Ageostrophic Forcing in a Height Tendency Equation

The role of the ageostrophic vorticity tendencies in the Zwack-Okossi vorticity tendency equation has been examined within the context that this term in the equation acts as a correction term for the vorticity tendency or height tendency calculations. In this paper, this term is shown to act as both a response to initial forcing and as a forcing mechanism itself in modulating the initial forcing. Using an earlier methodology for partitioning the wind field and the height field into their divergent and rotational parts, the ageostrophic wind is shown to contain components of both parts. Then, using an omega equation, an ageostrophic vorticity tendency is calculated, which has forcing mechanisms similar to a height tendency equation. This ageostrophic tendency is then used in this height tendency equation and provides for an improved method of calculating the ageostrophic wind rather than simply calculating it as a residual. Two case studies are then examined to demonstrate this point. For both case studies, the calculated height tendencies using the derived ageostrophic winds and those calculated as a residual overestimated the height tendencies, especially at the surface. While the correlation coefficients show that the two methods are comparable at 500 hPa, at the surface there is a notable degree of improvement in the pattern similarity. This resulted in an improved representation of the 500-hPa height tendencies for the onset of the Atlantic blocking event despite marginal improvements in the calculated field overall

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

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

The Variability in Atlantic Ocean Basin Hurricane Occurrence and Intensity as related to ENSO and the North Pacific Oscillation

The investigation of the effect of El Niño-related variability on hurricane activity has been a popular topic of study. Studies have shown that there are fewer Atlantic Ocean basin hurricanes during an El Nino year than during a La Nina year. Various atmospheric and oceanic parameters that influence hurricane development become significantly altered during an El Niño event, leading to suppressed easterly wave development and growth. The effect of the El Niño/La Niña cycle on hurricane intensity, however, is not straightforward. Studies addressing the interannual variability of hurricane intensity have captured far less attention than the more generalized subject of hurricane occurrence. This study examined the interannual variability of hurricane intensity (measured as wind speed and interpreted through the Saffir-Simpson Scale) from 1938 through 1999. These data were then compared with the occurrence of El Niño/La Niña events as defined using the Japan Meteorological Association (JMA) index. El Nino/La Nina variability superimposed on variability associated with the North Pacific Oscillation (NPO) was also examined here. Not surprisingly, during an El Niño year the intensity of Atlantic hurricanes was found to be weaker than during a neutral year or a La Niña year. There were also significant differences found in hurricane intensity between El Nino and La Nina years when the NPO was in phase 1, rather than when the NPO was in phase 2. Finally, this study also examined the interannual variation in hurricane intensity by genesis region (i.e. the eastern and western Atlantic Ocean Basins, the Caribbean, and the Gulf of Mexico)

The Application of a Simple Method for the Verification of Weather Forecasts and Seasonal Variations in Forecast Accuracy

The evaluation of weather forecast accuracy has always been a difficult subject to address for many reasons. In this study, a simple semiobjective method is used to examine the accuracy of zone forecasts issued by the Weldon Spring (Saint Louis) National Weather Service (NWS) Office for mid-Missouri over a period of 416 days with the goal of demonstrating the utility of this method. Zone forecasts were chosen because these forecasts are typically what the public will receive either directly or indirectly from various media outlets. Not surprising, the evaluation method used here demonstrates that forecasts issued by the NWS and the Nested Grid Model (NGM) model output statistics (MOS) represent a considerable improvement over persistence or climatological baseline forecasts. NWS forecasts were slightly better than NGM MOS forecasts, especially when considering temperature and precipitation only. All forecasts showed distinct seasonal variability. The NWS winter-season forecasts were superior to those issued in the summer season, and this superiority was found to be a function of the precipitation forecast parameter. This technique might represent an easily understandable and concise method for providing weather forecast performance information to the general public in such a way that it would instill or reinforce public confidence in the accuracy of weather forecasts

Interannual variability of tropical cyclone activity in the southern South China Sea

