : Change in precipitation patterns during the recent El Niño  The thick and thin white linesshow the 28˚C and 24˚C isotherms respectively.  (Taken from Quartly et al., 2000a.)1.2  COMPARISON OF CLIMATOLOGIESWe compared the TOPEX-derived climatology with that of the Global Precipitation ClimatologyProject (GPCP, see Huffman et al., 1997).  Although displaying the same spatial patterns, themagnitudes differ, with TOPEX returning only 70% of the GPCP values in the tropics, and much lessat higher latitudes.Figure 2 : Composite of rainfall for March-April 1993-1996. a) TOPEX climatology, b) GPCP.This apparent bias was investigated by comparing TOPEX rain rates to the near-simultaneousmeasurements from rain radars (McMillan et al., 2002).  The comparison with the TOGA-COARErain radar actually suggested that TOPEX was reading high by a factor of two; however, in anothercomparison using these 'ground truth' data, Ebert and Manton (1998) found the magnitudes of PM andIR rain algorithms also gave readings that were far too high.  The TOPEX rain algorithm is purelyphysical, with no empirical tuning; however, the TOGA-COARE analysis did imply that TOPEXwould underestimate due to inhomogeneities within the 6 km footprint (non-uniform beamfilling), andalso due to no account being taken of the unrecorded 'drizzle fraction' since the algorithm's detectionthreshold corresponds to a rain rate of ~2.3 mm hr-1.However, there are also sizeable differences between other climatologies, such as the TMI and PRdatasets from the TRMM satellite (Kakar and Kummerow, 2000), so we do not feel it appropriate toadjust our values empirically, but accept that there are some physical reasons for our underestimation.As the altimeter is not a swath instrument, it is not suitable for daily monitoring of the precipitationfield.  However its many year consistent dataset is useful for development and analysis of a rainfallclimatology (see Fig. 1), and its fine along-track sampling and simultaneous observation of sea statedo allow for analyses not possible with PM or IR sensors.  These are addressed in the succeeding twosections.1.3  MEASURING RAIN CELLSThe footprint for TOPEX's backscatter measurement is approximately 6  km in diameter, withconsecutive measurements being taken every 5.8 km (1s flight time) along track.  Thus compared to aPM footprint of ~40 km, TOPEX is able to provide fairly fine resolution measurements of the size ofrain cells.  We investigated this for 10˚ x 10˚ regions globally, and in all locations noted that themajority of events affect only isolated individual records along track (Fig. 3a)  However, if weexamine the tail of the probability distribution function, calculating a mean length scale of attenuationevents affecting more than one point, we find that the areas with the higher total rainfall receive theirrain from larger rain cells (Fig. 3b).  For example, the storms in the North Atlantic are larger in thewest, where they release most of their rain, than they are by the time they reach the east.  Markedzonal differences are also seen in the Pacific ITCZ.Figure 3 : a) Illustrative histogram of length of rain events at one 10˚ x 10˚ location.  b) Mean lengthof all events more than one point long (Jan. 1993 – Dec. 1996).Although poor at detecting low rain rates, the attenuation algorithm does not suffer any problems(such as saturation) at high rain rates.  Quartly et al. (2000a) used this property to investigate the meanintensity of rain events in the tropical Pacific.  As well as a greater occurrence of rain (see Fig. 1), the1997/98 El Niño was marked by a greater rain rate when raining.1.4  CORRELATION WITH SEA STATEThe TOPEX altimeter can make simultaneous measurements of wind speed, wave height and rain rate.Figure 4a shows the correlation between wind and wave measurements, with the majority ofobservations being of swell-dominated seas.  With the altimeter we can look at the 'triple correlation'i.e. for each particular wind speed / wave height condition, we can determine what is the probabilitythat it is also raining.  The second plot shows that for the latitude band 15˚S-45˚S, there is very littlechance of rain in swell conditions, but a probability of up to 10% in developing seas.  This is notsurprising given that mid-latitude rain is often associated with storms; a somewhat different picture isfound for tropical regions where much of the rain is associated with convective cells, for which thesurface wind speed is small.  TOPEX has helped us to quantify these differences in behaviour, andenables us to improve predictions of environmental acoustic noise and gas transfer which depend uponthe joint probabilities of these meteorological parameters.Figure 4 : a) Scatter plot of simultaneous wind and wave observations.  The black line shows thePierson-Moskowitz relation for waves in equilibrium with the winds.  Points above the line are 'Swell'(wave heights greater than that supported by the wind) and those below correspond to 'developingseas'.  b) Rain likelihood as a function of wind-wave conditions.  Data are for March-April averagedover 1993-1996. (Taken from Quartly et al., 1999)2.  