A24C-01
The summertime buildup and decay of lightning NOx and aged thunderstorm outflow above North America
This study explores the upper tropospheric anticyclone above eastern North America, and its influence on the summertime buildup and decay of lightning NOx (LNOx) and thunderstorm outflow. LNOx transport is simulated with a particle dispersion model that releases a LNOx tracer from the locations of millions of cloud- to-ground lightning flashes, during May-September 2004 and 2006. On average, upper tropospheric zonal flow in May transitions to a closed anticyclone above northern Mexico and the southern USA in July, that strengthens in August, and rapidly decays in September. Concentrations of the LNOx tracer reach a maximum above the southern USA and Gulf of Mexico in July and August. Fourteen study sites across North America exhibit high day-to-day variability of the LNOx tracer in the upper troposphere during summer, with the sites most heavily influenced by the North American summer monsoon having the greatest background concentrations. During late spring and September the western sites have low concentrations with little variability. In general, the west coast sites plus Barbados have the most aged thunderstorm outflow, while the east coast sites have the least aged outflow. More than 80% of summertime upper tropospheric NOx above the eastern USA is produced by lightning. To produce the best available observation-based view of upper troposphere NOx above North America, measurements from six aircraft campaigns are combined in a single composite plot. The modeled upper tropospheric NOx matches the general continental scale distribution of NOx in the composite plot, supporting the dominant role of LNOx in the simulations.
A24C-02
Impact of lightning NO emissions on North American photochemistry as determined using the GMI model
The impact of lightning NO emissions on tropospheric photochemistry will be evaluated using the Global Modeling Initiative Model driven by meteorological fields from version 4 of NASA's GEOS Data Assimilation System. The contribution of lightning-NO to interannual variations in odd nitrogen and ozone will be analyzed during the 2002 to 2004 time period. Comparisons will be made to measurements from INTEX-A, SHADOZ columns, and to satellite-retrievals from Aura. Simulations will be run with a new lightning parameterization. The sensitivity of ozone and odd nitrogen fields to the method of lightning parameterization will be emphasisized. We will show that the updated GMI lightning parameterization is able to capture day-to-day fluctuations in lightning activity over the United States.
A24C-03
Lightning NOx from Island Convection over the Maritime Continent - Local and Regional Perspectives
The Maritime Continent is one of the major areas of lightning occurrence in the tropics. On the local scale, cloud-resolving chemistry simulations have been performed using the WRF-AqChem model for a 'Hector' storm observed on 16 November 2005 over the Tiwi Islands during the SCOUT-O3/ACTIVE campaigns based in Darwin, Australia. Observations from research aircraft in undisturbed air were used to construct composite initial condition chemical profiles. Observed lightning flashes were input to the model and a lightning placement scheme was used to inject the resulting NO into the simulated cloud. Various scenarios of NO production per flash were used for cloud-to-ground and intracloud flashes. NOx mixing ratios from each simulation were compared with upper tropospheric anvil observations to determine the best fit with observed NOx, leading to a best estimate of NO production per flash. The model output illustrates the local impact of lightning in one storm anvil. Flash rate and anvil NOx observations from other Hector storms during the SCOUT-O3 and ACTIVE campaigns were also analyzed. On the broader scale we also examine the role of lightning in the upper tropospheric NOx and O3 budgets over this region with NASA's Global Modeling Initiative (GMI) chemical transport model during the pre-monsoon and monsoon conditions of the November 2005 to February 2006 period. Lightning flash rates in this model are tied to the magnitudes of upward convective mass fluxes from the GEOS-4 data assimilation system and scaled to the monthly OTD/LIS climatology. The GMI O3, NO2, and HNO3 fields will be evaluated with Aura satellite data.
A24C-04
Temperature Change and water vapor feedback. A Comprehensive assessment using the Atmospheric InfraRed Sounder
We investigate the horizontal and vertical structure of the covariance between water vapor and temperature in the tropical troposphere, using satellite measurements from the Atmospheric InfraRed Sounder (AIRS). Our analysis reveals large spatial gradients in the local covariance between water vapor and temperature. Positive correlations dominate the tropical lower and upper troposphere, while regions of negative correlation are common in the tropical middle troposphere. While regressions of the tropical mean water vapor and temperature profiles reveal slopes of the same order of magnitude of the Clausius-Clapeyron regime, the regression of local values can be up to an order of magnitude larger than the Clausius-Clapeyron prediction. Other mechanisms, such as deep convection, are investigated in order to explain the factors controlling water vapor in this regions, besides local temperature. Results from the NOAA GFDL global circulation model are also shown for comparison.