A study of tropical storm activity in the southern South China Sea region was carried out for the period of 1960 to 2006 using data obtained from the UNISYS website archive, which was provided to them from the Joint Typhoon Warning Center (JWTC) best track data. This study was motivated by two particularly costly storms that impacted Malaysia during the 1996-2001 period. This study demonstrated that November and December were the most active months for tropical cyclone activity in this region. A majority of these storms attained tropical storm intensity. Also, a majority of the tropical cyclones originated within the study area near Malaysia as opposed to moving into the area. The long-term trend showed that there has been a slight increase in tropical cyclone activity in the region, but the trend was not statistically significant. A study of the interannual variability revealed that there was more (less) tropical cyclone activity in the region during La Nin˜a (El Nin˜o) years. Longer-term variability, such as that related to the Pacific Decadal Oscillation, was not found in the analysis here. Using spectral methods confirms that there was significant El Nin˜o-related variability in climatological quantities such as monthly sea surface temperatures or pressures. Finally, the background climatological state was examined in order to determine whether or not the atmosphere in the region was more conducive to tropical cyclone formation or maintenance during active years. It was found that the most active years were associated with warmer SSTs in the study region, relatively weak 200-850 hPa wind shear, a warm-core structure, more water vapor, and more cyclonic low-level relative vorticity, and these were all La Nin˜a-type years. Nonactive years were associated with weaker wind shear, less water vapor, and a more anticyclonic (vorticity) background, regardless of whether the SSTs were warmer or cooler, and most of these were El Nin˜o-type years

The Interactions between a Midlatitude Blocking Anticyclone and Synoptic-Scale Cyclones That Occurred during the Summer Season

Using the Goddard Laboratory for Atmospheres Goddard Earth Observing System 5-yr analysis and the Zwack-Okossi equation as the diagnostic tool, the horizontal distribution of the dynamic and thermodynamic forcing processes contributing to the maintenance of a Northern Hemisphere midlatitude blocking anticyclone that occurred during the summer season were examined. During the development of this blocking anticyclone, vorticity advection, supported by temperature advection, forced 500-hPa height rises at the block center. Vorticity advection and vorticity tilting were also consistent contributors to height rises during the entire life cycle. Boundary layer friction, vertical advection of vorticity, and ageostrophic vorticity tendencies (during decay) consistently opposed block development. Additionally, an analysis of this blocking event also showed that upstream precursor surface cyclones were not only important in block development but in block maintenance as well. In partitioning the basic data fields into their planetary-scale (P) and synoptic-scale (S) components, 500-hPa height tendencies forced by processes on each scale, as well as by interactions (I) between each scale, were also calculated. Over the lifetime of this blocking event, the S and P processes were most prominent in the blocked region. During the formation of this block, the I component was the largest and most consistent contributor to height rises at the center point. It was also shown that the height- rise regions located on the anticyclonic side of the jet maxima associated with block development and intensification were primarily composed of the S and I components. Also, the precursor cyclones were associated with S or S and I height rises that contributed to the formation of this block. Finally, the results of this paper show that the forcing associated with summer-season blocking events are similar to that of their winter-season counterparts neglecting the natural case-to-case variability. In comparing these results to the results of other papers in this series, however, it is suggested that there may be two models for block development

The Planetary- and Synoptic-Scale Interactions in a Southeast Pacific Blocking Episode Using PV Diagnostics

The synoptic- and planetary-scale forcing in two blocking anticyclones occurring over the southeast Pacific Ocean was examined using potential vorticity diagnostics. While many studies have examined the dynamic and thermodynamic forcing associated with blocking events in the Northern Hemisphere (NH), very few studies have examined blocking in the Southern Hemisphere (SH). Climatological analysis suggests SH blocking events in the Pacific region have similar characteristics to their NH counterparts. However, the occurrence of blocking is rare elsewhere in the SH, and these events are relatively short-lived. Some studies of NH blocking dynamics have also shown that the extent to which the planetary- and synoptic-scale and planetary-synoptic-scale interaction forcing that contribute to the genesis and maintenance of Pacific and Atlantic region events can be different. Thus, a study of the relevant atmospheric dynamics associated with blocking events in the SH was carried out in order to determine whether or not these events are associated with similar dynamic mechanisms to those in the NH. Using the National Center for Atmospheric Research and National Centers for Environmental Prediction (NCAR-NCEP) reanalyses dataset and applying a low-pass filter to the relevant variables, the authors examined the scale interactions associated with two blocking events that occurred during July and August 1986 and applied potential vorticity diagnostics. Results demonstrate that blocking in the southeast Pacific was associated with similar synoptic features, and the forcing mechanisms on the planetary, synoptic scales, and interactions were more similar to North Pacific blocking events rather than those occurring over the NH Atlantic region. However, these results also demonstrated that blocking events in the NH were associated with synergistically interacting synoptic- and planetary-scale waves, while in the SH, blocking events resulted from the superposition of synoptic and planetary waves. This result may explain the paucity of blocking occurrences and their tendency to be weaker and less persistent over much of the SH