ACOUSTIC ANALYSIS2.1  BACKGROUND TO UNDERWATER ACOUSTIC SENSING OF RAINA series of laboratory measurements has shown that there are several mechanisms by which raincreates underwater sound.  First, there is the initial impact of the drop on the surface (Nystuen, 1986).Second, there is the generation of a sub-surface bubble — a very common and loud process for thesmall drops in drizzle, resulting in a characteristic peak in intensity around 14 kHz (Pumphrey et al.,1989).  Third, the largest raindrops generate splash products on impact, and these in turn add to thesound generated (Nystuen et al., 1993).There may be many other sources of underwater sound.  One of the key features of Acoustic RainGauges (ARGs) is their recording of sound levels over a wide range of frequencies, and their use ofthat information to distinguish between various possible sources (Nystuen and Selsor, 1997).Fig. 5 : Comparison of spectra from various sources.  a) Wind-only (in colour), wind and drizzle(black dashed) and heavy rain (solid black).  b) Heavy rain compared with two believed man-madesources.  [ All data recorded at Loch Etive. ]2.2  DESIGN AND DEPLOYMENT OF ACOUSTIC RAIN GAUGESSOC's initial trials of ARGs were with a surface unit ~1m long, with the hydrophone on a thin cable30m long (Fig. 6a).  The early easily-portable versions were used in a series of month-long trials inLoch long and Loch Etive (Quartly et al., 2000b).  The hardware has been changed over the years, toallow us to move from testing in a sheltered environment to long-time exposure in harsher situations.Our first coastal deployment was near Aberporth in Wales (Quartly et al., 2002), with more recenttrials on the Scotian Shelf off Canada and in the English Channel off Plymouth.  The present version is3.4m long, and requires a crane to move it (Fig. 6b).  However, the concept behind it is the same as forthe Mk I buoy.  An omni-directional hydrophone tens of metres below the surface is sensitive to rainover a large area of the sea surface (Fig. 6c); this enables it to be sensitive to the slightest rainfall anddetect short period changes.  As it is important to avoid the hydrophone being on a taut resonant line,we use small buoyancy spheres to keep the mooring line away from the hydrophone cable.Figure 6 : Developments in the SOC Acoustic Rain Gauge programme.  a) Mk I ARG, based on ahelicopter-deployable sonarbuoy, b) Mk IV ARG being deployed from CFAV Quest.  c) Schematic ofmooring arrangement and area over which rain is detected.2.3  SENSITIVITY OF ACOUSTIC RAIN GAUGES TO WIND SPEEDThe typical variation in sound level with wind speed is illustrated in Fig. 5a  As the lower frequenciesmay pick up distant shipping, and those above 10 kHz will respond to drizzle, it is the 8 kHz channelthat is normally compared to wind values.  Figure 7a compares the 8 kHz intensities we observed inLoch Etive with the wind speed measured by a nearby anemometer; Fig. 7b shows the same for a morerecent trial on the Scotian Shelf, east of Canada.  In both cases, the solid black line shows therelationship expected according to Vagle et al. (1990).  Our measurements in Loch Etive agree wellwith the established relationship, so the ARG measurements can be used accurately to infer windspeed.  However, our data for Canada are ~7 dB higher than the curve of Vagle et al. (1990);consequently application of their algorithm gives values twice that noted by the met buoy, butnevertheless echoing its temporal variation well.  The 7 dB offset may be a calibration problem orcould be caused by strong acoustic reflection from the ocean bottom (only 70 m deep at this site).[ Loch Etive is a similar depth, but the bottom there is believed to be softer and less reflective. ]  Thecause of the 7 dB offset is still being investigated.Figure 7 : Comparsion of 8 kHz acoustic intensity with local measurement of wind speed.  Expectedcurve of Vagle et al. (1990) given by black line; the crosses show the mean and the vertical bars ±1std. dev. of the ensemble of observations.  a) Loch Etive.  b) Scotian Shelf.2.4  DETECTION OF RAIN BY ACOUSTIC RAIN GAUGESThe theory behind the acoustical detection of rain is illustrated in Fig. 5a — bubbles produced by thesmall drops in light rain generate a characteristic 'drizzle peak' around 14 kHz; heavier rain reducedthe magnitude of the slope of the acoustic spectrum.  Our trials in Loch Etive, a generally quietlocation, often showed a reliable discrimination between wind and rain (Fig. 8a).  However there wereperiods for which the acoustic sensors reported 'heavy rain', but none of the other sensors showed anyrainfall.  The spectra on such occasions (Fig. 5b) were not similar to those for corroborated rainevents, and the events were confined to daylight hours.  We suspect that the anomalous spectra are dueto boating activity near the moorings and operation of machinery on the boats; neither of which havebeen automatically recognised as 'acoustic contamination'.  Experiences in the open ocean test weresimilar.  