ALBERT

Beschreibung: The forcing of tropical upwelling in the Brewer–Dobson circulation (BDC) on intraseasonal to seasonal time scales is investigated in integrations of an idealized general circulation model, ECMWF Interim Re-Analysis, and lower-stratospheric temperature measurements from the (Advanced) Microwave Sounding Unit, with a focus on the extended boreal winter season. Enhanced poleward eddy heat fluxes in the high latitudes (45°–90°N) at the 100-hPa level are associated with anomalous tropical cooling and anomalous warming on the poleward side of the polar night jet at the 70-hPa level and above. In both the model and the observations, planetary waves entering the stratosphere at high latitudes propagate equatorward to the subtropics and tropics at levels above 70 hPa over an approximately 10-day period, exerting a force at sufficiently low latitudes to modulate the tropical upwelling in the upper branch of the BDC, even on time scales longer than the radiative relaxation time scale of the lower stratosphere. To the extent that they force the BDC via downward as opposed to sideways control, planetary waves originating in high latitudes contribute to the seasonally varying climatological mean and the interannual variability of tropical upwelling at the 70-hPa level and above. Their influence upon the strength of the tropical upwelling, however, diminishes rapidly with depth below 70 hPa. In particular, tropical upwelling at the cold-point tropopause, near 100 hPa, appears to be modulated by variations in the strength of the lower branch of the BDC.

Beschreibung: The causes of the annual cycle and nonseasonal variability in the globally averaged, equator-to-pole Brewer–Dobson circulation (BDC; defined here as the equatorially symmetric component of the Lagrangian-mean meridional circulation) are investigated based on zonally averaged, lower-stratospheric temperature data from satellite-borne Microwave Sounding Unit (MSU) and Advanced Microwave Sounding Unit (AMSU). Time-varying vertical velocities in the BDC are inferred from departures of the meridional temperature profiles from the respective radiative equilibrium temperature profiles. Equatorward of ∼45°N/S, the annual-mean profile of lower-stratospheric temperature and the seasonal and nonseasonal variations about it project almost exclusively onto the equatorially symmetric component. The climatological-mean annual cycle accounts for nearly 90% of the month-to-month variance of the equatorially symmetric component of the temperature field; January/February is colder than July/August equatorward of ∼45°N/S and warmer than July/August poleward of that latitude. The equator-to-subpolar temperature contrast roughly doubles from July/August to January/February, implying an approximate doubling of the strength of the BDC. The nonseasonal variability is dominated by a similar pattern. Tropical upwelling in the BDC, as inferred from of the temperature field, varies in response to variations in eddy heat fluxes at high latitudes with comparable strength on the intraseasonal and interannual time scales; it does not appear to be correlated with equatorial tropospheric planetary wave activity or with variations in wave forcing in subtropical lower stratosphere. It is concluded that high-latitude wave forcing plays an important role in modulating tropical upwelling in the BDC across a wide range of frequencies.

Beschreibung: A method for obtaining high time and spatial resolution convective cloud top data for the TTL Leonhard Pfister, Eric Jensen, Rei Ueyama, Eliot Atlas, and Maria Navarro Convective systems in the tropics have a maximum in the cloud top altitude distribution of about 13.5 km. However, there is a significant tail to this distribution -- a few percent reach the cold point tropopause (CPT) at 16.5 km, and there has been clear evidence of convective mass deposited as high as 19 km in the tropics. The region between 13.5 km and the cold point tropopause is transitional, between the free tropical troposphere where convective mixing dominates, and the stratosphere where slow upward ascent dominates. In this region (the Tropical Tropopause Layer), convective injection, slow ascent, and mixing from midlatitudes all have similar time scales. So, even though only a few percent of convective systems reach the CPT, convection is important. Space Based Lidar and cloud radar measurements have yielded information about long term average statistical distributions of cloud altitude as a function of location. However, we also need time-dependent cloud top altitude and cloud top potential temperature information, primarily to understand the water vapor and TTL cloud distributions. This is because the effect of convection depends on the local temperature, and on the subsequent temperature history. Time dependent cloud top information is also needed to understand short-lived tracers because cross-isentropic flow is time and space dependent. This paper presents a method of obtaining time and space dependent convective cloud top theta (and altitude) information using 3-hourly geostationary brightness temperature data, coupled with global 3 -hourly rainfall estimates and temperature analyses. We explore different mixing algorithms to obtain the most reasonable agreement with near-simultaneous observations by cloudsat and calipso. Observations of short-lived tracers from ATTREX, coupled with short-term trajectories are used to test the method's accuracy. An important caveat is the ambiguity of evaluating convective cloud top altitudes under from combined cloudsat and calipso measurements.

Beschreibung: Trajectory calculations with convective influence diagnosed from geostationary-satellite cloud measurements are used to evaluate the relative importance of different Tropical Tropopause Layer (TTL) transport pathways for establishing the distribution of carbon monoxide (CO) at 100 hPa as observed by the Microwave Limb Sounder (MLS) on board the Aura satellite. Carbon monoxide is a useful tracer for investigating TTL transport and convective influence because the CO lifetime is comparable to the time require for slow ascent through the TTL (a couple of months). Offline calculations of TTL radiative heating are used to determine the vertical motion field. The simple trajectory model does a reasonable job of reproducing the MLS CO distributions during Boreal wintertime and summertime. The broad maximum in CO concentration over the Pacific is primarily a result of the strong radiative heating (indicating upward vertical motion) associated with the abundant TTL cirrus in this region. Sensitivity tests indicate that the distinct CO maximum in the Asian monsoon anticyclone is strongly impacted by extreme convective systems with detrainment of polluted air above 360 K potential temperature. The relative importance of different CO source regions will also be discussed.