Determining the Spring to Summer Transition in the Missouri Ozarks Using Synoptic Scale Atmospheric Data

http://solberg.snr.missouri.edu/gcc/There is abundant anecdotal evidence available to suggest that the transition from a spring to summer season flow regime is often quite abrupt. This same transition renders longrange forecasting problematic as the forecast time period crosses through the spring and summer seasons. Despite these problems, the transition from spring-to-summer flow regimes is a problem that has not been examined in detail in the published literature. In this study, the transition is examined from a regional perspective over a 20-year period (1981 - 2000) and includes the development of a criterion for identifying the transition based on using routinely available synoptic observations. Within the East-central Ozarks region of Missouri, the transition from spring-to-summer season flow regimes is often abrupt, and is identified as occurring in mid-June. The transition could also be identified for the entire Northern Hemisphere using the 500 hPa wave amplitude index for some years during the 1980's. The results found here are consistent with the results of the one other study found in the literature that also addresses the spring-to-summer transition issue for the entire Northern Hemisphere. Additionally, this study found that the average date of summer onset in the region is June, and the 20-year set of summer onset dates was normally distributed with respect to this mean. It was also shown that there is an abrupt change in the average period between heavy precipitation events. Finally, it is demonstrated that late arriving summers are generally associated with a transition in the phase of the El Nino and Southern Oscillation (ENSO), especially the La Nina phase, while early arriving summers are generally associated with steady-state ENSO conditions

Dynamics of Eddy-Driven Low-Frequency Dipole Modes. Part I: A Simple Model of North Atlantic Oscillations

A simple theoretical model is proposed to clarify how synoptic-scale waves drive the life cycle of the North Atlantic Oscillation (NAO) with a period of nearly two weeks. This model is able to elucidate what determines the phase of the NAO and an analytical solution is presented to indicate a high similarity between the dynamical processes of the NAO and zonal index, which is not derived analytically in previous theoretical studies. It is suggested theoretically that the NAO is indeed a nonlinear initial-value problem, which is forced by both preexisting planetary-scale and synoptic-scale waves. The eddy forcing arising from the preexisting synoptic-scale waves is shown to be crucial for the growth and decay of the NAO, but the preexisting low-over-high (high-over-low) dipole planetary-scale wave must be required to match the preexisting positive-over-negative (negative-over-positive) dipole eddy forcing so as to excite a positive (negative) phase NAO event. The positive and negative feedbacks of the preexisting dipole eddy forcing depending upon the background westerly wind seem to dominate the life cycle of the NAO and its life period. An important finding in the theoretical model is that negative-phase NAO events could be excited repeatedly after the first event has decayed, but for the positive phase downstream isolated dipole blocks could be produced after the first event has decayed. This is supported by observed cases of the NAO events presented in this paper. In addition, a statistical study of the relationship between the phase of the NAO and blocking activity over Europe in terms of the seasonal mean NAO index shows that blocking events over Europe are more frequent and long-lived for strong positive-phase NAO years, indicating that the positivephase NAO favors the occurrence of European blocking events

Dynamics of Eddy-Driven Low-Frequency Dipole Modes. Part II: Free Mode Characteristics of NAO and Diagnostic Study

Through calculating the scatter diagrams of the streamfunction ( P or T) versus potential vorticity (PV)(qP or qT), where P and T are the planetary-scale streamfunction and total streamfunction, respectively, and using a weakly nonlinear NAO model proposed in Part I of this paper, it is suggested that negative- and positive-phase NAO events may approximately correspond to free modes even though driven by synopticscale eddies. In a planetary-scale field, the qP( P) scatter diagram of an NAO event exhibits a linear multivalued functional relationship in a narrow region for the negative phase, but exhibits a linear singlevalued functional relationship during the positive phase. It was also found that there is no steepening of the slope of the main straight line in the qP( P) scatter diagrams for two phases of the NAO event. Instead, the slope of the straight line in the scatterplots is time independent throughout the life cycle of the NAO event. However, when synoptic-scale eddies are included in the streamfunction field, the qT( T) scatter diagram of the negative-phase NAO event shows a trend toward steepening during the intensification phase, and this tendency reverses during the decay phase. During the positive NAO phase the slope of the qt( T) scatter diagram shoals during the intensification phase and then steepens during the decay phase. Thus, it appears that the steepening and shoaling of the scatter diagrams of the streamfunction versus PV for the negative and positive-phase NAO events are attributed to the effect of synoptic-scale eddies that force NAO events to form. Diagnostic studies using both composite and unfiltered fields of observed NAO events are presented to confirm these conclusions