On 13 July 2002 a widespread, subvisible tropopause cirrus layer occurred over the Florida region. This cloud was observed in great detail with the NASA Cirrus Regional Study of Tropical Anvils and ...Cirrus Layers–Florida Area Cirrus Experiment (CRYSTAL‐FACE) instrumentation, including in situ measurements with the WB‐57 aircraft. In this paper, we use the 13 July cloud as a case study to evaluate the physical processes controlling the formation and evolution of tropopause cirrus layers. Microphysics measurements indicate that ice crystal diameters in the cloud layer ranged from about 7 to 50 μm, and the peak number mode was about 10–25 μm. In situ water vapor and temperature measurements in the cloud indicated supersaturation with respect to ice throughout, with ice saturation ratios as large as 1.8. Even when the ice surface area density was as high as about 500 μm2 cm−3, ice supersaturations of 20–30% were observed. Trajectory analysis shows that the air sampled near the tropopause on this day generally came from the north and cooled considerably during the previous few days. Examination of infrared satellite imagery along air parcel back trajectories from the WB‐57 flight track indicates that the tropopause cloud layer formation was, in general, not simply left over ice from recently generated anvil cirrus. Simulations of cloud formation using time‐height curtains of temperature along the trajectory paths show that the cloud could have formed in situ near the tropopause as the air was advected into the south Florida region and cooled to unusually low temperatures. If we assume a high threshold for ice nucleation via homogeneous freezing of aqueous sulfate aerosols, the model reproduces the observed cloud structure, ice crystal size distributions, and ice supersaturation statistics. Inclusion of observed gravity wave temperature perturbations in the simulations is essential to reproduce the observed cloud properties. Without waves, crystal number densities are too low, crystal sizes are too large, and the crystals fall out too fast, leaving very little cloud persisting at the end of the simulations. In the cloud simulations, coincidence of high supersaturations and high surface areas can be produced by either recent nucleation or sedimentation of crystals into supersaturated layers. The agreement between model results and observed supersaturations is improved somewhat if we assume that the steady state relative humidity within cirrus at T < 200 K is enhanced by about 30%. The WB‐57 measurements and the model results suggest that the cloud layer irreversibly dehydrated air near the tropopause.
Aircraft‐based in situ measurements of cirrus cloud ice water content (IWC) are important for cloud microphysical/radiative modeling and satellite validation studies, yet the measurements have proven ...challenging due, in part, to the large dynamic range of IWC values present in cirrus clouds. To date, three instruments designed for the measurement of IWC have been flown aboard the NASA WB‐57F research aircraft: the University of Colorado closed‐path laser hygrometer, the Droplet Measurement Technologies Cloud Spectrometer and Impactor, and the Harvard University Lyman‐α total water photofragment‐fluorescence hygrometer. This paper compares IWC measurements from these three instruments taken during the Midlatitude Cirrus Experiment (MidCiX) in 2004. At larger values of IWC (IWC > 10 mg m−3), the three instruments agree, on average, to within 20%, which is of the order of their estimated instrumental uncertainties. At smaller IWC values (<10 mg m−3), the agreement is worse, in part due to increasing instrument uncertainties. These results have implications for measurements in thin and subvisual cirrus. Particle sampling and evaporation, instrument background levels, and hysteresis are not found to be significant contributors to discrepancies among the measurements. For remote sensing validation studies where IWC data are vertically integrated to obtain ice water path, the agreement between the instruments is better than 20% for thick cirrus (τ > 1), implying that IWC measurements in thicker clouds are of sufficient accuracy for validation studies.
