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Kevin F. Trenberth and K. C. Mo

Abstract

The focus of this Paper is on the frequency and spatial distributions of blocking and persistent anomalies of geopotential height over the Southern Hemisphere. The analysis is based upon daily height fields at 1000 and 500 mb for both summer and winter. Histogram frequency distributions of height anomalies and maps of the skewness and kurtosis have been computed. Blocking events are objectively defined by requiring a large positive anomaly to exist for 5 days or more. Composite flow and anomaly fields for several cases are presented and examined in detail. The geographical distribution of the frequency of lame amplitude ⩾150 gpm anomalies at 500 mb that persist for only 1–3 days is very similar to that of the high-frequency band (2–8 day period) variances that identity the storm tracks in the Southern Hemisphere.

The primary location for blocking in the Southern Hemisphere is in the New Zealand sector and blocking occurs through a local enhancement of the climatological split in the mean westerlies on a spatial scale of 60° longitude. Other maxima occur southeast of South America and over the southern Indian Ocean. Sporadically, multiple blocking events occur at more than one location and one triple blocking event is examined in detail. Zonal wave 3 is strongly evident in such cases; it also plays a dominant role in the majority of blocking events. However, on most occasions, blocking occurs in isolation as a local phenomenon, and it appears that the local wave 3 may be but part of a wave train with a great circle rather than zonal orientation. Transient eddies appear to play an important role in sustaining a blocking event by either continually reinforcing the anticyclone on its western flank or by quickly reestablishing a new anticyclone as the old one breaks down or moves away.

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Kevin E. Trenberth and Won-Tae K. Shin

Abstract

An investigation is made into the presence of quasi-biennial oscillations (QBOs) in sea-level pressure fields over the Northern Hemisphere. Using 55 years (1925–79) of seasonally averaged sea-level pressure anomalies, a series of analyses has been performed in order to systematically isolate and describe the QBOs. A standard empirical orthogonal function (EOF) analysis of the seasonal anomalies reveals significant nonrandomness at QBO periods. The data were then band-pass filtered in order to focus on the QBO, and a second EOF analysis was performed. Finally, a new complex EOF analysis technique was applied to the data. Complex EOFs have the advantage of permitting both standing and propagating modes. The QBO variance in six standard EOFs is essentially confined to four EOFs in the filtered data set and compressed into three complex EOFs. The latter account for 56% of the filtered variance.

The dominant mode of the complex EOF analysis was common to all analyses and has been found in many other studies. It is essentially a standing wave pattern corresponding to a high-latitude zonal index with departures in pressure of opposite sign in low and high latitudes. This mode includes elements of the North Atlantic Oscillation, the Pacific–North American teleconnection pattern and the Southern Oscillation, but also differs somewhat from all three. It tends to be phase locked to the annual cycle.

The second complex EOF mode C2 exhibits clear propagating characteristics as it evolves in time. The third mode C3 also has some propagating characteristics and, at times, both modes strongly resemble wave trains of quasi-stationary Rossby waves. Neither C2 nor C3 is phase locked to the annual cycle.

It appears likely that all three complex EOFs are normal mode responses of the atmosphere to different kinds of forcing. Statistical evidence for phase locking of all three complex EOFs to the QBO in zonal winds in the equatorial stratosphere is not convincing, and the origin of the preferred QBO periodicity remains to be determined.

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R. Ware, M. Exner, D. Feng, M. Gorbunov, K. Hardy, B. Herman, Y. Kuo, T. Meehan, W. Melbourne, C. Rocken, W. Schreiner, S. Sokolovskiy, F. Solheim, X. Zou, R. Anthes, S. Businger, and K. Trenberth

This paper provides an overview of the methodology of and describes preliminary results from an experiment called GPS/MET (Global Positioning System/Meteorology), in which temperature soundings are obtained from a low Earth-orbiting satellite using the radio occultation technique. Launched into a circular orbit of about 750-km altitude and 70° inclination on 3 April 1995, a small research satellite, MicroLab 1, carried a laptop-sized radio receiver. Each time this receiver rises and sets relative to the 24 operational GPS satellites, the GPS radio waves transect successive layers of the atmosphere and are bent (refracted) by the atmosphere before they reach the receiver, causing a delay in the dual-frequency carrier phase observations sensed by the receiver. During this occultation, GPS limb sounding measurements are obtained from which vertical profiles of atmospheric refractivity can be computed. The refractivity is a function of pressure, temperature, and water vapor and thus provides information on these variables that has the potential to be useful in weather prediction and weather and climate research.

