Implications of trends and variability in low-level water vapour Richard P. Allan Department of Meteorology/NCAS climate, University of Reading Thanks to: Brian Soden, Viju John, William Ingram, Peter Good, Igor Zveryaev and Mark Ringer

Implications of increasing low-level water vapour Water vapour feedback –Small but positive contribution from low-level water vapour LW continuum SW absorption Hydrological cycle feedback –Extreme precipitation (surface specific humidity) –Global mean precipitation/evaporation (radiative constraint, boundary layer adjustment) –Contrasting wet/dry response (moisture transport) –Circulation

Clausius Clapeyron Strong constraint upon low-altitude water vapour over the oceans Land regions?

Global changes in water vapour Updated from OGorman et al. (2012) submitted; see also John et al. (2009) GRL

Declining RH over land? Simmons et al. (2010) JGR Land T global T Land RH Stalling of ocean temperatures in 2000s Continued warming of land Reduced relative humidity over land?

Radiative transfer Surface net longwave radiation determined by low level water vapour (self-) continuum absorption Surface longwave radiation Downward Upward

Surface net longwave and water vapour Allan (2009) J. Climate ERA40 NCEP SRB SSM/I Surface net longwave strongly dependent on column water vapour Increased water vapour enhances ability of atmosphere to cool to the surface tropical oceans

NCAS-Climate Talk 15 th January 2010 Radiative cooling, clear (Wm -2 K -1 ) Allan (2009) J. Clim Models simulate robust response of clear-sky radiation to warming (~2 Wm -2 K -1 ) and a resulting increase in precipitation to balance (~2 %K -1 ) e.g. Allen and Ingram (2002) Nature, Lambert and Webb (2008) GRL; Stephens & Ellis (2008) J. Clim

Feedback on atmospheric radiative cooling see also OGorman et al. (2012) Survey. Geophys. submitted + – radiative cooling due to water vapour increases (fixed RH) increased water vapour at lowest levels enhances radiative cooling to the surface Previdi (2010) Environ. Res. Lett.

NCAS-Climate Talk 15 th January 2010 CCWindT s -T o RH o Muted Evaporation changes in models are explained by small changes in Boundary Layer: 1) declining wind stress 2) reduced surface temperature lapse rate (T s -T o ) 3) increased surface relative humidity (RH o ) Richter and Xie (2008) JGR Evaporation

Changes in net atmospheric radiative cooling and precipitation Updated from OGorman et al. (2012) submitted; see also John et al. (2009) GRL AMSRE

Extreme Precipitation Clausius-Clapeyron constraint –e.g.Trenberth et al. (2003) BAMS; Pall et al. (2007) Clim Dyn Changes in intense rainfall also constrained by moist adiabat –OGorman and Schneider (2009) PNAS Low-level water vapour constraint Does extra latent heat release within storms enhance rainfall intensity above Clausius Clapeyron? –e.g. Lenderink and van Meijgaard (2010) Environ. Res. Lett.; Haerter et al. (2010) GRL

Changes in Extreme Precipitation Determined by changes in low-level water vapour and updraft velocity Above: OGorman & Schneider (2008) J Clim Aqua planet experiment shows extreme precipitation rises with surface q, a lower rate than column water vapour Right: Gastineau and Soden (2009) GRL Reduced frequency of upward motion offsets extreme precipitation increases.

Increases in the frequency of the heaviest rainfall with warming: daily data from models and microwave satellite data (SSM/I) Reduced frequencyIncreased frequency Allan et al. (2010) Environ. Res. Lett.

Increase in intense rainfall with tropical ocean warming SSM/I satellite observations at upper range of substantial model spread (see also OGorman and Schneider 2009 PNAS) Turner and Slingo (2009) ASL: dependence on convection scheme? Observational evidence of changes in intensity/duration (Zolina et al GRL) Links to physical mechanisms/relationships required (Haerter et al GRL)

Contrasting precipitation response expected Precipitation Heavy rain follows moisture (~7%/K) Mean Precipitation linked to radiation balance (~3%/K) Light Precipitation (-?%/K) Temperature e.g.Held & Soden (2006) J. Clim; Trenberth et al. (2003) BAMS; Allen & Ingram (2002) Nature

Contrasting precipitation response in wet and dry regions of the tropical circulation Updated from Allan et al. (2010) Environ. Res. Lett. descent ascent ModelsObservations Precipitation change (%) Sensitivity to reanalysis dataset used to define wet/dry regions