We use in situ measurements of water vapor (H2O), ozone (O3), carbon dioxide (CO2), carbon monoxide (CO), nitric oxide (NO), and total reactive nitrogen (NOy) obtained during the CRYSTAL‐FACE ...campaign in July 2002 to study summertime transport in the subtropical lowermost stratosphere. We use an objective methodology to distinguish the latitudinal origin of the sampled air masses despite the influence of convection, and we calculate backward trajectories to elucidate their recent geographical history. The methodology consists of exploring the statistical behavior of the data by performing multivariate clustering and agglomerative hierarchical clustering calculations and projecting cluster groups onto principal component space to identify air masses of like composition and hence presumed origin. The statistically derived cluster groups are then examined in physical space using tracer‐tracer correlation plots. Interpretation of the principal component analysis suggests that the variability in the data is accounted for primarily by the mean age of air in the stratosphere, followed by the age of the convective influence, and last by the extent of convective influence, potentially related to the latitude of convective injection (Dessler and Sherwood, 2004). We find that high‐latitude stratospheric air is the dominant source region during the beginning of the campaign while tropical air is the dominant source region during the rest of the campaign. Influence of convection from both local and nonlocal events is frequently observed. The identification of air mass origin is confirmed with backward trajectories, and the behavior of the trajectories is associated with the North American monsoon circulation.
We observed a plume of air highly enriched in carbon monoxide and particles in the stratosphere at altitudes up to 15.8 km. It can be unambiguously attributed to North American forest fires. This ...plume demonstrates an extratropical direct transport path from the planetary boundary layer several kilometers deep into the stratosphere, which is not fully captured by large-scale atmospheric transport models. This process indicates that the stratospheric ozone layer could be sensitive to changes in forest burning associated with climatic warming.
Convective systems are an important mechanism in the transport of boundary layer air into the upper troposphere. The Cirrus Regional Study of Tropical Anvils and Cirrus Layers-Florida Area Cirrus ...Experiment (CRYSTAL-FACE) campaign, in July 2002, was developed as a comprehensive atmospheric mission to improve knowledge of subtropical cirrus systems and their roles in regional and global climate. In situ measurements of carbon monoxide (CO), water vapor (H2Ov), and total water (H2Ot) aboard NASA's WB-57F aircraft and CO aboard the U.S. Navy's Twin Otter aircraft were obtained to study the role of convective transport. Three flights sampled convective outflow on 11, 16 and 29 July found varying degrees of CO enhancement relative to the free troposphere. A cloud-resolving model used the in situ observations and meteorological fields to study these three systems. Several methods of filtering the observations were devised here using ice water content, relative humidity with respect to ice, and particle number concentration as a means to statistically sample the model results to represent the flight tracks. A weighted histogram based on ice water content observations was then used to sample the simulations for the three flights. In addition, because the observations occurred in the convective outflow cirrus and not in the storm cores, the model was used to estimate the maximum CO within the convective systems. In general, anvil-level air parcels contained an estimated 20-40% boundary layer air in the analyzed storms.
In an effort to better constrain atmospheric water vapor mixing ratios and to understand the discrepancies between different measurements of water vapor in the stratosphere and troposphere, we have ...carefully examined data from the Harvard Lyman-alpha photofragment fluorescence hygrometer, which has flown on the NASA ER-2 aircraft from 1992 through 1998. The instrument is calibrated in the laboratory before and after each deployment, and the calibration is checked by direct absorption measurements in the troposphere. On certain flights, the ER-2 flew level tracks during which water vapor varied by up to 80 ppmv, under nearly constant atmospheric conditions. These flights provide a stringent test of our calibration via direct absorption and indicate agreement to within 3%. During the 1997 Photochemistry of Ozone Loss in the Arctic Region In Summer (POLARIS) mission, our Lyman-alpha instrument was compared with a new diode laser hygrometer from the Jet Propulsion Laboratory. Overall agreement was 5% during the June/July deployment and 1% for potential temperatures of 490 to 540 K. The accuracy of our instrument is shown to be +/-5 %, with an additional offset of at most 0.1 ppmv. Data from this instrument, combined with simultaneous measurements of CH4, and H2, are therefore ideal for studies of the hydrogen budget of the lower stratosphere.
