51 research outputs found
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Sub-seasonal variations in lower stratospheric water vapor
Observations of water vapor with high temporal and spatial resolution and good horizontal coverage just above the tropical tropopause have been scarce, but a preliminary version of such data has been developed using radiance measurements of the Microwave Limb Sounder. These data reveal distinct variations with periods in the ranges 10-25 days and 30-70 days, consistent with (respectively) slow Kelvin waves and the tropical intraseasonal oscillation
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Intraseasonal variations of water vapor in the tropical upper troposphere and tropopause region
We show the signature of the tropical intraseasonal oscillation (TIO) in upper tropospheric moisture and dynamical fields, roughly between 200 and 100 hPa. Relationships among these fields are examined using lag-correlation analysis and using multivariate extended empirical orthogonal functions (MEEOFs), which maximize the shared explained variance among several fields for both spatial and temporal variations. The MEEOFs show that all of the fields respond to the TIO and that the TIO is the dominant factor influencing each of the fields on these timescales. Convection associated with the TIO moistens the upper troposphere up to about 150 hPa, as expected; the behavior at 100 hPa is more complex. Over the longitude range where the TIO is associated with convection, roughly 60°-180°E, 100-hPa temperature and water vapor tend to be reduced above convection on TIO timescales. East of 180°, though, the temperature and water vapor variations at 100 hPa become decoupled. The water vapor variations, like those of 200-hPa velocity potential, appear to speed up at about 180°E
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Vertical velocity, vertical diffusion, and dilution by midlatitude air in the tropical lower stratosphere
Air passing upward through the tropical tropopause is âmarkedâ by an annually varying water vapor mixing ratio much as a tape recorder marks a magnetic tape; as the air ascends in the tropical stratosphere, these marks are effaced by a combination of vertical diffusion within the tropics and dilution of tropical air by sideways (isentropic) mixingâin of midlatitude air. We represent these processes using a oneâdimensional advectionâdiffusionâdilution model, which we inverseâsolve for the vertical profiles of three unknowns (vertical advection velocity, vertical diffusion coefficient, and dilution rate coefficient) after prescribing the vertical profiles of time mean methane [CHâ] and of amplitude and phase of the annually varying tape recorder signal in 2[CHâ]+[HâO]. When tested on synthetic data generated by forward solving the same model, the method for inverse solution proved to be well conditioned and to give accurate results above 18 km. Applying the method to 5 years of smoothed data from the Halogen Occultation Experiment, we find a vertical advection velocity with a minimum of about 0.2 mm sâ»Âč near 20 km, and both dilution rate coefficient and vertical diffusion coefficient with remarkably low minima near 22 km, 1/(6â7 year) and roughly 0.02 m2sâ»Âč, respectively. Our derived profile of vertical advection velocity agrees well, between 18 and 24 km, with an independent, radiatively derived, massâbudgetâconstrained transformed Eulerian mean calculation. Despite the relatively modest values of the diffusion coefficient, vertical diffusion plays a significant role in attenuating the tape recorder signal, according to our model. The minimum value of the dilution rate coefficient corresponds to a relaxation timescale of 6â7 years, much longer than the timescales found in other studies. The long relaxation timescale at 20â24 km is, however, consistent with (1) the minimum in vertical velocity, (2) a reduced attenuation rate in the tape recorder signal, and (3) a decrease, hitherto unremarked, in the tropical vertical gradient of [CHâ] there
On the existence of the logarithmic surface layer in the inner-core of hurricanes
Q. J. R. Meteorol. Soc., 1394128
Extratropical Impacts on Atlantic Tropical Cyclone Activity
With warm sea surface temperature (SST) anomalies in the tropical Atlantic and cold SST anomalies in the east Pacific, the unusually quiet hurricane season in 2013 was a surprise to the hurricane community. The authors' analyses suggest that the substantially suppressed Atlantic tropical cyclone (TC) activity in August 2013 can be attributed to frequent breaking of midlatitude Rossby waves, which led to the equatorward intrusion of cold and dry extratropical air. The resultant mid- to upper-tropospheric dryness and strong vertical wind shear hindered TC development. Using the empirical orthogonal function analysis, the active Rossby wave breaking in August 2013 was found to be associated with a recurrent mode of the midlatitude jet stream over the North Atlantic, which represents the variability of the intensity and zonal extent of the jet. This mode is significantly correlated with Atlantic hurricane frequency. The correlation coefficient is comparable to the correlation of Atlantic hurricane frequency with the main development region (MDR) relative SST and higher than that with the Niño-3.4 index. This study highlights the extratropical impacts on Atlantic TC activity, which may have important implications for the seasonal predictability of Atlantic TCs
Biographical Sketch, Timothy J. Dunkerton
Naval Postgraduate School Faculty Resum
Decadal Variability and Temperature Trends in the Middle Atmosphere From Historical Rocketsonde Data
Observational studies were performed using historical rocketsonde data to investigate long-term temperature trends, solar-cycle variations, and interactions between tropical and extratropical latitudes in the middle atmosphere. Evidence from tropical, subtropical, and midlatitude North American rocketsonde stations indicated a consistent downward trend over 25 years, with a solar cycle component superposed. The trend is about -1.4 to -2.0 K per decade and the amplitude of the decadal oscillation is about 1.1 K. Prior to trend derivation it was necessary for us to correct temperatures for aerodynamic heating in the early years. The empirically derived correction profile agrees well with a theoretical profile of Krumins and Lyons. A study was also performed of the correlation between equatorial winds and north polar temperatures in winter, showing that the entire stratospheric wind profile near the equator -- including the quasi-biennial oscillation (QBO) and stratopause semiannual oscillation (SAO) -- is important to the extratropical flow, not merely the QBO component as previously thought. A strong correlation was discovered between winter polar temperatures and equatorial winds in the upper stratosphere during the preceding September, suggesting a role for the second cycle of the SAO
Application of the Marsupial Paradigm to Tropical Cyclone Formation from Northwestward Propagating Disturbances
Mon. Wea. Rev., 140 66-76The article of record as published may be located at http://dx.doi.org/10.1175/2011MWR3604.
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