A22C-01 INVITED
Midlatitudes precipitation and the global atmospheric circulation
The global atmospheric circulation transports energy from the equatorial regions to higher latitudes. Due to the turbulent nature of the flow, describing a 'mean' circulation depends strongly on the averaging method and coordinates system. When averaged in isentropic coordinates, the circulation appears as a single overturning cell with a poleward flow of high entropy air and return flow at lower entropy. However, the entropy of a parcel of moist air is not uniquely defined, and different expression for the entropy yield different mean circulations. Here, the global circulation in the NCEP/NCAR Reanalysis is computed on surfaces of constant potential temperature, or dry isentropes, and on surfaces of constant equivalent potential temperature, or moist isentropes. The two analyses are qualitatively similar but differ quantitatively in that the circulation on moist isentropes is between 1.5 and 3 times larger than the circulation on dry isentropes. It is shown that the additional mass transport on moist isentropes corresponds to a poleward flow of warm, moist air near the Earth's surface that moves from the subtropics into the midlatitudes and rises in the upper troposphere within the stormtracks. In the subtropics, this flow is characterized by a low potential temperature but a much higher equivalent potential temperature. It does not appear in the circulation on dry isentropes, as it is hidden by the presence of a larger equatorward flow of drier air at same potential temperature. However, as the equivalent potential temperature in this low-level poleward flow is close to the potential temperature of the air near the tropopause, it is included in the total circulation on moist isentropes. The thermodynamic properties of this low-level poleward flow indicates that these poleward moving air parcels should ascend into the upper troposphere within the midlatitude stormtracks. Based on these findings, we propose a revised version of the global circulation. We argue that, in addition to a global equator-to-pole overturning cell similar to the 'dry'-isentropic circulation, there is a second 'moist' branch. This moist branch starts with low-level warm, moist air parcels being advected from the subtropics and into the stormtracks by the midlatitudes eddies. These air parcels ascent into the upper troposphere within the stormtracks, where they then merge with the poleward flow at high level. The rest of the circulation is then similar to the dry branch, with air subsiding over the polar regions, and returning toward the equator near the Earth's surface. This second branch of the circulation accounts for half of the global atmospheric circulation. The stormtracks and the associated high precipitation zones in the midlatitudes mark the ascent of this second branch of the circulation into the upper troposphere.
A22C-02 INVITED
Climate impacts on short and long timescales on water isotope distribution, precipitation source, and water vapor source in a coupled general circulation model
Water isotopes records collectively provide some of the most extensive proxy evidence for past climate. Required for the interpretation of these records is a known or assumed relationship between water isotopes and climate. Climate variability on various timescales (orbital, decadal, inter-annual (ENSO), annual, etc.) impacts the hydrologic cycle and influences water isotope distribution, with varying impacts on individual climate variables and water isotopes. As such, the relationship between water isotopes and climate may not remain constant through time. We assess the relationship between water isotopes and climate and infer the primary mechanisms controlling water isotope variability on various time scales using multiple simulations of current and past (Holocene through glacial) climate using GISS ModelE-R, a fully coupled atmosphere-ocean GCM equipped with water isotope as well as other tracers, ideal tracing the source of water isotope variability. We further investigate the source of water isotope variability through the addition of 144 tracers that allow us to explicitly track the water vapor source and precipitation source. We find that the relationships between water isotopes and climate (i.e., surface air temperature, salinity, precipitation, etc.) are different at the various timescales and that this relationship can also change during abrupt climate change events.
A22C-03 INVITED
Mean and Variability of Vertical Structure of Cloud Liquid and Ice Water: Observations from CloudSat and Comparisons with GCMs
The pathway of water through the atmosphere involves evaporation from the surface, horizontal transport mainly as vapor, condensation into, and evaporation from, clouds, and precipitation out of the column that can involve additional phase changes to the hydrometeors. Characterization of the cloud component of this pathway has been difficult due to the sparseness of in-situ observations, the poorly penetrating capabilities of visible and infrared satellite observations, and the coarse vertical resolving power of contemporary passive microwave satellite observations. With the launch of Cloudsat, and thus the availability of spaceborne radar observations of clouds, there are altogether new opportunities to examine the vertical structure of clouds, including their phase and precipitating characteristics. In this presentation, we present characterizations of the vertical structure of clouds, including the distinction between liquid and water clouds, as well as an indication of how these characterizations change under precipitating vs non-precipitating conditions. The presentations will be made in the context of climatology as well as an example of variability that is associated with the Asian summer monsoon. The presentation will also briefly highlight GCM performance of some of these characteristics and mention the cautions to be considered in drawing conclusions from the data, the models, and the model-data comparisons.
A22C-04
Evaluation of the Present and Past Hydrologic Cycles in the Tropics Using Stable Isotopic Measurements from the Surface, Aircraft and Satellites
Precipitation and water vapor samples from tropical cyclones, rain bands in the ITCZ, meso-scale convective systems and isolated convection cells have been analyzed for their stable isotopic compositions. Analyses have been made not only on the Earth's surface but also on samples gathered by aircraft and remotely using satellites. Interpretations of the isotopic variations were made using (1) surface and aircraft radar (2) GOES images (3) the TRMM rainfall instruments (4) the AIRES and TES instruments in the A-train satellite group (5) backward trajectories using NCEP reanalysis data and (6) meteorological data from the NWS including radiosonde data. The result has been that we have an improved knowledge of the underlying physical and dynamical drivers of isotopic variations in the hydrologic cycle. Specifically, the recycling of water in storms, especially in the tropics, results in significant deviations of the stable isotope values of both rain and water vapor from those predicted by the commonly used Raleigh Distillation model. The important processes include (1) evaporation of rain (2) isotopic exchange between rain and water vapor (3) lofting of ice followed by sublimation and (4) partial to complete evaporation of sea spray. The very low isotope ratios discovered in surface waters from ephemeral ponds in the dry subtropical regions after the passage of tropical cyclones suggest that fossil carbonate in sediment cores may provide a record of past tropical cyclone activity. The very low isotope ratios of both precipitation and water vapor during the passage of tropical cyclones suggests that fine scale isotopic analysis of tree ring cellulose in subtropical coastal areas may provide a record of past tropical cyclone activity. The very low isotope ratios of water vapor injected into the mid to upper levels of the troposphere by tropical cyclones may be transported by upper level winds to ice core sites. The above approaches used in the study of tropical cyclones also apply to the study of monsoons, El Nino and La Nina events and even droughts.
