The Effect of Deep Convection on Temperatures in the Tropical Tropopause Layer and Its Implications to the Regulation of Tropical Lower Stratospheric Humidity PDF Download
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Languages : en
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Languages : en
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Author: J. S. Hosking
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Languages : en
Pages :
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Category : Dissertations, Academic
Languages : en
Pages : 918
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Author: Kathleen Anne Schiro
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ISBN:
Category :
Languages : en
Pages : 148
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Constraining precipitation processes in climate models with observations is crucial to accurately simulating current climate and reducing uncertainties in future projections. This work presents robust relationships between tropical deep convection, column-integrated water vapor (CWV), and other thermodynamic quantities analyzed with data from the DOE Atmospheric Radiation Measurement (ARM) Mobile Facility in Manacapuru, Brazil as part of the GOAmazon campaign and are directly compared to such relationships at DOE ARM sites in the tropical western Pacific. A robust relationship between CWV and precipitation, as explained by variability in lower tropospheric humidity, exists just as strongly in a tropical continental region as it does in a tropical oceanic region. Given sufficient mixing in the lower troposphere, higher CWV generally results in greater plume buoyancies through a deep convective layer. Although sensitivity of convection to other controls is suggested, such as microphysical processes and dynamical lifting mechanisms, the increase in buoyancy with CWV is consistent with the sharp increase in precipitation observed. Entraining plume buoyancy calculations confirm that CWV is a good proxy for the conditional instability of the environment, yet differences in convective onset as a function of CWV exist over land and ocean, as well as seasonally and diurnally over land. This is largely due to variability in the contribution of lower tropospheric humidity to the total column moisture. Over land, the relationship between deep convection and lower free tropospheric moisture is robust across all seasons and times of day, whereas the relation to boundary layer moisture is robust for the daytime only. Using S-Band radar, these transition statistics are examined separately for unorganized and mesoscale-organized convection, which exhibit sharp increases in probability of occurrence with increasing moisture throughout the column, particularly in the lower free troposphere. An observational basis for an integrated buoyancy measure from a single plume buoyancy formulation that provides a strong relation to precipitation can be useful for constraining convective parameterizations. A mixing scheme corresponding to deep inflow of environmental air into a plume that grows with height provides a weighting of boundary layer and free tropospheric air that yields buoyancies consistent with the observed onset of deep convection across seasons and times of day, across land and ocean sites, and for all convection types. This provides a substantial improvement relative to more traditional constant mixing assumptions, and a dramatic improvement relative to no mixing. Furthermore, it provides relationships that are as strong or stronger for mesoscale-organized convection as for unorganized convection. Downdrafts and their associated parameters are poorly constrained in models, as physical and microphysical processes of leading order importance are difficult to observe with sufficient frequency for development of robust statistics. Downdrafts and cold pool characteristics for mesoscale convective systems (MCSs) and isolated, unorganized deep precipitating convection in the Amazon are composited and both exhibit similar signatures in wind speed, surface fluxes, surface equivalent potential temperature () and precipitation. For both MCSs and unorganized convection, downdrafts associated with the strongest modifications to surface thermodynamics have increasing probability of occurrence with decreasing height through the lowest 4 km and show similar mean downdraft magnitudes with height. If is approximately conserved following descent, a large fraction of the air reaching the surface likely originates at altitudes in the lowest 2 km. Mixing computations suggest that, on average, air originating at heights greater than 3 km would require substantial mixing, particularly in the case of isolated cells, to match the observed cold pool . Statistics from two years of surface meteorological data at the GOAmazon site and 15 years of data at the DOE ARM site on Manus Island in the tropical western Pacific show that conditioned on precipitation levels off with increasing precipitation rate, bounded by the maximum difference between surface and its minimum in the profile aloft. Robustness of these statistics observed across scales and regions suggests their potential use as model diagnostic tools for the improvement of downdraft parameterizations in climate models.
