Abstract
Abstract. We examine the co-variations of tropospheric water vapor, its isotopic composition and cloud types and relate these distributions to tropospheric mixing and distillation models using satellite observations from the Aura Tropospheric Emission Spectrometer (TES) over the summertime tropical ocean. Interpretation of these process distributions must take into account the sensitivity of the TES isotope and water vapor measurements to variations in cloud, water, and temperature amount. Consequently, comparisons are made between cloud-types based on the International Satellite Cloud Climatology Project (ISSCP) classification; these are clear sky, non-precipitating (e.g., cumulus), boundary layer (e.g., stratocumulus), and precipitating clouds (e.g. regions of deep convection). In general, we find that the free tropospheric vapor over tropical oceans does not strictly follow a Rayleigh model in which air parcels become dry and isotopically depleted through condensation. Instead, mixing processes related to convection as well as subsidence, and re-evaporation of rainfall associated with organized deep convection all play significant roles in controlling the water vapor distribution. The relative role of these moisture processes are examined for different tropical oceanic regions.
Highlights
Stable isotopic observations of water vapor and precipitation are useful in quantifying global or local distributions of exchange processes between vapor, ice and water clouds, and precipitation and characterizing sources of water because lighter isotopes preferentially evaporate and heavier isotopes preferentially condense, leading to an isotopic fingerprint of condensation history (e.g., Kuang et al, 2003; Dessler and Sherwood, 2003; Noone and Simmonds, 2004; Gettelman and Webster, 2005; Schmidt et al, 2005)
We sub-divide the International Satellite Cloud Climatology Project (ISCCP) categories into more general categories of nonprecipitating clouds (COD greater than 0.2 and less than 3.6, e.g., cumulus and cirrus), boundary layer clouds (COD greater than 3.6 and cloud top pressure (CTP) greater than 680 hPa, e.g. stratus and stratocumulus) and precipitating clouds associated with deep convection (COD greater than 3.6 and CTP less than 680 hPa), as well as clear sky (COD less than 0.2) (Liu et al, 2008)
These definitions are chosen (1) in order to best match the Tropospheric Emission Spectrometer (TES) measured cloud optical properties to the ISCCP cloud definitions and (2) because the sensitivity of the TES water isotope measurements varies with the optical properties of these different cloud types
Summary
Stable isotopic observations of water vapor and precipitation are useful in quantifying global or local distributions of exchange processes between vapor, ice and water clouds, and precipitation and characterizing sources of water because lighter isotopes preferentially evaporate and heavier isotopes preferentially condense, leading to an isotopic fingerprint of condensation history (e.g., Kuang et al, 2003; Dessler and Sherwood, 2003; Noone and Simmonds, 2004; Gettelman and Webster, 2005; Schmidt et al, 2005). Observations of the isotopic composition of precipitation, for example, GNIP database (IAEA/WMO, 2006), have been used to characterize moisture sources (Masson-Delmotte et al, 2005), and to infer cloud processes (Ciais and Jouzel, 1994; Lawrence and Gedzelman, 1996; Gedzelman et al, 2003; Lee and Fung, 2007; Bony et al, 2008; Risi et al, 2008a). Measuring the isotopic composition of water vapor can provide a more direct link to understanding cloud processes (e.g., Moyer et al, 1996; Webster and Heymsfield, 2003; Lawrence et al, 2004; Worden et al, 2007; Lee et al, 2009; Frankenberg et al, 2009) because of a shorter history between the phase changes related to the cloud and because the isotopic composition of precipitation can equilibrate to boundary layer values as it falls (e.g., Gat, 1996, 2000; Lee and Fung, 2007). Risi et al (2008a) used a single column model to explain a short term, “amount effect”, in which isotopically depleted rainfall in tropical convective regions is linked to reevaporation of the falling rain, diffusive exchanges with the surrounding vapor and the injection of vapor from the unsaturated downdraft into the subcloud layer
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