Dynamical and Radiative Processes in the Tropical Tropopause Layer PDF Download

Author: Kai-Wei Chang
Publisher:
ISBN:
Category : Stratospheric circulation
Languages : en
Pages : 0

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Book Description
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.