Comparison of atmospheric hydrology over convective continental regions using water vapor isotope measurements from space

Abstract

[1] Global measurements of the 500-825 hPa layer mean HDO/H 2 O ratio from the Tropospheric Emission Spectrometer (TES) are used to expose differences in the dominant hydrologic processes in the Amazon, north Australian, and Asian monsoon regions. The data show high regional isotopic variability and numerous values unexpected from classical Rayleigh theory. Correlation analysis shows that mixing with boundary layer air, enhanced isotopic fractionation during precipitation, and subsiding air parcels contribute to intraseasonal isotopic variability. These local controls explain only 8-30% of total regional variance, which suggests that the isotopes are primarily indicators of moist processes that occur upstream. Seasonal trajectory analysis demonstrates that Rayleigh distillation in a Lagrangrian framework underestimates the observed isotopic depletion during the monsoons and suggests substantial recycling of water within or below clouds. The trajectory results for the dry seasons reveal that subsiding air parcels periodically introduce isotopically depleted air into the north Australia and Asian monsoon regions, whereas vigorous low-level convection over the Amazon basin acts to quickly enrich and moisten dry subsiding air. The analysis indicates variations in the strength of convective detrainment into the lower to middle troposphere over all regions, which, during the dry seasons of the north Australian and Asian monsoon regions, correlate with increases in relative humidity. This study shows that isotopic measurements provide unique diagnostics of mechanisms that control the seasonal sources of water and that these provide a refined understanding of the differences in the characteristics of hydrologic budgets in these monsoonal regions.Citation: Brown, D., J. Worden, and D. Noone (2008), Comparison of atmospheric hydrology over convective continental regions using water vapor isotope measurements from space,

H<sub>2</sub><sup>16</sup>O and HDO measurements with IASI/MetOp

“…It plays an important role in many atmospheric processes, such as radiative transfer, circulation dynamics Hanisco et al, 2007;Kuang et al, 2003;Johnson et al, 2001;Moyer et al, 1996), stratospheric chemistry (Steinwagner et al, 2007;Rosenlof et al, 2001Rosenlof et al, , 2003McCarthy et al, 2004;Franz et al, 2005;Coffey et al, 2006), cloud formation (Schmidt et al, 2005), precipitation (Bowen and Revenaugh, 2003) and the greenhouse effect (Schneider et al, 1999;Hartmann, 2002). For all of these processes it is important to answer questions such as: What is the distribution of moisture sources and sinks?…”

“…Likewise, distributions of the tropospheric HDO/H 16 2 O ratio were reported using measurements from the Tropospheric Emission Spectrometer (TES) (Worden et al, 2006(Worden et al, , 2007. The HDO/H 16 2 O ratios derived from TES have been applied to improve understanding of the characteristics of hydrologic budgets in monsoonal regions (Brown et al, 2008). IASI is an operational meteorological instrument onboard the European MetOp-A platform, which targets primarily temperature and water vapour profile measurements in the troposphere Schlüssel et al, 2005).…”

Relating tropical ocean clouds to moist processes using water vapor isotope measurements

“…As such, we restrict our analysis to lower troposphere or free troposphere (850-500 hPa) mean values except in convective regions where significant water amounts increases the sensitivity of the TES data to HDO at higher altitudes. (Worden et al, 2007;Brown et al, 2008).…”

“…TES data used in this work come from June 2005 to August 2008 over the tropical ocean (latitude between −15 • and 15 • N) and one data set is generated from June to August (JJA) of each year. For the analysis shown here, we only use the data where the Degrees-ofFreedom for signal (DOF) for the HDO retrievals are larger than 0.5 (Worden et al, 2007;Brown et al, 2008) in all sky conditions. Note that only the HDO DOF's are used as a sensitivity metric because the sensitivity of the TES H 2 O estimate will always vertically overlap that of the HDO measurement sensitivity but not necessarily the reverse.…”

“…The Af region is relatively dry, but relatively enriched in heavy isotopes, which indicates frequent mixing with fresh oceanic vapor or vapor from evapotranspiration and less subsequent precipitation then the other regions. (Flanagan et al, 1991;Worden et al, 2007;Brown et al, 2008). observations averaged between 500 and 800hPa.…”