Both in situ measurements and satellite remote sensing data show that water vapor in the extratropical lowermost stratosphere has a pronounced seasonal cycle, with a maximum in summer. In this paper ...we show that the seasonal cycles derived from satellite measurements by the Stratospheric Aerosol and Gas Experiment (SAGE) II and the Microwave Limb Sounder (MLS) and ER‐2 aircraft measurements are in reasonable agreement when compared using potential vorticity (PV) and potential temperature (θ) binned monthly averages. The annual means and amplitudes of the water vapor seasonal cycle are derived from the SAGE II data for 320–360 K and 2–7 PV units (covering latitudes near and poleward of the tropopause). The high moisture content and the summer maxima indicate that the effect of transport from the troposphere across the extratropical tropopause can be seen in nearly the entire range examined and is more significant in the lower PV‐θ bins. We further investigate mechanisms that control the amount and seasonal cycle of water vapor in the lowermost stratosphere. One important issue is whether local temperature near the extratropical tropopause limits isentropic transport of water vapor into the lowermost stratosphere, similar to the freeze‐out of water vapor crossing the tropical tropopause. A quantitative comparison of saturation vapor mixing ratios (derived from National Centers for Environmental Prediction temperature analyses) with the water vapor measurements shows that the amount of water vapor near the extratropical tropopause is substantially lower than that given by saturation over ice. This demonstrates that local temperature effects do not set the upper limit of water vapor in this region. Analyses using ER‐2 measurements confirm that although saturation and supersaturation do occur, on average, air near the extratropical tropopause is under saturated. The seasonal cycle of the fraction of the overworld air in the lowermost stratosphere is inferred using the water vapor and saturation vapor mixing ratio climatology.
This study derives effective ice particle densities (ρe) from data collected by the NASA WB‐57F aircraft near the tops of Florida anvils during the Cirrus Regional Study of Tropical Anvils and Cirrus ...Layers (CRYSTAL) Florida Area Cirrus Experiment (FACE). The ρe‐ice particle mass divided by the volume of an equivalent diameter liquid sphere‐, is obtained for particle populations () and single sizes from a few to 200–300 μm in maximum dimension using measurements of condensed water content and particle size distributions. Density values are needed for numerical modeling of ice cloud microphysical properties and remote sensing retrievals, and have not up to now been characterized for cold ice clouds containing mixed particle habits. The decrease with increasing slopes of gamma size distributions fitted to the size distributions, ranging from 0.15–0.91 g cm−3. For single sizes, ρe obeys a power‐law with an exponent of about −0.4.
Aircraft observations from a recent campaign spanning 0–40N latitude are compared to coincident observations from satellite sensors on board the Aqua satellite of temperature, ozone, water vapor and ...cloud properties in the upper troposphere and lower stratosphere. Satellite observations compare well to aircraft data; temperature is generally within ±1.5 K and water vapor is within ±25% of aircraft observations for pressures above 150 hPa and mixing ratios above ∼10 ppmv. Satellite ozone has a positive bias in the upper troposphere, and clouds observed by the aircraft are qualitatively well represented in the satellite data. These data and analyses provide critical validation of satellite observations, which promise new global insights into this region of the atmosphere.
The chemical composition of the lowermost stratosphere exhibits both spatial and temporal variability depending upon the relative strength of (1) isentropic transport from the tropical tropopause ...layer (TTL), (2) diabatic descent from the midlatitude and northern midlatitude stratosphere followed by equatorward isentropic transport, and (3) diabatic ascent from the troposphere through convection. In situ measurements made in the lowermost stratosphere over Florida illustrate the additional impact of equatorward flow around the monsoon anticyclone. This flow carries, along with older stratospheric air, the distinct signature of deep midlatitude convection. We use simultaneous in situ measurements of water vapor (H2O), ozone (O3), total odd nitrogen (NOy), carbon dioxide (CO2), and carbon monoxide (CO) in the framework of a simple box model to quantify the composition of the air sampled in the lowermost stratosphere during the mission on the basis of tracer mixing ratios ascribed to the source regions for these transport pathways. The results show that in the summer, convection has a significant impact on the composition of air in the lowermost stratosphere, being the dominant source of water vapor up to the 380 K isentrope. The implications of these results extend from the potential for heterogeneous ozone loss resulting from the increased frequency and lifetime of cirrus near the local tropopause, to air with increased water vapor that as part of the equatorward flow associated with the North American monsoon can become part of the general circulation.
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