A21A-0709 0800h
Convectively Generated Meso-Scale Gravity Waves in ER-2 Observations During CRYSTAL-FACE
The MMS and MTP data from ER-2 observations during the CRYSTAL-FACE campaign are analyzed to retrieve meso-scale gravity wave information at the aircraft flight level. For a given flight segment, the S-transform is used to locate small-scale (10-25 km) gravity wave events. The Stokes method and the MTP method are then used to determine the horizontal propagation directions, and the vertical scales of the wave events, respectively. Other wave parameters, such as horizontal scales, group velocities, can also be derived. From the estimated propagation directions, group velocities, and the ground-based radar reflectivity observations, some wave events are traced back to convectively active regions, suggesting convection as the source of the waves.
A21A-0710 0800h
Laboratory Calibration and Flight Validation of an Aircraft Based Instrument to Measure Water Isotopes in the Upper Troposphere and Lower Stratosphere.
The relative abundance of the hydrogen isotopes of water, H$_2$O and HDO, is a sensitive indicator of the condensation history of air in the near-tropopause region. The observations of the isotopes present a particular challenge because of the very high probability of sampling artifacts in the detection of water vapor itself and the certainty that the isotopes bear those same errors. We have developed a fluorescence-based instrument that has the sensitivity to measure the relative abundance of H$_2$O and HDO without the sampling artifacts associated with large sampling volumes. The instrument combines a new water photolysis system with our pre-existing instrument for laser induced fluorescence detection of OH. Water is photolyzed with an excimer lamp source at $172\;nm$, producing ground state OH and OD radicals that are detected with state selective laser induced fluorescence at $287\;nm$. The experiment has three notable characteristics. The first is the high sensitivity afforded by laser induced fluorescence detection. At stratospheric mixing ratios of H$_2$O ($4\;ppm$ at $50\;mbar$), the relative abundance of H$_2$O and HDO can be measured with a S/N $> 12$ in a $16\;s$ acquisition cycle. The second is a reduction in the exchange of water isotopes on surfaces within the instrument: the OH and OD radicals are removed with near unity efficiency after collisions with walls in the system and are not detected. The third is a rigorous laboratory evaluation of artifacts in the sampling of water vapor and its isotopes, and an empirical demonstration of the instrument's capabilities. The instrument layout and unique detection scheme virtually eliminates the possibility of contamination and resulting measurement hysteresis. These characteristics enable an independent validation of the absorption-based water isotope instruments \emph{in situ}. Laboratory calibration will be discussed in depth, and data will be presented from recent test flights where the laser induced fluorescence instrument was flown alongside an absorption-based water isotope instrument and a Lyman-$\alpha$ water vapor instrument.
A21A-0711 0800h
Low Humidity in the Tropical and Subtropical Atmosphere
The occurrence of dry intrusions in the tropical troposphere and their origins are investigated. The velocity fields from ERA-40 are used to calculate the back trajectories or air masses from radiosonde profiles. The humidity observed by the sonde is compared with humidity profiles reconstructed by taking ERA-40 specific humidity from the origin of the trajectories, reducing the value only if saturation occurs along the trajectory. The agreement between the two provides a validation of the trajectory technique used. The origin of dry air masses arriving in the tropical and subtropical region during January 1993 is discussed.
A21A-0712 0800h
The Role of Deep Convection in Establishing the Isotopic Composition of Water Vapor in the Tropics
We employ a three-dimensional large eddy simulation with bin-resolved microphysics to simulate the distribution of water isotopomers within deep convection. Deep convection produces vertical profiles of HDO vapor that deviate significantly from the Rayleigh distillation model. These deviations are caused by the lofting of HDO-rich cloud ice within convective updrafts and the subsequent sublimation of this ice in cumulonimbus anvils. The model results agree with recent remote sensing observations of the tropical transition layer.
