Near surface temperature sampling from Apex floats

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1 Near surface temperature sampling from Apex floats AST-12, March 2011 Jon Turton, UK Met Office

Diurnal variation of SST The Argo array of profiling floats provides profiles of temperature and salinity from 2,000 m depth to around 4 or 5 m below the surface However there

2 Diurnal variation of SST The Argo array of profiling floats provides profiles of temperature and salinity from 2,000 m depth to around 4 or 5 m below the surface However there is a need from the SST community for additional measurements closer to the surface to better describe the diurnal variation in near-surface temperature structure under light winds

3 Diurnal variation The magnitude of the associated excursions in the temperature at the surface is typically 1 K to 2 K and has been observed in geostationary SST imagery to reach 5 K The location of diurnal variation is determined by areas of light winds and high insolation and diurnal variability has been seen on a case-by-case basis in satellite imagery to vary over horizontal scales of 10 to 1000 km The following figures are taken from Kettle and Merchant (2006) Report on preliminary analyses of diurnal variability in sea surface temperature observed from a geostationary sensor (SEVIRI)

4 DV 0.5K Diurnal warming of at least 0.5 K affects more or less all of the Atlantic on about 5 days per month or more The summer hemisphere around 30 to 40 displays such events on half or more of days per month. The most clear-sky seas show such variation virtually every observable day.

5 DV 1K Diurnal warming events exceeding 1 K are much rarer, occurring on 5% to 30% of days in the summer hemisphere and observable equatorial zone, and very rarely in the winter hemisphere. Even in the Mediterranean in summer, such events occur no more than 50% of days except in some coastal waters

DV 2K Diurnal warming events exceeding 2 K occur on fewer than 5 days per month in the climatological subtropical high pressure areas of the summer hemisphere and in the equatorial zone, and are not

6 DV 2K Diurnal warming events exceeding 2 K occur on fewer than 5 days per month in the climatological subtropical high pressure areas of the summer hemisphere and in the equatorial zone, and are not observed in the winter hemisphere The apparent signals off the coasts of South Africa in the July to September maps are most likely artefacts of the sampling scheme in the presence of the strong temperature gradients associated with features such as the Agulhas Current

7 Unpumped measurements from Apex floats Normally, pumping of the CTD sensor is turned off between 5 and 10 m before the float reaches the surface in order to avoid contamination of the salinity sensor by pollutants at the sea surface NST capable floats continue to sample temperatures after the CTD pump has been switched off At 20 dbar, 15 dbar, 10 dbar and 5 dbar cross-calibration samples are collected which consist of a non-pumped PT immediately followed by a pumped PTS At depths less than 5 dbar, 12 non-pumped PT samples are taken at 6 seconds intervals. This allows for sampling up to the surface at ~0.6 dbar intervals The additional data can be transmitted with 2 extra Argos messages (e.g. 15 regular plus 2 for near surface data)

8 Results from preliminary analysis (as at June 2010) Based on 8 NST capable floats deployed by UK Argo, as shown in figure 2 (left). However, figure 3 (right) shows that only 2 of these floats (81220 and 81221) sampled near-surface temperatures during afternoon conditions when near surface warm layers are most likely to occur.

9 Comparison of crosscalibration pumped & nonpumped measurements Mean and RMS differences between the corresponding pumped and nonpumped measurements (at 5, 10, 15 and 20 dbar) Difference is small for all of the floats with the exception of (sensor problem where non-pumped temperature were consistently warmer) While this suggests that the nonpumped temperature measurements are reliable, the good agreement is not surprising as in most profiles there was relatively little temperature gradient at those levels

10 Floats and For float ( ) (left) and ( ) (right) non-pumped minus the uppermost pumped temperature differences showing the existence of near-surface temperatures up to 2.5 C warmer than the uppermost pumped temperature (at nominally 4 or 5m depth). Over 50 profiles in each case.

