The Impact of Deep Convection on the Structure Of, and Transport Through, the Tropical Tropopause Layer PDF Download
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Author: J. S. Hosking
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Languages : en
Pages :
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Author: J. S. Hosking
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Languages : en
Pages :
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Languages : en
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Author: Alisa H. Young
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Category : Atmosphere
Languages : en
Pages :
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Several studies suggest that deep convection that penetrates the tropical tropopause layer may influence the long-term trends in lower stratospheric water vapor. This thesis investigates the relationship between penetrating deep convection and lower stratospheric water vapor variability using historical infrared (IR) observations. However, since infrared observations do not directly resolve cloud vertical structure and cloud top height, and there has been some debate on their usefulness to characterize penetrating deep convective clouds, CloudSat/Calipso and Aqua MODIS observations are first combined to understand how to best interpret IR observations of penetrating tops.
Author: Jing Feng
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Languages : en
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"Upper-tropospheric and lower-stratospheric (UTLS) water vapor plays an essential role in controlling the outgoing longwave radiation. Convection can affect UTLS water vapor through cross-tropopause transport and by modulating the tropopause temperature. However, this mechanism remains poorly quantified due to a lack of observations above convective storms. In this thesis, we aim to understand the role of convection in shaping the UTLS humidity structure by developing a novel retrieval method.Firstly, we develop a cloud-assisted retrieval method to obtain thermodynamic fields above a thick cloud layer using hyperspectral infrared observations. Based on the Optimal Estimation approach, this method iteratively retrieves temperature and water vapor profiles above a thick cloud layer, which is approximated as a slab with uniform ice density. The retrievability of water vapor is examined by using simulations that represent different instrument settings. These experiments demonstrate that observations from operational infrared sounders, such as AIRS and IASI, contain considerable information for retrieving UTLS water vapor above thick clouds. Interestingly, we find that the underlying cloud layer improves the performance of the retrieval method, compared to clear-sky conditions. Using AIRS observations, further validation with collocated aircraft data shows that this method can detect the elevated water vapor concentration due to convective moistening.We further discover that the cloud properties near the top of convective clouds lead to non-negligible spectral uncertainties in infrared radiances. These uncertainties can be alleviated, but not fully eliminated, by assuming a slab-cloud as in the cloud-assisted retrieval. To overcome this issue, we develop a synergistic method, which incorporates observations from active sensors and nearest reanalysis products in synergy with hyperspectral infrared observations. The improved synergistic method retrieves temperature, water vapor, and cloud properties simultaneously. A simulation experiment is designed to investigate whether retrieval methods can capture anomalous atmospheric conditions above deep convective clouds using existing instruments. We find that the synergistic method reduces the root-mean-square-errors in temperature and column-integrated water vapor by more than half and accurately reproduces the spatial distribution of temperature and humidity anomalies above convective storms.Finally, we implement the improved method by combining infrared radiance observations from AIRS L1B product, ice water content (IWC) profile and effective radius from DARDAR-Cloud, and also atmospheric profiles from ERA5 in the nearest grid. Applying this method to satellite overpasses over tropical cyclones (TCs), we construct a dataset that contains retrieved temperature, humidity above TCs. With a focus on the tropical tropopause layer (TTL), the influence of tropical cyclones on the TTL is investigated by creating composites of temperature, humidity, IWC, and radiative heating rates with respect to the distance to the cyclone centers. We find that overshooting convective clouds (DCC-OTs) greatly impact the TTL water budget. DCC-OTs contribute to 80% of the TTL cloud ice above cyclones and increase the column-integrated water vapor above the tropopause by up to 40% compared to the climatology. Other non-overshooting TTL clouds are found to be collocated with TTL temperature minimum and dehydration. Overall, the synergistic retrieval reveals that cyclones increase the stratospheric humidity above them. However, further radiative transfer calculations show that the increased moisture is typically associated with radiative cooling of the TTL, which inhibits the diabatic ascent of the moistened air. Therefore, the radiative balance of the TTL under the impact of the cyclone is not in favor of maintaining the moist anomalies in the TTL or transporting water vertically to the stratosphere"--
Author: Felix Plöger
Publisher: Forschungszentrum Jülich
ISBN: 3893366954
Category :
Languages : en
Pages : 129
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Author: Jason R. Schroeder
Publisher:
ISBN: 9781339124100
Category :
Languages : en
Pages : 167