The presence of a 'drizzle peak' and changes in spectral slope did coincide with some of theindependent observations of rain (Fig. 8b), but the automatic acoustic classification was not alwaysreliable.  This may be due to the increased sound levels, noted in Fig. 7b.Figure 8 : Acoustic observations during rain events.  a) Comparison of acoustic classification duringLoch Etive trial, with nearby tipping bucket data (each 'X' marks a tip equivalent to 0.1 mm of rain.b) Spectral slope and drizzle peak during Scotian Shelf trial compared with an optical rain gauge anddistant rain radar.  (Note, in the absence of rain, the residual at 14 kHz is usually slightly negative.)3.  SUMMARYTRMM and its successor GPM should lead to great advances in our understanding of the globalprecipitation field.  However there is much that can still be done with existing sensors to address thevariability on various temporal and spatial scales.  Southampton Oceanography Centre has beenheavily involved in the development of a rainfall climatology based on dual-frequency altimeter data,and in the testing of acoustic rain gauges.  The first technology gives a global picture allowing studiesof  interannual variations, length scales of events and the associated wind-wave conditions.  Thesecond method may yield frequent rainfall estimates over a sizeable area, which should contribute tosatellite validation, as it will be important for calibration of GPM to have measurements over the deepocean, as well as at coastal sites and over land.  Acoustic Rain Gauges could also provide real-timemonitoring of selected locations for the purpose of weather forecasting.  However, further tests arestill needed in order to address differences in performance for different locations.4.  REFERENCESEBERT, E.E., and MANTON, M.J., (1998) Performance of satellite rainfall estimation algorithmsduring TOGA COARE. J. Atmos. Sci., 55, pp 1537-1557.FU, L-L. et al., (1994) TOPEX/POSEIDON mission overview. J. Geophys. Res., 99, pp 24369-24381.GOLDHIRSH, J. and WALSH, E.J., (1982) Rain measurements from space using a modified Seasat-type radar altimeter. IEEE Trans. Antennas and Propagation, 30, pp 726-733.HUFFMAN G.J. et al., (1997) The Global Precipitation Climatology Project (GPCP) combinedprecipitation dataset. Bull. Amer. Met. Soc., 78, pp 5-20.KAKAR, R. and KUMMEROW, C., (2000) The tropical rainfall measuring mission (TRMM) - Whathave we learned and what does the future hold? Proc. of PORSEC '00, Goa, India, 5th-8th Dec.2000, Vol. I, pp 138-142.MCMILLAN, A.C., QUARTLY, G.D., SROKOSZ, M.A. and TOURNADRE, J., (2002) Validation ofTOPEX rain algorithm: Comparison with ground-based radar. J. Geophys. Res., 107, 3.1-3.10.(DOI 10.1029/2001JD000872)NYSTUEN, J.A., (1986) Rainfall measurements using underwater ambient noise. J. Acoust. Soc. Am.,79, pp 972-982.NYSTUEN, J.A., MCGLOTHIN, C.C., and COOK, M.S., (1993) The underwater sound generated byheavy rainfall. J. Acoust. Soc. Am., 93, pp 3169-3177.NYSTUEN, J.A. and SELSOR, H.D., (1997) Weather classification using passive acoustic drifters. J.Atmos. Oceanic Technol., 14, pp 656-666.PUMPHREY, H.C., CRUM, L.A., and BJØRNØ, L., (1989) Underwater sound produced byindividual drop impacts and rainfall. J. Acoust. Soc. Am., 85, pp 1518-1526.QUARTLY, G.D., GUYMER, T.H., and SROKOSZ, M.A., (1996) The effects of rain on Topex radaraltimeter data.  J. Atmos. Oceanic Tech. 13, pp 1209-1229.QUARTLY, G.D., SROKOSZ, M.A., and GUYMER, T.H., (1999) Global precipitation statistics fromdual-frequency TOPEX altimetry. J. Geophys. Res.104 (D24), pp 31489-31516.QUARTLY, G.D., SROKOSZ, M.A., and GUYMER, T.H., (2000a) Changes in oceanic precipitationduring the 1997-98 El Niño. Geophys. Res. Lett., 27, pp 2293-2296.QUARTLY, G.D., GUYMER, T.H., BIRCH, K.G., SMITHERS, J., GOY, K., and WADDINGTON,I., (2000b) Listening for rain: Theory and practice. Proc. of 5th European Conference onUnderwater Acoustics, Lyon, France, 11th-13th July 2000, Vol. I, pp 723-728.QUARTLY, G.D., SHANNON, K.M., GUYMER, T.H., BIRCH, K.G., and CAMPBELL, J.M.,(2002) Testing acoustic rain gauges in inland and coastal waters. Proc. of the 6th EuropeanConference on Underwater Acoustics, 24th-27th June 2002, Gdansk, pp 659-664.VAGLE, S., LARGE, W.G., and FARMER, D.M., (1990) An evaluation of the WOTAN technique ofinferring oceanic winds from underwater ambient sound. J. Atmos. Oceanic Technol., 7, pp 576-595.

An evaluation of the WOTAN technique of inferring oceanic winds from underwater ambient sound.

Changes in oceanic precipitation during the 1997-98 El Niño.

Global precipitation statistics from dual-frequency TOPEX altimetry.

Listening for rain: Theory and practice.

Performance of satellite rainfall estimation algorithms during TOGA COARE.

Rain measurements from space using a modified Seasattype radar altimeter.

Testing acoustic rain gauges in inland and coastal waters.

The effects of rain on Topex radar altimeter data.

The Global Precipitation Climatology Project (GPCP) combined precipitation dataset.

The tropical rainfall measuring mission (TRMM) - What have we learned and what does the future hold?

Measuring rainfall from above and below the sea surface

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