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Airborne Observations of a Front Near a Col During FASTEX

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Airborne Observations of a Front Near a Col During FASTEX 1898 MONTHLY WEATHER REVIEW VOLUME 130 Airborne Observations of a Front near a Col during FASTEX ROGER M. WAKIMOTO AND HUAQING CAI Department of Atmospheric Sciences, University of California, Los Angeles, Los Angeles, California (Manuscript received 24 August 2001, in ®nal form 31 December 2001) ABSTRACT An analysis of an oceanic front situated near a col de®ned by the surface pressure ®eld is presented. There have been few observational examples of this type of front presented in the literature. The primary source of information for this study was data recorded by an aircraft equipped with a Doppler radar. The front was approximately two-dimensional and the cross-frontal scale at low levels was 30±40 km. A prefrontal low-level jet was identi®ed in the high-resolution analyses and was shown to be supergeostrophic. Surface pressure measurements and the horizontal temperature gradients were used to calculate the geostrophic wind and the thermal wind imbalance (TWI) in the alongfront direction. Large negative values of TWI (the vertical shear is less than predicted for the given horizontal temperature gradient) were located near a region of frontogenesis. The strong ageostrophic component of the wind parallel to the front suggests that the alongfrontal component of the wind may not have been in geostrophic balance at the time of the observations. 1. Introduction in approximate thermal wind balance in the alongfront direction. To maintain this balance under frontogenetic One of the classical conceptual models of frontogen- and frontolytic conditions requires the formation of a esis is the evolving temperature ®eld embedded within cross-frontal circulation that is ageostrophic. As noted ¯ow that is dominated by horizontal stretching defor- by Thorpe and Clough (1991), the semigeostrophic as- mation. Deformation tends to change the temperature sumption does not require that the imbalance be zero, ®eld so that the isotherms become closely spaced along rather that the acceleration of the secondary circulation the axis of dilatation provided that the initial tempera- ture ®eld has a ®nite gradient along the axis of con- be small compared to the accelerations across the front traction (Bergeron 1928). Since it was ®rst introduced, due to the Coriolis and pressure gradient forces. They deformation frontogenesis has received wide theoretical used high-resolution dropsonde data deployed perpen- treatment in the literature (e.g., see Davies and MuÈller dicular to several cold fronts (spatial resolution ranging 1988; Bluestein 1993); however, there have been rela- from 20 to 60 km) to show that the fronts were in tively few observational studies of this type of front. approximate thermal wind balance with regions of im- Ostdiek and Blumen (1995) have provided the most balance restricted to horizontal dimensions less than 50 comprehensive study to date but it was primarily re- km. A similar analysis was performed by Lagouvardos stricted to the high-resolution surface measurements et al. (1993) using serial rawinsonde ascent over a 30- collected during the Stormscale Operational and Re- h period during the passage of a front. search Meteorology-Fronts Experiment Systems Test On 26 January 1997, airborne data were collected of (STORM-FEST) and, therefore, lacked information on an oceanic cold front during the Fronts and Atlantic the front's vertical structure. Storm-Track Experiment (FASTEX; Joly et al. 1997) The seminal theoretical papers describing the fron- Intensive Observation Period (IOP) 7. The front was togenetical process using the semigeostrophic equations positioned within a horizontal deformation zone as the were ®rst introduced by Hoskins (1971) and Hoskins aircraft recorded re¯ectivity and Doppler velocity in- and Bretherton (1972). The basis of semigeostrophic formation as well as high-resolution in situ thermody- theory is that the front is treated as a quasi-two-dimen- namic information. Accordingly, detailed horizontal and sional ¯ow in which there is approximate geostrophic vertical cross sections of the thermodynamic and ki- balance in the alongfront direction; that is, the ¯ow is nematic characteristics of this front are presented. The study by Ostdiek and Blumen (1995) is the only known observational case study to examine a front within a Corresponding author address: Dr. Roger M. Wakimoto, Depart- deformation zone. Unfortunately, the vertical structure ment of Atmospheric Sciences, University of California, Los Angeles, 405 Hilgard Ave., Los Angeles, CA 90095-1565. was not well documented in their study owing to limited E-mail: [email protected] upper-air soundings. The analysis of the FASTEX IOP q 2002 American Meteorological Society Unauthenticated | Downloaded 09/29/21 11:23 PM UTC JULY 2002 WAKIMOTO AND CAI 1899 TABLE 1. Characteristics of the ELDORA. Descriptions ELDORA Antenna rotation rate (8 s21) 91 No. of samples 18 PRF (Hz) 2000/1600 Gate length (m) 150 Sweep-angle resolution (8) 1.0 Along-track resolution (m) ;400 Max range (km) 75 Max unambiguous velocities (6ms21) 63.78 7 front also provides an opportunity to assess the thermal wind balance in the frontal region with higher horizontal and vertical resolution than was possible with past stud- ies. Section 2 provides a brief overview of FASTEX and the airborne radar platform used in this study. Section 3 provides a description of IOP 7 and the ¯ight track of the aircraft. The detailed thermodynamic and kine- matic structure of the front is shown in section 4. A summary is presented in section 5. 