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Measurements of trace gases were taken onboard the NASA DC-8 during the Deep Convective Clouds and Chemistry (DC3) field project with the goal of understanding the role that midlatitude deep convection plays in altering the vertical distribution of atmospherically-relevant species. Measurements of VOCs were obtained via UC Irvine’s whole air sampler (WAS) instrument, while measurements of CH4, O3, NOx [subscript], N2O, water vapor, CO, and meteorological variables were performed by a variety of other instruments operated by collaborators onboard the DC-8. Using known VOC atmospheric lifetimes and measured VOC mixing ratios in the PBL, a tracer for rapid vertical lofting of air from the planetary boundary layer (PBL) to the upper troposphere/lower stratosphere region (UT/LS) by convection was created. In this study, it was found that light hydrocarbons associated with oil and natural gas (O&NG;) and vehicular sources were widespread throughout the PBL of the DC3 study regions. In the UT/LS, enhanced levels of these light hydrocarbons were strongly correlated with water vapor, indicating a convective source. On the other hand, decreases in the measured mixing ratios of CFCs, HCFCs, and other long-lived halocarbons (LLHCs) in the UT were used as tracers for stratosphere-to-troposphere transport (STT). These two sets of tracers were used to divide the DC3 WAS merge into many subsets of data corresponding to: the PBL, convective outflow in the UT, convective outflow in the LS (i.e. overshooting tops), STT-influenced air in the troposphere, background UT air, and background LS air. Using these derived subsets of data, interactions and mixing between stratospheric intrusions and tropospheric air masses was investigated. A large number of stratospherically-influenced samples were found to have reduced levels of O3 and elevated levels of CO (both relative to background stratospheric air); indicative of mixing with anthropogenically-influenced air. Using n-butane and propane as tracers of anthropogenically-influenced air, it is shown that this type of mixing was present both at low altitudes and in the UT. At low altitudes, this mixing resulted in O3 enhancements consistent with those reported at surface sites during deep stratospheric intrusions, while in the UT, two case studies were performed to identify the process by which this mixing occurs. In the first case study, stratospheric air was found to be mixed with aged outflow from a convective storm, while in the second case study, stratospheric air was found to have mixed with outflow from an active storm occurring in the vicinity of a stratospheric intrusion. From these analyses, it was concluded that deep convective events may facilitate the mixing between stratospheric air and polluted boundary layer air in the UT. Throughout the entire DC3 study region, this mixing was found to be prevalent: 72% of all samples that involve stratosphere-troposphere mixing show influence of polluted air. Applying a simple chemical kinetics analysis to these data, it is shown that the instantaneous production of OH in these mixed stratospheric-polluted air masses was 11 ± 8 (± 1?) times higher than that of stratospheric air, and 4.2 ± 1.8 times higher than that of background upper tropospheric air, which could result in a quick, high magnitude pulse of O3 production and reduced lifetimes of OH-controlled species in the UT. These derived subsets of data were used to investigate the effects of deep convection on mixing ratios of organic chlorine and organic bromine in the UT and LS over the DC3 study region. In the LS, it was found that mixing ratios of organic chlorine in overshooting tops were higher than mixing ratios of organic chlorine in the background LS by an average of 217 ± 179 pptv (6.3 ± 5.2% enhancement), while the total organic bromine mixing ratio in overshooting tops was higher than that of the background LS by an average of 2.8 ± 3.2 pptv (17.4 ± 19.9% enhancement). In both cases, short-lived halocarbons made up a large portion of this enhancement. In the UT, convection was found to play a much more complicated role on the organic halogen content of the region. Model back trajectories and analysis of the chemical composition of the background UT revealed that long-range transport of outflow from East Asia and from the central Pacific affected the background UT of the DC3 study region to varying degrees on different days. When the background UT was affected by East Asian outflow, mixing ratios of organic chlorine in convective outflow were lower than those of the local background UT by up to 150 ± 115 pptv (3.7 ± 2.9% enhancement). On the other hand, when the background UT was affected by clean outflow from the central Pacific, mixing ratios of organic chlorine in convective outflow were higher than the local background UT by up to 115 ± 98 pptv (3.2 ± 2.6% enhancement). Mixing ratios of organic bromine in the background UT were unaffected by these long-range transport processes. However, mixing ratios of organic bromine in convective outflow were highly variable and were affected by the transport of brominated very short-lived halocarbons (VSLH) from the Gulf of Mexico to the surface of the DC3 study region. When organic bromine enhancements in convective outflow were calculated on a flight-by-flight basis, organic bromine mixing ratios in convective outflow were higher than those in the background UT by an average of 1.7 ± 1.6 pptv (8.5 ± 8.1%). Based on these results, it is speculated that deep convection may play an indirect role in climate change by introducing pulses of short-lived halocarbons into the UT/LS, which in turn result in relatively quick (on the order of a few months) pulses of O3 loss in the region.