A21A-0713 0800h
Possible contributions of pileus clouds to TTL dehydration
Measurements from CRYSTAL-FACE show that thin tropopause cirrus (TTC) was frequently present above anvil cirrus. TTC was typically a hundred times more tenuous, about 20 K colder, and had similar horizontal dimensions to the anvil. Photography, as well as analysis of the cloud dynamics, chemistry, and isotopic ratios has led to a hypothesis that the tropopause cirrus formed initially as cap-shaped pileus clouds over deep convective turrets. The pileus spread as stratiform layers over the anvil, where they were shielded from heating from terrestrial infrared radiation. Here we propose possible implications of pileus cloud formation to the process of stratospheric dehydration. In the moist stratified tropopause transition layer (TTL), isentropes are bent upwards ahead of deep convection. A thin pileus cloud may form if isentropic surfaces are raised sufficiently that air is cooled to the point of homogeneous ice nucleation. If the TTL was initially supersaturated with respect to ice (as is often observed), once the convection subsides and the isentropes flatten, a stratiform TTC cloud lingers. As the anvil beneath the TTC thins, the TTC is increasingly exposed to radiative heating from the earth's surface, and it is lofted upwards. If it precipitates it may contribute to the dessication of the lower stratosphere.
http://www.met.utah.edu/tgarrett/Publications/CloudStructure/TTC.pdf
A21A-0714 0800h
Tropical Cirrus Cloud Characteristics Using MODIS Level-3 Data
The knowledge of the optical and microphysical properties of cirrus clouds is an important aspect of radiative and climate research. Cirrus clouds may substantially regulate the longwave energy balance near the tropical tropopause, yet the net effect of such clouds is not yet known. In fact, cirrus clouds have been identified as one of the most uncertain components in climate research. In this study, we detail a method for inferring tropical cirrus cloud optical thickness from MODIS level-3 derived cirrus reflectance and solar/satellite view geometry data. We then demonstrate the applicability of this method using independent MODIS level-3 data files from NASA's Aqua satellite to obtain the average daily tropical cirrus optical thickness. A preliminary study has also been conducted to ascertain the general characteristics of tropical cirrus cover using two years of Aqua MODIS data. This study includes the frequency of occurrence (number of days with cirrus optical thickness greater than zero), the spatial distribution of the optical thickness fields, as well as the latitudinal distribution of tropical cirrus clouds. The retrieval method described here is complimentary to the MODIS cloud retrieval product (included in level-3 data) for the case of tropical cirrus clouds.
A21A-0715 0800h
{\it In Situ} Measurement of Water Vapor Isotopic Composition Near the Tropopause: First Results From the Harvard ICOS Isotope Instrument
In recent years the atmospheric science community has placed high priority on obtaining measurements of the isotopic composition of water vapor at near-tropopause altitudes. Because water isotopic composition is altered by all processes involving condensation or evaporation, it can serve as a tracer of the processes governing the water content of air ascending to the stratosphere. Obtaining scientifically useful measurements is made difficult, however, by the scarcity of HDO, the most useful water isotopologue; by the precision and accuracy required to differentiate between dehydration scenarios; and by the potential for contamination. We report here on the first flights of a new instrument designed to meet these needs and to provide high-sensitivity {\it in situ} measurements of HDO, H$_{2}^{18}$O, and H$_2$O in the near-tropopause region. The Harvard ICOS Isotope Instrument uses the relatively new technique of Integrated Cavity Output Spectroscopy (ICOS) to provide mid-infrared absorption spectra with a pathlength of 4 km, as opposed to the $< 100$ m of traditional multipass cells. The instrument offers more than an order of magnitude improvement in sensitivity over traditional {\it in situ} mid-infrared spectroscopic instruments. Integration times are 1-3 s, short enough to capture the small spatial scales of tropical deep convection plumes and cirrus layers. Instrument design also focuses on minimizing measurement hysteresis due to contamination, using pharmaceutical-grade fittings and welds to eliminate trapped volumes. We discuss the specific measurement needs for studying near-tropopause dehydration and show validation of the performance of this instrument on a series of test flights in autumn 2004 on NASA's WB-57 high-altitude aircraft. The aircraft payload includes for comparison a second instrument measuring HDO and H2O by a fully independent method (the Harvard Hoxotope Instrument, using photofragment laser-induced fluorescence) as well as two well-established and validated instruments measuring water vapor and total water. The Harvard ICOS Isotope Instrument is proposed for use on upcoming science missions studying water vapor in the near-tropopause region.