Implications for moisture transport and P-E patterns Projected (top) and estimated (bottom) changes in Precipitation minus Evaporation d(P-E) Held and Soden (2006) J Climate See also Muller & OGorman (2011) NCC ~

First argument: P ~ Mq. So if P constrained to rise more slowly than q, this implies reduced M Second argument: ω=Q/σ. Subsidence (ω) induced by radiative cooling (Q) but the magnitude of ω depends on (Г d -Г) or static stability (σ). If Г follows MALR increased σ. This offsets Q effect on ω. See Held & Soden (2006) and Zelinka & Hartmann (2010) JGR P~Mq Tropical Circulation

Models/observations achieve muted precipitation response by reducing strength of Walker circulation. Vecchi and Soden (2006) Nature; see also Gastineau & Soden (2011) GRL P~Mq Tropical Circulation

Moisture transports from ERA Interim Moisture transport into tropical ascent region Use ERA Interim Significant mid-level outflow Zahn and Allan (2011) JGR; see also Sohn and Park (2010) JGR Instantaneous field

Low-level water vapour –Powerful Clausius Clapeyron constraint over ocean –Agreement between ground-based and satellite observations –Ocean source of land moisture (e.g. Gimeno et al GRL) –Decadal variation in ocean/land temperature and relative humidity over land? (e.g. Simmons et al JGR) Radiative cooling and Precipitation –radiative impact of temperature and water vapour increases changes in mean precipitation and evaporation –Low level water constrains intense precipitation, but large model uncertainty in the tropics (e.g. OGorman & Schneider) –Moisture budget constraint can explain contrasting wet/dry tropical responses. (Held & Soden 2006 J Clim) Conclusions

Outstanding issues Decadal variability: –surface temperature and relative humidity over land –Atmospheric circulation Observing system: –satellite sensors: retrieval, drift & calibration –surface radiation budget Changes in extreme precipitation in tropics are uncertain Changes in boundary layer humidity: –Importance for surface fluxes –Links to low-altitude cloud feedback?

Clausius-Clapeyron Radiative transfer Surface Evaporation Atmospheric radiative cooling Extreme Precipitation Wet get wetter, dry get drier Lower tropospheric humidity Water vapour feedback Cloud feedback? Global Precipitation Q r (K/day): P (hPa) Free/upper tropospheric humidity

Water vapour open issues, needs/possibilities for coordinated efforts, upcoming satellite missions Moisture transports and atmospheric circulation Water vapour reanalysis? Surface radiation budget? Upper tropospheric water vapour datasets – Retrieval/drifts/calibration Links to cloud feedback – High cloud and FAT/PHAT hypothesis – Low cloud and boundary layer processes Changes in relative humidity over land Low level water vapour and extreme precipitation events: dynamics and thermodynamics

Evidence for recent increased strength of tropical Hadley/Walker circulation since 1979? –Sohn and Park (2010) JGR Walker circulation index (top) and sea level pressure anomalies (bottom) over equatorial Pacific ( ) Hadley circulation index over 15 o S-30 o N band

Is the mean state important? Models appear to overestimate water vapour –Pierce et al. (2006) GRL; John and Soden (2006) GRL –But not for microwave data? [Brogniez and Pierrehumbert (2007) GRL] This does not appear to affect feedback strength –John and Soden (2006) What about the hydrological cycle? –Symptomatic of inaccurate simulation? Pierce et al. (2006) GRL

Does low-level moisture rise at 7%/K? Specific humidity trend correlation (left) and time series (right) Willett et al. (2007) Nature Robust relationships globally. Less coherent relationships regionally/over land/at higher altitudes? Evidence for reductions in RH over land (Simmons et al JGR) which are physically plausible. LandOcean Willett et al. (2008) J Clim

Links to radiative cooling 7-8%/K rises in global column moisture Associated with rises in clear-sky longwave radiative cooling

Current trends in wet/dry regions of tropical oceans Wet/dry trends remain – GPCP record may be suspect for dry region –SSM/I dry region record: inhomogeneity 2000/01? GPCP trends –Wet: 1.8%/decade –Dry: -2.6%/decade –Upper range of model trend magnitudes Models DRY WET Allan et al. (2010) Environ. Res. Lett.