A24C-05
Wavelet Analysis of Mountain Waves Crossing the Tropopause
During the Terrain-Induced Rotor Experiment (T-REX) field campaign of March and April 2006, the new NCAR High-performance Airborne Platform for Environment Resarch (HIAPER) Gulfstream V aircraft was used to observe mountain waves in the upper troposphere and lower stratosphere. The introduction of fast- response Differential GPS allowed for the measurement of key atmospheric properties such as energy and momentum fluxes. Such analysis has not previously been possible using aircraft data in gravity wave experiments. The computed gravity wave vertical energy fluxes revealed a surprising vertical energy flux reversal in the stratosphere during one of the flights. In previous work, the linear relationship between momentum and energy fluxes, analytically predicted for gravity waves by Eliassen and Palm, was confirmed. Wavelet techniques were employed to document spatial and spectral properties of mountain waves using in- situ data. Wavelet cross-spectrum techniques allowed for the isolation of packets of up-going and down- going energy. Wavelet cospectra were used to examine the vertical and horizontal energy transport of propagating waves while the addition quadrature spectra were used to examine wave trapping. Energy flux signatures of vertically propagating waves, trapped waves, and down-going waves were shown have unique signatures that allow for classification of wave packets on the basis of their associated vertical and horizontal energy fluxes. Wavelet cross-spectra phase relations also allow for the determination of the direction of wave propagation. Wave properties over the Sierra Nevada were analyzed for twelve cases with cross-barrier flow. Surprisingly, down-going gravity waves were found to be nearly as common as up-going waves. Additional investigation has revealed at least three more cases of down-going waves produced from secondary wave generation collocated with vertically propagating mountain waves, all associated with the largest amplitude wave cases. Large amplitude vertically propagating waves were found to commonly to be superposed with a down-going wave in the same location. Down-going waves were also sometimes stronger than up-going mountain waves in their wavelet power. A case from a T-REX ferry flight over the Wasatch Mountains is suggestive of in-situ sampling of gravity wave breaking and an associated vertical energy flux reversal.
A24C-06
Cirrus Cloud Variation in the Tropics During the Southern Stratospheric Sudden Warmings in 2006 and 2007
Tropical cirrus cloud variation during the southern stratospheric sudden warming (SSW) events in October 2006 and September 2007 was investigated by using the CALIOP data. The results shown that the variations of the cirrus cloud top and the tropopause height (temperature) is positively (negatively) correlated during both SSW events. Averaged tropical cirrus cloud top and tropopause height increased by approximately 600m and 1km after the SSW, respectively. There were, however, important differences between the two events. In the case of the SSW of October 2006, after the SSW the convection became more active around the equator, in particular, in the middle Pacific and Indian Ocean. In the case of the SSW in September 2007, the cirrus cloud frequency and the top height increased at the south side of the equator. However, the convective activity was increased in the ITCZ and Asian monsoon region, and the northerly wind across the equator became stronger in the upper troposphere. It is suggested that high frequency cirrus clouds in the tropical region after the SSW 2007 is caused by the moist air from the convection in the northern hemisphere and low temperature caused by the diabatic cooling due to the tropical upwelling driven by the SSW as well as the Kelvin wave response to a deep convection in the northern tropical region triggered by the SSW.
A24C-07
Enhancement of the Brewer Dobson circulation associated with ENSO
Recent studies have shown an enhancement of the Brewer Dobson (BD) circulation in the stratosphere due to the increase in GHG.. We use the Whole Atmosphere Community Climate Model WACCM3 to show that the BD circulation is also intensified in the stratosphere during warm ENSO events. We study the mechanisms involved in the warm ENSO case and compare them with those operating under GHG increase. Results show that, while in the GHG case resolved Rossby waves play a principal role and the effect of gravity waves is negligible in the lowermost stratosphere, during warm ENSO events, gravity waves become essential to explain the actual acceleration of the BD circulation in the lowermost stratosphere.
A24C-08
The Lyapunov Diffusivity: a new diagnostic to detect local mixing events in the UTLS
A new diagnostic, the Lyapunov Diffusivity, is presented that has the ability to quantify isentropic mixing in diffusion units and detects local mixing events by describing latitude-longitude variability. It is a hybrid diagnostic combining Nakamura's tracer-based effective diffusivity with the particle-based Lyapunov exponent. Application to consider isentropic mixing on the 350K surface shows there to be significant longitudinal variation to the strength of the barrier at the Northern subtropical jet, with the strongest barrier over Asia and the Western Pacific, a weaker barrier over the Western Atlantic, and mixing regions at the jet exits over the Eastern Pacific and Atlantic. The diagnostic highlights the seasonal cycle of the longitudinally varying mixing properties, in particular those associated with the monsoon circulations and with the westerly ducts in the subtropics. The mixing structure is shown to be modulated by modes of atmospheric variability. ENSO is shown to influence mixing in the equatorial Pacific and in the subtropics of the Pacific and Atlantic in association with changes to the westerly ducts. In the Eastern Atlantic and Mediterrenean, mixing is observed to be enhanced during the positive phase of the NAO. Variations associated with the QBO are also highlighted.