Because of the dependence of refractivity on both temperature and water vapor, it is generally impossible to compute both variables from a refractivity sounding. However, if either temperature or water vapor is known from independent measurements or from model predictions, the other variable may be calculated. In portions of the atmosphere where moisture effects are negligible (typically above 5–7 km), temperature may be estimated directly from refractivity.

This paper compares a representative sample of 11 temperature profiles derived from GPS/MET soundings (assuming a dry atmosphere) with nearby radiosonde and high-resolution balloon soundings and the operational gridded analysis of the National Centers for Environmental Prediction (formerly the National Meteorological Center). One GPS/MET profile was obtained at a location where a temperature profile from the Halogen Occultation Experiment was available for comparison. These comparisons show that accurate vertical temperature profiles may be obtained using the GPS limb sounding technique from approximately 40 km to about 5–7 km in altitude where moisture effects are negligible. Temperatures in this region usually agree within 2°C with the independent sources of data. The GPS/MET temperature profiles show vertical resolution of about 1 km and resolve the location and minimum temperature of the tropopause very well. Theoretical temperature accuracy is better than 0.5°C at the tropopause, degrading to about 1°C at 40-km altitude.

Above 40 km and below 5 km, these preliminary temperature retrievals show difficulties. In the upper atmosphere, the errors result from initial temperature and pressure assumptions in this region and initial ionospheric refraction assumptions. In the lower troposphere, the errors appear to be associated with multipath effects caused by large gradients in refractivity primarily due to water vapor distribution.

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R. A Anthes, P. A Bernhardt, Y. Chen, L. Cucurull, K. F. Dymond, D. Ector, S. B. Healy, S.-P. Ho, D. C Hunt, Y.-H. Kuo, H. Liu, K. Manning, C. McCormick, T. K. Meehan, W J. Randel, C. Rocken, W S. Schreiner, S. V. Sokolovskiy, S. Syndergaard, D. C. Thompson, K. E. Trenberth, T.-K. Wee, N. L. Yen, and Z Zeng

The radio occultation (RO) technique, which makes use of radio signals transmitted by the global positioning system (GPS) satellites, has emerged as a powerful and relatively inexpensive approach for sounding the global atmosphere with high precision, accuracy, and vertical resolution in all weather and over both land and ocean. On 15 April 2006, the joint Taiwan-U.S. Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC)/Formosa Satellite Mission 3 (COSMIC/FORMOSAT-3, hereafter COSMIC) mission, a constellation of six microsatellites, was launched into a 512-km orbit. After launch the satellites were gradually deployed to their final orbits at 800 km, a process that took about 17 months. During the early weeks of the deployment, the satellites were spaced closely, offering a unique opportunity to verify the high precision of RO measurements. As of September 2007, COSMIC is providing about 2000 RO soundings per day to support the research and operational communities. COSMIC RO data are of better quality than those from the previous missions and penetrate much farther down into the troposphere; 70%–90% of the soundings reach to within 1 km of the surface on a global basis. The data are having a positive impact on operational global weather forecast models.

With the ability to penetrate deep into the lower troposphere using an advanced open-loop tracking technique, the COSMIC RO instruments can observe the structure of the tropical atmospheric boundary layer. The value of RO for climate monitoring and research is demonstrated by the precise and consistent observations between different instruments, platforms, and missions. COSMIC observations are capable of intercalibrating microwave measurements from the Advanced Microwave Sounding Unit (AMSU) on different satellites. Finally, unique and useful observations of the ionosphere are being obtained using the RO receiver and two other instruments on the COSMIC satellites, the tiny ionosphere photometer (TIP) and the tri-band beacon.

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