A23B-01
Air-Sea Interaction in the Gulf of Tehuantepec
Measurements of meteorological fields and turbulence were made during gap wind events in the Gulf of Tehuantepec using the NSF C-130 aircraft. The flight patterns started at the shore and progressed to approximately 300km offshore with low-level (30m) tracks, stacks and soundings. Parameterizations of the wind stress, sensible and latent heat fluxes were obtained from approximately 700 5 km low-level tracks. Structure of the marine boundary layer as it evolved off-shore was obtained with stack patterns, aircraft soundings and deployment of dropsondes. The air-sea fluxes approximately follow previous parameterizations with some evidence of the drag coefficient leveling out at about 20 meters/sec with the latent heat flux slightly increasing. The boundary layer starts at shore as a gap wind low-level jet, thins as the jet expands out over the gulf, exhibits a hydraulic jump, and then increases due to turbulent mixing.
A23B-02
East Pacific Tropical Cyclogenesis: Differences Between Developing and Non-developing Disturbances and Contrasts With the Atlantic.
The eastern North Pacific basin (EPAC) is the world's most concentrated tropical cyclogenesis region and is an ideal location for studying tropical cyclogenesis. A database of 116 developing (DEV) and 614 non-developing (NOD) disturbance tracks has been derived from ERA-40 Reanalysis for the EPAC and north Atlantic for May to October, 1998-2001. The relative vorticity, daily genesis parameter, vertical wind shear, and temperature and moisture anomalies are calculated from ERA-40 for each disturbance. The area of low cloud top temperature is calculated from ISCCP. The maximum rain rate, raining area, and area of heavy rain rates in the vicinity of each disturbance are derived from the 3B42 dataset, and the 85 GHz polarization corrected temperature (PCT) from the Special Sensor Microwave Imager (SSMI) is used as a measure of ice production and intensity of convective systems near the disturbance center. Statistically significant differences in several variables are presented for EPAC NOD versus DEV systems, and contrasts are made with with the corresponding statistics for the Atlantic NOD and DEV disturbances.
A23B-03
Relationships between eastern Pacific tropical cyclones and convective rainfall in Baja California, Mexico
The influence of tropical cyclones in rainfall patterns over the Baja California peninsula is examined. The impact of these systems, over the southern portion of the peninsula, is analyzed and the study period is limited to the summer of 2004. This is associated with the field phase of the North American Monsoon Experiment (NAME), which is intended to improve predictions of warm season precipitation over North America. We used the best-track dataset from the U.S. National Hurricane Center to classify, based on distance from the circulation center, systems that approached the peninsula at ranges of 400-, 800- and 1200-km. During the season of 2004, activity was below the long-term annual mean with nine systems developing between July and September. Four cases were selected for subsequent analysis: Tropical Storm Blas, Hurricane Frank, Hurricane Howard, and Hurricane Javier. Data from the upper-air station at La Paz were used to evaluate humidity changes during the storm approach and rain gauge reports provided information to determine the spatial distribution of the convective precipitation. Our analysis shows that, when compared with a base period of 15 years, 2004 resulted in below normal precipitation over the southern peninsula. In contrast, above normal conditions occurred in the central peninsula.
A23B-04
Vorticity balance in East Pacific tropical cyclones
There are still questions about how a tropical cyclone develops out of an unorganized region of convection over the tropical oceans. In this study we investigate the evolution of a few cyclones that developed in the East Pacific during the EPIC2001 and IFEX2005 projects. We have analyzed in those cyclones the relative importance of the terms derived from the vorticity equation: the time rate of change of its absolute circulation, the convergence of absolute vorticity into the system at each level and the spin-down tendency (friction term). We use FNL analyses for September-October 2001 and GFS model runs for June-July 2005 to calculate the two first terms, while the friction term was calculated as a residual from the other two terms and also using a simple bulk formulation from the FNL analyses. The output of the GFS model runs (which were especially for this study run by Jongil Han from NOAA/NCEP) includes the time evolution of the friction term, which can also then be compared with the residual calculation. Results from the FNL data show that the agreement between the residual calculation and the bulk formula is better during the early stages of cyclone development, when a not too deep (up to 2 km) layer is considered. As the tropical cyclones go from the tropical depression (TD) stage to the hurricane stage, the agreement is better for a shallower layer (0.5-1 km). Results from the GFS runs show that the balance is observed in a shallow layer with almost the same frequency as in a very deep layer. The results indicate that as the tropical cyclones start to intensify, the convergence of absolute vorticity dominates in the vorticity balance producing the consequent increment circulation in the system. As storm evolution proceeds the friction term becomes more important in the balance, resulting in a decrease in the circulation and leading to the final decay of the cyclone. Furthermore, the convergence of mass seems to dominate in the convergence of absolute vorticity term. However, in the hurricane stage, the absolute vorticity seems to have a stronger role on this term that causes the intensification of tropical storms.
A23B-05
Tropical cyclones in east part of Pacific Ocean and their demonstration on the Mexican shelf.
The trajectories of tropical cyclones in the Northeastern Pacific during the period 1969-2004, calculated from historical data are discussed. Upwelling structure corresponding to those generated by tropical cyclones on the continental shelf of Jalisco is analyzed. Measurements from CTD profiler, ADCP and an automatic weather station on the variations of thermal structure and coastal currents during the pass of hurricanes Juliette (September 2001) and Marty (August 2003) are displayed. It is proved that the tropical cyclones generate an intensive mixing of the water column on the continental shelf of the Mexican Pacific. At the same time, this causes a significant change in currents patterns, nutrients concentrations and, as a result, a change in the primary productivity of coastal waters.
A23B-06
New Wind Information From a Single Doppler Radar
In this work, we present a technique to extract horizontal strain rates as a function of height from data collected by a single Doppler radar. At each level, the strain is calculated using all data available within a radius of 80 km. Hence, they are representative of the overall mesoscale behavior of the convective region. We present results for the East Pacific obtained by applying this technique to data collected during EPIC-2001 by the Doppler radar on board the vessel Ronald H. Brown of the National Oceanic andAtmospheric Administration (NOAA).
A23B-07
Climatology of Marine Stratocumulus Cloud Fraction in the South-East Pacific Using Surface Longwave Radiative Flux Observations
Although marine stratocumulus cloud coverage is regarded as one of the most significant regulators of the radiation budget over the eastern subtropical oceans, it is poorly represented in global climate models. The Woods Hole Oceanographic Institute (WHOI)'s Ocean Reference Station (WORS) located at 20° S 85° W has collected observations of surface thermodynamic variables and broadband radiative fluxes continuously since October 2000. A technique to derive stratocumulus cloud fraction using the WORS observations is presented and applied to the buoy observations to give a three-year climatology (for 2001-2003). The technique uses the surface downward longwave (LW) fluxes along with the clear sky LW flux estimated from a model calibrated during five WORS maintenance cruises conducted in the region. The results are compared with the satellite derived monthly cloud fraction and the ceilometer derived zenith cloud fraction (available during the maintenance cruises). The initial results and analysis show that the technique can be effectively applied to the longer datasets and the continuous observation from the WORS. Seasonal, inter-seasonal and average diurnal variations in cloud cover are obtained for the initial three years of observations.