Mobile Observations of a Quasi-Frontal Transient Moisture Boundary in the Deep South

Loren D. White Jackson State University, Jackson, Mississippi

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Abstract

A protocol for the collection and analysis of high-resolution (≤300 m) temperature and humidity data along a transect using an unmodified passenger vehicle is described. A case is investigated of a weak dissipating cold front that became disconnected from its upper-tropospheric support and interacted with a developing Gulf of Mexico return flow. The changing relationships between the thermal and moisture gradients are described through cross-section analyses of the mobile, surface synoptic, and radiosonde data, extending from Texas to Alabama. The mobile transect data facilitated description of subsynoptic-scale airmass transition zones in the vicinity of the frontal remnant, as well as other variations within air masses. Similarities and differences are noted relative to previous studies of cold fronts, drylines, and coastal fronts.

Corresponding author address: Loren D. White, Dept. of Physics, Atmospheric Science, and Geoscience, Jackson State University, P.O. Box 17660, Jackson, MS 39217. E-mail: loren.d.white@jsums.edu

Abstract

A protocol for the collection and analysis of high-resolution (≤300 m) temperature and humidity data along a transect using an unmodified passenger vehicle is described. A case is investigated of a weak dissipating cold front that became disconnected from its upper-tropospheric support and interacted with a developing Gulf of Mexico return flow. The changing relationships between the thermal and moisture gradients are described through cross-section analyses of the mobile, surface synoptic, and radiosonde data, extending from Texas to Alabama. The mobile transect data facilitated description of subsynoptic-scale airmass transition zones in the vicinity of the frontal remnant, as well as other variations within air masses. Similarities and differences are noted relative to previous studies of cold fronts, drylines, and coastal fronts.

Corresponding author address: Loren D. White, Dept. of Physics, Atmospheric Science, and Geoscience, Jackson State University, P.O. Box 17660, Jackson, MS 39217. E-mail: loren.d.white@jsums.edu

1. Introduction and literature review

The detailed structure of atmospheric discontinuities such as classical synoptic fronts, the tropopause, drylines, sea-breeze fronts, and outflow boundaries has been a matter of interest since the beginnings of synoptic meteorology. For dynamical reasons, most emphasis has been on relatively intense density fronts (e.g., Bergeron 1937), commonly idealized as first-order discontinuities (Reed and Albright 1997). Classic examples are the Sanders (1955), Brundidge (1965), and Shapiro (1984) descriptions of strong cold fronts in the southern Great Plains, north Texas, and Colorado, respectively. In recent decades, large field campaigns have made greater use of aircraft and remotely sensed data (radar and lidar in particular) to probe kinematic structures of cold fronts and drylines (Neiman et al. 1998; Colle et al. 1999; Demoz et al. 2005; Geerts et al. 2006). In the southern Great Plains region, measurements from the mobile facilities of the National Severe Storm Laboratory’s (NSSL) Mobile Mesonet (Straka et al. 1996) have complemented the locally enhanced surface observational capabilities of the Oklahoma Mesonet (Brock et al. 1995) and the Atmospheric Radiation Measurement Program Cloud and Radiation Test Bed (ARM-CART) Southern Great Plains (SGP) (Stokes and Schwartz 1994) facilities for studies of the dryline and other boundary features (Pietrycha and Rasmussen 2004; Arnott et al. 2006).

Investigation of other meso-γ-scale features (Orlanski 1975) at the surface has typically been limited by spacing and logistical constraints of predefined observing sites and assumptions of stationarity for time–space conversion of single-site time series. Use of mobile observing platforms at the surface has not been widely exploited for meteorological research beyond the large field campaigns of the southern Great Plains. One of the earliest was the relatively coarse observations from vehicle intercepts of a quasi-stationary front in North Carolina that were utilized by Businger et al. (1991). Notable applications of the NSSL Mobile Mesonet to describe the surface details of supercell thunderstorms during the Verification of the Origins of Rotation in Tornadoes Experiment (VORTEX; Rasmussen et al. 1994) and VORTEX-2 were reported by Markowski (2002) and Ziegler (2013), respectively. In a very different vein, Mayr et al. (2002) designed a mobile vehicular observing system to study foehn and gap flows in the Alps during the Mesoscale Alpine Programme (MAP; Bougeault et al. 2001). Mobile transects have also been made of the Phoenix, Arizona, urban heat island (Sun et al. 2009). At even smaller scales, microscale temperature variations within an urban park in fair weather have been measured by mounting sensors on a bicycle (Chow et al. 2011; Vanos et al. 2012). In recent years, studies in the United States have considered the potential for a nationwide fleet of “connected vehicles” collecting real-time measurements for road weather management (Mahoney et al. 2010; Anderson et al. 2012; Hill 2013), though without exploring applications of the data for atmospheric research.