2. FASTEX and the ELDORA The ®eld phase of FASTEX occurred in January and February 1997. The primary goal of the experiment was to improve forecasts of end-of-storm-track cyclogenesis over the northeastern Atlantic Ocean. Primary scienti®c objectives were to test hypotheses on frontal cyclogen- esis, to understand and improve the predictability of cyclones, and to document the meso- and microscale FIG. 1. Upper-air analysis at the 500- and 700-mb levels at 0000 organization of cyclone cloud systems including the UTC 26 Jan 1997. Black lines are geopotential height contours (dam) and the thin-dashed lines are the isotherms. Wind vectors are plotted frontal structure. Major observing facilities for FASTEX with the following notation: ¯ag 5 25ms21, barb 5 5ms21, and included four ships capable of releasing soundings, six half-barb 5 2.5 m s21. research aircraft, and a number of buoys deployed over the Atlantic Ocean. For additional information on FAS- TEX, the reader is referred to Joly et al. (1997, 1999). not calculated in the studies by Thorpe and Clough The observing platform used in the present study is (1991) and Ostdiek and Blumen (1995). the National Center for Atmospheric Research (NCAR) Electra Doppler Radar (ELDORA), which was deployed 3. IOP 7 and the Electra ¯ight track from the airport located in Shannon, Ireland. The Electra is equipped with two X-band Doppler radars and a suite The trough axis of an intense baroclinic wave at upper of probes capable of recording in situ measurements at levels was positioned over the northeastern United ¯ight level (see Jorgensen et al. 1983; Hildebrand et al. States at 0000 UTC 24 January 1997. All times, here- 1994, 1996; Wakimoto et al. 1996). The scanning pa- after, are in UTC. The trough became elongated in the rameters for the radars are shown in Table 1 and a dis- north±south direction as it crossed the Atlantic Ocean cussion of the wind synthesis technique is presented in and approached the FASTEX domain (Fig. 1). The pack- the appendix. The Doppler velocities recorded by the ing of the isotherms associated with the front can be radars were synthesized into a three-dimensional wind seen at both the 500- and 700-mb levels west of Ireland. ®eld of the ¯ow surrounding the front. The in situ data Surface analyses for 0000 and 0600 26 January 1997 collected at ¯ight level supplemented the kinematic in- superimposed on infrared satellite images are shown in formation from the Doppler radars but its most impor- Figs. 2 and 3. These two times were chosen since they tant role was the reconstruction of the thermal ®elds. In bracket the takeoff and landing times of the Electra at addition, the ¯ight-level data collected near the surface Shannon Airport and they are also the times associated of the ocean provided an accurate estimate of the surface with the highest density of surface reports over the pressure ®eld that was used to calculate the alongfront ocean. The front was located at the leading edge of a geostrophic wind at the surface. The latter variable was prominent quasi-linear cirrus cloud band seen on the Unauthenticated | Downloaded 09/29/21 11:23 PM UTC 1900 MONTHLY WEATHER REVIEW VOLUME 130 FIG. 2. (a) Infrared satellite image at 0000 UTC 26 Jan 1997 with the ¯ight track of the Electra superimposed. (b) Subjective analysis of the surface pressure ®eld superimposed onto the satellite image at 0000 UTC 26 Jan 1997. (c) Subjective analysis of potential temperature superimposed onto the satellite image at 0000 UTC 26 Jan 1997. Surface reporting stations are shown as black dots. Wind vectors are plotted when available (one barb 5 5ms21, half barb 5 2.5ms21). satellite image and is analyzed as a cold front in the (not shown) revealed weak (,15 dBZ) and scattered northern section of the domain owing to its eastward echoes along the cold front. movement (5.9 m s21 from 3058 based on numerous in A horizontal deformation pattern can be seen in Figs. situ penetrations by the Electra and an examination of 2b and 3b with an axis of dilatation located at the same sequential surface analyses). Numerous small cumulus approximate position as the front. The total deformation clouds are prevalent west of the surface frontal position was calculated (not shown) and its maximum values in the satellite images indicating the presence of a cold were collocated with the axis of dilatation.
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