A22C-05
The Role of Condensate Evaporation in Setting Water Vapor, HDO, and H2O18 amounts in a GCM
The standard configuration of the Goddard Institute for Space Studies ModelE contains two parallel hydrologic cycles. The primary hydrologic cycle interacts with the model physics, while the other is implemented offline and used as a carrier for stable water isotopes. These two cycles are normally identical, but their segregation allows the offline cycle to be modified without affecting model physics and dynamics. We have performed two model simulations, one in which cloud and precipitation can evaporate in the offline cycle and one in which it cannot. The resulting differences in atmospheric humidity and stable water isotope content can be directly attributed to condensate evaporation. We find that the suppression of condensate evaporation leads to specific humidity decreases of between 5% and 25% in this model. Furthermore, condensate evaporation is found to generally deplete the HDO and H2O18 content of vapor in the lower and middle troposphere, while it enriches vapor in the upper troposphere and high latitudes in the winter hemisphere. The locations that are most sensitive to the suppression of condensate evaporation are identified, and the contributions of different components of the model atmospheric hydrologic cycle are examined.
A22C-06 INVITED
The atmospheric water vapor line.
We have measured the hydrogen and oxygen isotope composition of atmospheric water vapor periodically across the American Southwest through most of 2007. Samples were primarily collected over Albuquerque, NM on the roof of the 3-story UNM geology building on a near-daily basis with occasional sampling in southern Arizona and southern Texas. Water vapor was captured by pumping ~60 to ~600 liters of air (amount depending on dew point) through a cold trap, producing ~1mL of water. Precipitation samples were also collected in Albuquerque throughout the year and analyzed for hydrogen and oxygen isotopic composition. Isotopic compositions of both vapor and precipitation were determined by CO2 equilibration for oxygen and chromium reduction for hydrogen, with resulting gasses analyzed on a mass spectrometer. Nearly all water vapor samples lie parallel to the Global Meteoric Water Line (GMWL) but with a deuterium excess of ~17 (δD = 8δO + 17). This is true regardless of relative humidity, dew point, location, time of day, or season. Precipitation samples fall to the right of the GMWL with a slope of ~5. Within our dataset we have identified 10 pairs of vapor and precipitation samples that were collected within 24 hours. Half of these sample pairs have values consistent with equilibrium conditions at ground temperature, while the other half are not in equilibrium at any temperature. Simple modeling of nonequilibrium fractionation processes suggests that the array of precipitation samples can be derived from the array of vapor samples by equilibrium condensation followed by partial evaporation of falling raindrops. Our data suggests that atmospheric water vapor has a relatively constant deuterium excess value regardless of moisture source, degree of rainout, and/or evapotranspiration contributions.
A22C-07
Variability in the Hourly Deuterium Excess of Water Vapor Near the Ground
Net and gross fluxes of methyl chloride (CH3Cl) and methyl bromide (CH3Br) were measured between 2006 and 2008 from a variety of vegetation types and climatic conditions at a tallgrass prairie (Konza Prairie, Manhattan, KS). Gross consumption and production rates were calculated using a stable isotope tracer technique that entailed a small addition of 13C labeled methyl halides, and an inert tracer, at the beginning of each chamber experiment. Overall, the tallgrass prairie was found to be a net sink for both atmospheric CH3Cl and CH3Br (gross rates were –581.1 ± 249.7 nmol m-2d-1 and –13.8 ± 5.5 nmol m-2d-1). The average uptake ratio of CH3Cl to CH3Br was 42.7 ± 9.2, which was lower than the lower tropospheric concentration ratio of 59.3 ± 5.5 at this site. Daytime and nighttime fluxes were not significantly different. Uptake rates were higher but more variable during the non-growing season outings, which were cooler and wetter than the growing season sampling times. Uptake fluxes ranged widely over small spatial scales and were correlated better with soil moisture than temperature. Burn frequency and total aboveground biomass of each site did not appear to influence uptake of CH3Cl or CH3Br. Tallgrass prairie consumed CH3Cl and CH3Br at a lower rate than the average published values for grassland soils. Live Amorpha canescens and A. fruticosa produced both CH3Cl and CH3Br but at significantly different CH3Cl:CH3Br ratios (27.5 ± 1.9 and 316.0 ± 82.5 nmol per day per gram dry biomass, respectively). Senescing A. canescens, however, showed no measurable production. Therefore the tallgrass prairie provides regional sources of CH3Cl and CH3Br during the Amorpha spp. growing season (May-September). Rainfall manipulations showed that production rates continued to increase for up to 48 hours after wetting. Amorpha spp. showed the highest production rate of CH3Cl and CH3Br of any published values from grassland vegetation.