A21A-0716 0800h
Humidity Measurements in the Tropical Tropopause Layer from Pre-AVE
Upper tropospheric and lower stratospheric relative humidity and temperature were characterized by instruments on the NASA WB-57F high-altitude aircraft during the NASA Pre-Aura Validation Experiment (Pre-AVE). On three flights in late January 2004, the aircraft made extensive measurements in the Tropical Tropopause Layer (TTL) near the Galapagos Islands. Humidity in the TTL is important to climate through its direct radiative impact, its role in ice cloud formation, and its role in determining stratospheric-entry water mixing ratios. Ultimately, this research is directed toward answering the following question: What are the physical mechanisms that control (and cause) long-term changes in the humidity of the upper troposphere and lower stratosphere in the tropics and subtropics? We report here relative humidity calculated from water vapor measurements by the JPL Laser Hygrometer (JLH) and temperature measurements by the NOAA Pressure-Temperature (PT) instrument. The distribution of humidity and accuracy of the water measurements will be discussed. We will also discuss previous tropical humidity measurements from the NASA CRYSTAL-FACE mission.
http://laserweb.jpl.nasa.gov/earthinstruments/h2owb57.html
A21A-0717 0800h
The Ticosonde/NAME 2004 Experiment: A Program of High-Frequency Rawinsonde Observations over Costa Rica During Summer
Ticosonde/NAME 2004 is a collaboration between NASA, the North American Monsoon Experiment (NAME), the Instituto Meteorologico Nacional and four other academic and scientific institutions in Costa Rica to characterize the vertical structure and temporal variability of the atmosphere over Central America during summer. The Ticosonde/NAME observations were made four times per day (00, 06, 12 and 18 UT) from Juan Santamaria International Airport (WMO station 78762) between June 16 and September 6, 2004. 318 successful ascents were made over the course of the 83-day campaign with Vaisala GPS rawinsondes, of which 220 were made with the RS-90 sonde equipped with a dual-humicap system for measuring relative humidity; the remaining ascents used the RS80-15G sonde. Despite the frequent presence of deep convective clouds locally, most of the sondes ascended well into the stratosphere, with an average burst altitude of over 23 km or ~30 hPa. Data every two seconds were archived from the sondes, permitting the application of corrections to remove the known time-lag errors in the RS-90 humicap system. Since ascents were routinely made within 90 minutes of both the ascending and descending nodes of the Aqua satellite, the corrected data will provide an opportunity to validate the AIRS water vapor measurements, particularly in the tropical tropopause layer, where accurate humidity measurements are not obtained on a routine basis. The temperature data through the first half of the experiment show a relatively weak cycle in the characteristics of the tropopause, with the time-mean tropopause potential temperature through the end of July ranging from a minimum of 360.6 K at 6 PM (00 UT) to a maximum of 364 K at local noon (18 UT). The tropopause through this same period had a mean pressure, temperature and potential temperature of 115.3 hPa, -78.1 °C and 362.4 K, respectively. Fluctuations about these means were substantial, with the tropopause as high as 88.9 hPa and as low as 141.8 hPa, a variation of nearly 3 kilometers and temperatures varying from -72.8°C to -83.5°C. The preliminary findings suggest that the local convection may have some effect on the tropopause, but that the rawinsonde measurements at Juan Santamaria made during Ticosonde-NAME 2004 as well as those proposed for the upcoming Tropical Cloud Systems Processes experiment in the summer of 2005 are useful for characterizing the temporal variability on a regional scale within the intertropical convergence zone lying over Costa Rica during this period.
http://bocachica.arc.nasa.gov/Ticosonde-NAME/
A21A-0718 0800h
The vapor pressures of supercooled water and ice
The vapor pressures of supercooled water and ice are reviewed with an emphasis on atmospheric applications. Surprisingly, none of the commonly used parameterizations for the vapor pressure of supercooled water were originally considered valid below the freezing point. In addition, the World Meteorological Organization definition of the vapor pressure of supercooled water contains a typographical error. Recent data on the molar heat capacity of supercooled water are used to derive its vapor pressure. Still, the uncertainty is such that measurements of the deliquescence and freezing behavior of aerosol particles are beginning to be limited by uncertainties in the thermodynamics of supercooled water. The vapor pressure of ice is better known and the experimental data are in good agreement with a derivation from the Clapeyron equation. Below about 200 K the different vapor pressures of hexagonal and cubic ice need to be considered.
