The characteristics of explosively developing extratropical cyclones in the northwestern Pacific region are analyzed using the global objectively Analyzed dataset (GANAL) provided by the Japan Meteorological Agency (JMA). In the present paper, these cyclones are classified into three types, depending on positions of formation and of rapid development: OJ cyclones originate over the eastern Asian continent and develop over the Sea of Japan or the Sea of Okhotsk; PO-L cyclones are also formed over the Asian continent and develop over the northwestern Pacific Ocean; and PO-O cyclones are formed and develop over the northwestern Pacific Ocean. Statistical analyses suggest that OJ cyclones frequently appeared in late fall and had the smallest deepening rates of the three types; PO-L cyclones had medium deepening rates and frequently occurred in early and late winter; and PO-O cyclones mainly occurred in midwinter and had the largest deepening rates. Two kinds of composite analyses were conducted to un...

Structures and Environment of Explosively Developing Extratropical Cyclones in the Northwestern Pacific Region

Synoptic-scale incursions of cold, midlatitude air that penetrate deep into the Tropics are frequently observed to the east of the Andes cordillera. These incursions are a distinctive year-round feature of the synoptic climatology of this part of South America and exhibit similar characteristics to cold surges observed in the lee of the Rocky Mountains and the Himalayan Plateau. While their large-scale structure has received some attention, details of their mesoscale structural evolution and underlying dynamics are largely unknown. This paper advances our understanding in these matters on the basis of a mesoscale numerical simulation and analysis of the available data during a typical case that occurred in May of 1993. The large-scale environment in which the cold air incursion occurred was characterized by a developing midlatitude wave in the middle and upper troposphere, with a ridge immediately to the west of the Andes and a downstream trough over eastern South America. At the surface, a migra...

http://www.dgf.uchile.cl/rene/PUBS/csurges_case_MWR.pdf

Cold Air Incursions over Subtropical and Tropical South America: A Numerical Case Study

A systematic and objective procedure is developed for applying the simple cyclone classification scheme of Petterssen and Smebye. This method uses a height-attributable solution of the quasi-geostrophic ! equation to identify and quantify the relative importance of upper- and lower-tropospheric forcing and also the time trend in the horizontal spacing of the forcing at these two levels. By applying this classification method to a sample of cyclogenesis events, during their maximum intensification stage, from the Fronts and Atlantic Storm-Track EXperiment field experiment, the Type A and Type B scheme of Petterssen and Smebye is reproduced and extended to include a Type C. Type C consists of upper-level dominated cyclones that form at high latitudes and in their initial stages resemble comma-cloud-type polar lows. Detailed examples of each of the three types are presented. Some cyclones can undergo more than one period of development and it is found that each development period can be classified as a different cyclogenesis category, A or B: these cyclones are classified as hybrid Type A=B. There is some evidence that cyclone forecast accuracy depends on cyclone type, thereby suggesting the potential for this method to be used to assign confidence levels for forecasts of cyclogenesis produced by numerical weather-prediction models. Type B cyclones appear more difficult to predict, because their development depends, initially, on the interaction between significant features in the upper and lower troposphere. Copyright © 2002 Royal Meteorological Society. T. D. Hewson's contribution is Crown copyright.

A classification of FASTEX cyclones using a height-attributable quasi-geostrophic vertical-motion diagnostic

The formation of a blocking anticyclone over the North Atlantic has been examined over its entire life-CyCle using the Zwack-Okossi (Z-O) equation as the diagnostic tool. This blocking anticyclone occurred in late October and early November of 1985. The data used were provided by the NASA Goddard Laboratory for Atmospheres on a global 2.O degree latitude by 2.5 degree longitudinal grid. The horizontal distribution of the atmospheric forcing mechanisms that were important to 500 mb block formation, maintenance and decay were examined. A scale-partitioned form of the Z-O equation was then used to examine the relative importance of forcing on the planetary and synoptic scales, and their interactions. As seen in previous studies, the results presented here show that upper tropospheric anticyclonic vorticity advection was the most important contributor to block formation and maintenance. However, adiabatic warming, and vorticity tilting were also important at various times during the block lifetime. In association with precursor surface cyclogenesis, the 300 mb jet streak in the downstream (upstream) from a long-wave trough (ridge) amplified significantly. This strengthening of the jet streak enhanced the anti-cyclonic vorticity advection field that aided the amplification of a 500 mb short-wave ridge. Tile partitioned height tendency results demonstrate that the interactions between the planetary and sn,noptic-scale through vorticity advection was the most important contributor to block formation. Planetary-scale, synoptic-scale. and their interactions contributed weakly to the maintenance of the blocking anticyclone with the advection of synoptic-scale vorticity by the planetary-scale flow playing a more important role. Planetary-scale decay ofthe long-wave ridge contributed to the demise of this blocking event.

/pdf/planetary-and-synoptic-scale-interactions-during-the-life-2x5cr4gpg9.pdf

Planetary and synoptic-scale interactions during the life cycle of a mid-latitude blocking anticyclone over the North Atlantic

Using the Goddard Laboratory for Atmospheres Goddard Earth Observing System-1 analyses, the horizontal distribution of atmospheric forcing processes involved in the growth, maintenance, and decay of two Northern Hemisphere midlatitude blocking anticyclones that occurred simultaneously were examined, with the goal of determining whether there was a dynamic connection between the two cases. The complete form of the Zwack‐ Okossi equation, which is a generalization of the Petterssen‐Sutcliffe development equation, was used as the primary diagnostic tool. The basic analyses were partitioned into their planetary-scale and synoptic-scale components (noninteraction) 500-hPa height tendencies, as well as scale interaction 500-hPa height tendencies. Based on the results of this diagnosis, there was no dynamic connection between these blocking anticyclones. This result agrees with findings of other studies based on a statistical analysis of simultaneous blocking. These results also imply that blocking may be a local phenomenon. The diagnostic results from these blocking events were, in many respects, similar to those of previous studies. However, some key differences were found. For example, it was found here that for both mode 2 blocking events, temperature advection was an important mechanism in block formation. Earlier results showed vorticity advection as the primary atmospheric forcing process in block formation for a mode 1 block. Also, the scalepartitioned results show that upstream cyclones contributed to block formation and intensification in these events directly through the synoptic-scale component, often with the support of the interaction component, of the total forcing. Earlier scale-partitioned results showed that the interaction component of the total forcing was most important in the formation of a mode 1 block.

/pdf/a-diagnosis-of-two-blocking-events-that-occurred-5avf7fs2kk.pdf

A Diagnosis of Two Blocking Events That Occurred Simultaneously in the Midlatitude Northern Hemisphere

A new method for numerically solving the classical quasi-geostrophic omega equation is proposed. The method, in effect, integrates the omega equation upward using two bottom boundary conditions at the top of the boundary layer: ω and ∂ω/∂p. The former can be specified by using methods that use only surface data while the latter is calculated using information contained in the horizontal Laplacian of the surface pressure tendency field. As opposed to traditional solutions, the proposed method requires no explicit horizontal boundary condition, requires no data above the level at which ω is desired, nor constrains the static stability. When integrated to the level near the top of the atmosphere where ω is normally taken to be zero, the equation becomes a development equation that is similar to (and is shown to be more complete than) the Petterssen-Sutcliffe development equation. The new method to calculate omega is tested in a simple, analytic atmosphere and the solution is found to satisfy the cla...

Benoit Okossi

Papers

A New Method for Solving the Quasi-Geostrophic Omega Equation by Incorporating Surface Pressure Tendency Data