Author: Kai-Wei Chang
Publisher:
ISBN:
Category : Stratospheric circulation
Languages : en
Pages : 0
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As a gateway of troposphere-to-stratosphere transport, the tropical tropopause layer (TTL) plays a key role in determining the concentration and distribution of water vapor in the upper troposphere and lower stratosphere (UTLS). This dissertation presents three studies on the dynamical and radiative processes that influence the TTL and also the Brewer-Dobson circulation (BDC) in the global UTLS. Water vapor in the tropical lower stratosphere is strongly correlated with TTL temperatures, which are closely associated with latent heating (LH) in tropical convection. The first study examines the role of latent heating (LH) vertical distribution in TTL cooling and upper-tropospheric warming associated with equatorial wave responses. Using cross-spectral analysis on time series of LH and UTLS temperature, we show that heating above 6 km was found to have the highest coherence with the equatorial wave cooling and warming pattern in the mean temperature profile. We distinguish the effects of convective and stratiform LH, whose heating altitudes differ. Stratiform LH exhibits higher coherence with temperature throughout the UTLS, especially in the equatorial Rossby wave response as seen in the cross-spectral analysis. Highest coherences occur mostly at time scales of the Madden-Julian Oscillation (MJO), suggesting the importance of MJO convection in TTL cooling and subsequent dehydration processes. The second study explores the relationship of TTL cirrus clouds to gravity and Kelvin waves. Motivated by the recent interest in understanding how the vertical gradient of temperature anomalies (dT'/dz) from waves influence clouds, we collocate lidar observations of TTL clouds and wave temperature anomalies from radio occultation to understand how cloud occurrence relates to wave anomalies. Throughout the TTL, 57% of clouds were found in the wave phases where both the temperature anomaly (T') and dT'/dz were negative. In contrast, 24% of clouds were in the phase of negative T' but positive dT'/dz, suggesting that regions of negative dT'/dz significantly promote the formation and/or maintenance of clouds. We show that larger (smaller) values T' are associated with a lower (higher) probability of cloud occurrence, demonstrating connection of wave amplitude to TTL cloud formation. The BDC is a balance between wave-mean-flow interaction and radiative heating rates in the middle atmosphere. Since clouds modulate the amount of upwelling radiation, they can also influence the radiative heating in the UTLS. Using the CloudSat/CALIPSO 2B-FLXHR-LIDAR data set and the MERRA-2 reanalysis, the final study evaluates cloud effects on the BDC by comparing the mass circulation diagnosed from clear-sky and all-sky radiative heating rates. Cloud effects are strongest during boreal winter when the vertical and meridional components of the BDC below 80 hPa exhibit differences on the order of 0.1 mm/s and 10 cm/s, respectively. These magnitudes are comparable to the BDC itself, illustrating that cloud effects on radiative heating rates can have a significant influence on the strength of tropical upwelling and meridional mixing. TTL cirrus, which tends to impose weak heating in the TTL, were found to enhance the tropical upwelling and also the poleward transport, while the aggregate effect of all other cloud types was to weaken them instead.
Author: Katrina S. Virts
Publisher:
ISBN:
Category : Convection (Meteorology)
Languages : en
Pages : 126
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Deep convection is a major heat source in the tropical atmosphere, driving circulation cells that are an important aspect of Earth's climate. The environmental conditions that modulate the occurrence of lightning, which is associated with vigorous convection, are examined using observations from WWLLN, a ground-based lightning network. Diurnal lightning climatologies illustrate the interplay between sea breezes, mountain-valley wind regimes, and remotely forced gravity waves in touching off thunderstorms in a wide variety of geographical settings. Over the Maritime Continent, 850-hPa wind speed and area-averaged cloudiness are shown to be inversely related to day-to-day lightning frequency over land. Both lightning and rainfall, which is observed by the TRMM satellite, are suppressed windward of, and enhanced leeward of mountain ranges. These relationships are also observed during the active and break periods of the intraseasonal Madden-Julian Oscillation (MJO). The relationship between lightning and nitrogen oxide radicals, which are associated with ozone production, over the Maritime Continent is examined based on WWLLN observations and tropospheric NO2 data from the GOME-2 satellite. Composites of the daily NO2 regressed onto lightning frequency reveal a plume of enhanced NO2 following a day of enhanced lightning. Lightning and NO2 also vary coherently with the MJO, with variations of up to ~50% of the annual mean. MJO-related deep convection induces planetary-scale Kelvin and Rossby waves in the stably stratified tropical tropopause transition layer (TTL). The structure of these waves is investigated using satellite observations from COSMIC, CALIPSO, and MLS, as well as ERA-Interim wind and humidity fields. Regions of ascent in the planetary waves are associated with anomalously low temperatures, high radiative heating rates, enhanced cirrus occurrence, and high carbon monoxide and low ozone concentrations. Low water vapor concentrations lag the low temperature anomalies by ~1-2 weeks. Anomalies in each field tilt eastward with height in the TTL and propagate downward from the lower stratosphere to the upper troposphere. As the Kelvin wave front propagates eastward across equatorial South America and Africa, equatorially-symmetric, anomalously low temperature and water vapor mixing ratio and enhanced TTL cirrus are observed above ~100 hPa in the zonal-mean.
Author:
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Category : Aeronautics
Languages : en
Pages : 704
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Author:
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Category : Atmospheric chemistry
Languages : en
Pages : 524
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