A21A-0719 0800h
Odin/SMR Observations of Water Vapor and its Isotopes in the Middle Atmosphere
Water vapor, a strong greenhouse gas, plays an important role for the dynamics of the middle atmosphere. It is also linked to many chemical processes like the natural destruction of ozone (through the HO$_x$ family). Improving our knowledge of the amount of water and its variability in the stratosphere and in the mesosphere is thus of primary importance. The Sub-Millimetre Radiometer (SMR) on board the Odin satellite, launched in February 2001, measures thermal emissions from the Earth's limb in the 485-580\,GHz spectral range. In particular, a band around 489\,GHz is used to study water vapor and its isotopes, on the basis of 4 observation days per month. Vertical profiles of H$_2$O-16, H$_2$O-18, and HDO are retrieved between roughly 20 and 70\,km. In addition to the interesting picture of water vapor provided in the entire middle atmosphere, the unique and original measurements of HDO and H$_2$O-18 allow to study the isotopic depletion\,/\,enrichment of water, potentially supplying information on the origin of stratospheric water vapor: transport of tropospheric air through the tropical tropopause layer (TTL) vs in-situ chemical production by methane oxydation. The presentation aims to describe the global Odin/SMR dataset of H$_2$O, HDO, and the derived quantity $\delta$D\,(H$_2$O). Particular emphasis is put on the variability seen during one year of analyzed observations. Odin is a Swedish-led satellite project funded jointly by Sweden, Canada, Finland and France.
A21A-0720 0800h
Implications regarding the global distribution of water vapor isotopes observed by Odin/SMR: An estimation of the $\delta$D at the overhead of the TTL
Odin is a Swedish-led satellite project funded jointly by Sweden, Canada, Finland and France. The SubMillimeter Radiometer (SMR) onboard the Odin satellite, launched in February 2001, employs 4 tunable single-sideband Schottky-diode heterodyne receivers in the $\sim$485-580GHz spectral range. In aeronomy mode, various target bands are dedicated to observations of trace constituents relevant to stratospheric/mesospheric chemistry and dynamics such as O$\_{3}$, ClO, N$\_{2}$O, HNO$\_{3}$, H$\_{2}$O, CO, NO, as well as isotopes of H$\_{2}$O and O$\_{3}$. The global distribution of water vapor isotopes and its seasonal variation were obtained for the first time by Odin/SMR measurements. The $\delta$D of water vapor in the stratosphere agrees with the past measurements and a model. It increases with altitude from the TTL to the top of stratospehre. The methane contribution to the increase of $\delta$D is discussed ans the $\delta$D at the overhead of the TTL was estimated.
A21A-0721 0800h
Cloud Radiative Heating, Transport, and Dehydration in the Tropical Tropopause Layer
Recent studies have shown that thin cirrus formed in situ within the tropical tropopause layer (TTL) can effectively freeze-dry air entering the stratosphere. An important part of this process is the pathways taken by air parcels through the TTL due to rapid horizontal transport and slow ascent in balance with radiative heating. Under clear-sky conditions, the TTL experiences net radiative heating with a peak magnitude of about 0.5 K/day, corresponding to ascent rates of a few tenths of a mm/s. It has been shown that thick cirrus below can decrease (or even reverse) the TTL radiative heating, and thin cirrus within the TTL can substantially increase radiative heating. We use fully interactive simulations of transport and cloud formation in the TTL with radiative transfer calculations including clouds to evaluate the impact of clouds on TTL radiative heating, transport, and dehydration.
A21A-0722 0800h
A new look at - Tropical Middle-Troposphere Clouds
Cloud vertical structure observations within the tropical convective atmosphere made from a cloud radar during two ship-board field campaigns are presented and interpreted using soundings, large-scale divergences calculated from precipitation radar Doppler velocities, and surface rainfall rates. The Eastern Pacific Investigation of Climate (EPIC), held in the eastern Pacific ITCZ, documented mid-troposphere layers of low relative humidity occurring within an otherwise moist environment. The dry layers, centered at about 7 km, are thought to originate from near the equator and to represent a climatological feature. The dry layers work not only to suppress surface-based convection, but also to sublimate cirrus ice from previous outflow, and may be implicated in the generally lower cloud-top heights found for the eastern Pacific ITCZ compared to other tropical convective environments. Similar data gathered during the Joint Air-Sea Monsoon Interaction Experiment (JASMINE) held within the Bay of Bengal in May of 1999 document mid-troposphere relative humidities that rarely fell below 60% once the monsoon onset began, along with more upper-level cloud. Buoyancy-sorting model calculations document more detrainment near the top of the boundary layer during EPIC than during JASMINE, suggesting air of lower initial buoyancy that is then also more sensitive to inhibition of its vertical movement by free troposphere low-relative-humidity layers.