Author: Kai-Wei Chang
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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: Kai (M.S. in statistics) Zhang
Publisher:
ISBN:
Category :
Languages : en
Pages : 350
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Convection and its interactions with transport of heat, water and momentum are essential in understanding many aspects of the climate system, for example, tracer transport in both the troposphere and lower stratosphere, the modelling of rainfall and prediction of climate change. The goal of this dissertation is to examine the role of convection variability in the related atmospheric composition transport and rainfall processes using both observations and climate models. The relationships between deep convection and the global diabatic heat budget are studied to understand the coupling between convection, temperature and general circulation. By examining the influence of convection on diabatic heating in the troposphere and stratosphere, we can gain a comprehensive understanding of the tropical convection and its influence on large-scale atmospheric circulation and stratosphere composition, especially on the cross-tropospuase transport of water vapor. We show that transient variability in deep convection is highly correlated with diabatic heating throughout the troposphere and stratosphere. Enhanced deep convection is linked to amplified heating in the tropical troposphere and in the mid-latitude storm tracks, tied to latent heat release. Enhanced convection is also linked to radiative cooling in the lower stratosphere, due to weaker upwelling longwave (LW) from lower altitudes. Transient deep convection modulates LW and shortwave (SW) radiation in the troposphere, with compensating effects that are linked to variations in cloud fraction and liquid and ice water content. Then we explore the variability of lower stratospheric (LS) water vapor in the Northern Hemisphere (NH) monsoon regions based on satellite observations and trajectory model simulations driven by diabatic heating in reanalyses. The links between stratospheric water vapor, fluctuations in deep convection and large-scale circulation and temperature are quantified. Results suggest that temperature plays a dominant role on water vapor variations with stronger convection leads to cold dehydration temperatures and a relatively dry stratosphere. Besides, the seasonal increase of stratospheric water vapor can be attributed to the geographic variations of convection and resultant variations of the dehydration center, of which the influence is comparable to the influence of the local dehydration temperature increase. Specifically, the seasonal geographic shift of the dehydration center from the east to the west Asian monsoon region with warmer tropopause temperatures could increase water vapor significantly. Dry biases over Southern Amazonia are observed in Community Atmosphere Model version 5 (CAM5). We use hindcast simulations to track the root causes for the biases. Results suggest that the dry bias is present by day 2 (24 to 48 hours) of model integrations and is very robust for all the seasons with largest bias magnitude during the southern summer (Dec-Feb, wet season). The near-surface-warm biases and low biases of humidity in the lower troposphere that exist since day 2 may be significant factors influencing the dry biases. The low biases of humidity are contributed by both physical components (shallow convective scheme and Zhang-McFarlane convective scheme, and maybe weak turbulence term), and dynamics (weak moisture convergence). We further evaluate the CAM5 with a higher-order turbulence closure scheme, named Cloud Layers Unified By Binomials (CLUBB), and a Multiscale Modeling Framework, referred as the "super-parameterization" (SP) with two different microphysics configurations to investigate their influences on rainfall simulations over Southern Amazonia. The two different microphysics configurations in SP are the one-moment cloud microphysics without aerosol treatment (SP1) and two-moment cloud microphysics coupled with aerosol treatment (SP2). Results show that both SP2 and CLUBB effectively reduce the low biases of rainfall, mainly during the wet season, and reduce low biases of humidity in the lower troposphere with further reduced shallow clouds and increased surface solar flux. These changes increase moist static energy, contribute to stronger convection and more rainfall. SP2 appears to realistically capture the observed increase of relative humidity prior to deep convection and it significantly increases rainfall in the afternoon; CLUBB significantly delays the afternoon peak time and produces more precipitation in the early morning, due to more gradual transition between shallow and deep convection. In CAM5 and CAM5 with CLUBB, occurrence of deep convection appears to be a result of stronger heating rather than higher relative humidity.
Author: Mekonnen Gebremichael
Publisher: Springer Science & Business Media
ISBN: 904812915X
Category : Science
Languages : en
Pages : 327
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With contributions from a panel of researchers from a wide range of fields, the chapters of this book focus on evaluating the potential, utility and application of high resolution satellite precipitation products in relation to surface hydrology.
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.
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Category : Aeronautics
Languages : en
Pages : 704
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Author: Daniel P. Grosvenor
Publisher:
ISBN:
Category :
Languages : en
Pages : 169
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