Evolution of the water stable isotopic composition of the rain sampled along Sahelian squall lines

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Letitia Andrews
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1 Evolution of the water stable isotopic composition of the rain sampled along Sahelian squall lines Camille Risi, Sandrine Bony, Françoise Vimeux, Luc Descroix AMMA meeting, Toulouse 14 october 28

2 Water stable isotopes 16 H 2 16 H 2 18 H 2 18 H 2 water=light molecules (H2 16 O) + heavy molecules (H2 18 O, HDO) isotopic fractionation applications: past climates, present water cycle convective processes: a large uncertainty

3 Goals How does convection inuence the isotopic composition of precipitation? Collection of water samples along squall lines In turn, what information about squall line dynamics and water budgets can be infered from isotopic measurements?

4 Goals How does convection inuence the isotopic composition of precipitation? Collection of water samples along squall lines In turn, what information about squall line dynamics and water budgets can be infered from isotopic measurements?

5 Goals How does convection inuence the isotopic composition of precipitation? Collection of water samples along squall lines In turn, what information about squall line dynamics and water budgets can be infered from isotopic measurements?

6 ÔÖ Ô Ø Ø ÓÒ Ö Ø µ δ 18 O µ ÑÑ» µ Example of the 11 August squall line front to rear flow squall line propagation high reevaporation convective descent high reevaporation rear to front flow high reevaporation convective zone transition zone stratiform zone time (minutes after the start of the rain) d excess = δd 8 δ 18 O

15 µ µ δ 18 O µ µ δ 18 O µ δ 18 O µ δ 18

7 µ Robust isotopic features for the dierent lines relative humidity µ µ δ 18 O µ µ δ 18 O µ δ 18 O µ δ 18 O µ water isotopes 6 August August August August time (minutes after the start of the rain)

8 µ What processes explain this evolution? high reevaporation convective front to rear flow condensation in higher altitude descent high high reevaporation reevaporation rear to front flow Reevaporation diffusive exchanges H 16 2 O rain drop HDO H 18 2 O convective zone transition zone stratiform zone δ 18 O µ time (minutes after the start of the rain) Hypotheses: rain reevaporation, meso-scale subsidence, condensation altitude... Quantication?

9 2D isotopic model of transport and microphysics Û Ò Ð

10 2D isotopic model of transport and microphysics Æ È ÓØÓÔ ÔÖÓ Ð ÊÅ Æ È ÓØÓÔ ÔÖÓ Ð ÊÅ ÓÙÒ ÖÝ ÓÒ Ø ÓÒ

11 2D isotopic model of transport and microphysics advection and microphysics

12 2D isotopic model of transport and microphysics isotopic fields

13 2D model results Z (dbz) at 1km z (km) z (km) d )» µ Ö %µ δ 18 O )È ÑÑ» µ qc ½ ½ ½¾ ½¼ ¾ ½ ½ ½¾ ½¼ ¾ time (minutes after passage over the radar)

14 What processes explain the isotopic evolution in the 2D model? δ 18 O µ rain vapor Ø Ñ Ñ ÒÙØ Ø Ö Ô ÓÚ Ö Ø Ö Öµ Ø Ñ Ñ ÒÙØ Ø Ö Ô ÓÚ Ö Ø Ö Öµ front to rear flow convective descent rear to front flow isotopic reequilibration with the vapor

15 What processes explain the isotopic evolution in the 2D model? δ 18 O µ rain vapor condensate Ø Ñ Ñ ÒÙØ Ø Ö Ô ÓÚ Ö Ø Ö Öµ Ø Ñ Ñ ÒÙØ Ø Ö Ô ÓÚ Ö Ø Ö Öµ front to rear flow convective descent rear to front flow isotopic reequilibration with the vapor

16 What processes explain the isotopic evolution in the 2D model? δ 18 O µ rain vapor condensate advection vapor Ø Ñ Ñ ÒÙØ Ø Ö Ô ÓÚ Ö Ø Ö Öµ Ø Ñ Ñ ÒÙØ Ø Ö Ô ÓÚ Ö Ø Ö Öµ convective front to rear flow subsidence of dry and depleted air descent isotopic rear to front flow reequilibration with the vapor

17 What processes explain the isotopic evolution in the 2D model? 1 1 δ 18 O µ rain vapor condensate advection vapor reevaporation Ø Ñ Ñ ÒÙØ Ø Ö Ô ÓÚ Ö Ø Ö Öµ Ø Ñ Ñ ÒÙØ Ø Ö Ô ÓÚ Ö Ø Ö Öµ convective front to rear flow descent rear to front flow humidification and reenrichment through reevaporation subsidence of dry and depleted air isotopic reequilibration with the vapor

18 ¹ Ü µ What controls d-excess in the 2D model? 1 5 rain drop H 16 2 O HDO H 18 2 O relative humidity at low levels d-excess depends mainly on relative humidity at low-levels

19 µ Interpretation of the data using the 2D model convective zone transition zone δ 18 O µ convective front to rear flow descent rear to front flow humidification and reenrichment through reevaporation stratiform zone time (minutes after the start of the rain) subsidence of dry and depleted air enrichment and decrease of d in the rain through reevaporation δ 18 O : dynamics and low-level humidication by reevaporation d-excess: low-level relative humidity

20 Conclusion and perspectives Both data and 2D model suggest that rain reevaporation and meso-scale subsidence are key processes. 2D model limits and uncertainties vapor sampling? use of water stable isotopes to better constrain the water budget in squall line and their representation in models?

Evolution of the stable water isotopic composition of the rain sampled along Sahelian squall lines

Evolution of the stable water isotopic composition of the rain sampled along Sahelian squall lines QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY Q. J. R. Meteorol. Soc. 136(s1): 7 4 () Published online 8 September 9 in Wiley InterScience (www.interscience.wiley.com) DOI:./qj.485 Evolution of