11 Float 81220/ Temperature profiles showing the corresponding pumped and non-pumped measurements and the non-pumped NST measurements (pumped measurements in black with crosses and non-pumped measurements in blue with triangles) for (left) 20/12/ :51 GMT (right) 8/2/ :29 GMT

12 Float 81221/ Temperature profiles showing the corresponding pumped and non-pumped measurements and the non-pumped NST measurements (pumped measurements in black with crosses and non-pumped measurements in blue with triangles) for (left) 10/1/ :31 GMT (right) 6/11/ :46 GMT.

13 Float 92723/ After ~12 cycles the non-pumped temperatures became consistently warmer than the pumped temperatures ( ) referred to UoW Profile 26 21/12/09 Profile 33 1/3/10

14 Conclusions from initial study Agreement between the pumped and non-pumped measurements looks very good for all floats except 92723/ (this float is believed to have a sensor problem) In most profiles there was relatively little temperature gradient between 20 and 5 dbar, so any lag in the non-pumped measurements was impossible to identify Although relatively few stratified profiles were observed, the results show the NST capable Apex floats are able to observe near-surface temperature stratification that would otherwise remain undetected and so provide useful additional information To better quantify the benefit and accuracy of such near-surface temperature measurements, more NST capable Apex floats should be deployed in regions where stratification is most likely and timed to surface during the afternoon hours when the stratification is likely to be strongest

15 Recent status of nearsurface capable floats 4, 18 and 85 NS capable Apex deployed in 2008, 09 and 10 (US, UK, Japan and India)

16 GHRSST requirements Useful 50 cm sampling in upper 3 m, with 10 cm depth accuracy 100 cm sampling from 3 m to ~10-15 m, with 15 cm depth accuracy) Goal (ideal) 10 cm sampling in upper 3 m, with 2.5 cm depth accuracy 50 cm sampling below 3 m to ~10-15 m depth, with 10 cm depth accuracy

17 GHRSST clarification Pressure measurement is appropriate for depth But need to determine when float is at the surface and correct Largest stratification/diurnal variation is with very low winds, so waves should be small (although there may be longer wavelength swell) With higher waves (stronger winds) depth attribution is likely to be harder but conditions are likely to be wellmixed

18 Further investigation Following the initial study the scientist involved left the Met Office and with government recruitment restrictions could not be replaced, so further analysis of the data has not been possible However, a replacement scientist is now expected in April Also an MSc dissertation project (at University of Reading) for summer 2011 has been proposed with myself and Andrea Kaiser-Weiss (GHRSST PO) as supervisors

19 Un-pumped temperature measurements from Apex Using Argos each P, T pair is 4 bytes, so 7 pairs would necessitate an additional Argos message Don t suggest any significant change (which would increase the number of Argos messages) to the present Apex near-surface sampling scheme at this stage Suggest the near-surface data are now made available via the GDACs with cautions (after appropriate QC) Can we get closer to the surface than 5dbar before switching off the pump?

20 Iridium floats Iridium floats typically record at 2 dbar vertical resolution Given the response time of the T sensor (~½ s) then it s feasible to sample every second (typically 0.1 dbar) but given the resolution of the P sensor (0.1 dbar) it is probably not worth sampling faster than every 2 s (~0.2 dbar spacing) Possible to meet GHRSST goal of 50 cm sampling from 5 to 15m depth Can to meet GHRSST useful sampling of 50 cm for less than 5 m depth, but unlikely to achieve the 10 cm goal accuracy

Some perspective on nearsurface T measurements Standard drifting buoys These currently report SST (at ~25 cm depth) to the GTS with a resolution of 0.

21 Some perspective on nearsurface T measurements Standard drifting buoys These currently report SST (at ~25 cm depth) to the GTS with a resolution of 0.1K (limitation of GTS BUOY code), and an accuracy no better than 0.2K; ~34,000 reports/day High res drifting buoys Accuracy 0.05K, resolution 0.01K (WMO BUFR format), report design depth in calm water to ± 5 cm Voluntary Observing Ships (Mean) RMS 1.5 ± 0.1 K, depths variable; ~2,200 reports/day Argo provides fewer (~300 reports/day) but to high accuracy, with vertical information so very complimentary to other sources

How DBCP Data Contributes to Ocean Forecasting at the UK Met Office

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