Relatively few observational efforts have been made to describe the detailed structure of occluded or decaying fronts (Saarikivi and Puhakka 1990), or of moisture boundaries not associated with the dryline. Such features are often difficult to analyze using standard synoptic observations (Sanders and Doswell 1995) and of uncertain thermodynamic character. Especially within the forested environment of the southern United States, surface frictional dissipation and distance from the major midlatitude baroclinic forcing often tend to result in relatively weak surface wind flow (Klink 1999), which at many sites may be more strongly determined by local microscale obstacles than by the mesoscale and synoptic-scale processes of the airmass boundary. However, significant thermodynamic contrasts can still be detected if observations are measured with sufficient horizontal resolution across the boundary. The most economical and practical means of obtaining such finescale resolution is by mobile transects.

Using observations from a mobile vehicular platform to complement standard synoptic data sources, the small-scale structure of a remnant moisture boundary that was intercepted in northern Louisiana at ~0000 UTC 12 January 2013 is described. Observations of this intense gradient featuring moist air to the east and drier air to the west prompted further investigation. The complex three-dimensional structure and evolution of the unusual Gulf return-flow pattern that developed over the Deep South in the wake of a decaying surface frontal system and upper-level short-wave trough is presented. In section 2, the synoptic context of the measurements will be discussed, followed by an overview of the data collection and processing procedures in section 3. Detailed analysis of the mobile data and related synoptic datasets will be covered in section 4. Last, section 5 will summarize and further consider the relation to conceptual models and mechanisms.

2. Broad synoptic patterns

The synoptic pattern at 250 hPa (Fig. 1) rapidly evolved from a mostly cutoff circulation centered over west Texas at 0000 UTC on 10 January 2013 to a sharp short-wave trough over eastern Oklahoma. An approaching long-wave trough over the southwestern United States then dominated the large-scale pattern on 12 January, with related ridging over the south-central states. By 0000 UTC 12 January, the original short-wave trough is barely Tdetectable along the Atlantic Seaboard. These upper-tropospheric pattern changes are relevant to the reorientation of surface pressure gradients and near-surface flow for advection of Gulf of Mexico return flow moisture on 11 and 12 January. A local geopotential height anomaly (relative to the undisturbed flow) of about 15 dam and temperatures about 10°C cooler was seen in the 500-hPa cyclonic circulation (Fig. 2) at 0000 UTC 10 January 2013. Similar height and temperature anomalies remained on 11 January, although no longer isolated from the main westerly flow and somewhat smaller in scale. More specifically, the 500-hPa temperature at Little Rock, Arkansas, decreases from −12°C at 1200 UTC 10 January to −15°C at 0000 UTC 11 January with the approach of the trough and, then, quickly rebounds to −9°C by 1200 UTC 11 January as the weakening upper trough exits the region.

Fig. 1.
Fig. 1.

The 250-hPa charts from NOAA/Storm Prediction Center (http://www.spc.noaa.gov) with isotachs [25-knot (kt; 1 kt = 0.51 m s−1) interval above 75 kt] and streamlines at (a) 0000 UTC 10 Jan, (b) 0000 UTC 11 Jan, (c) 1200 UTC 11 Jan, and (d) 0000 UTC 12 Jan 2013. Station model shows temperature (red) and dewpoint (green) (°C).

Citation: Weather and Forecasting 29, 6; 10.1175/WAF-D-14-00009.1

Fig. 2.
Fig. 2.

The 500-hPa charts with geopotential height contours (solid) and temperatures (dashed) at (a) 0000 UTC 10 Jan, (b) 0000 UTC 11 Jan (c) 1200 UTC 11 Jan, and (d) 0000 UTC 12 Jan 2013. Station model as in Fig. 1.

Citation: Weather and Forecasting 29, 6; 10.1175/WAF-D-14-00009.1

Near the surface, the initially deep baroclinic circulation is evident at 925 hPa at 0000 UTC 10 January (Fig. 3) over central Texas, with a developing warm conveyor belt of moist air streaming northward off the Gulf of Mexico. The cyclonic circulation has almost completely disappeared by 0000 UTC 11 January, though a thermal contrast of 9°C still exists between Fort Worth, Texas (FWD), and Jackson, Mississippi. Broad influence of the system at this time is evidenced by a tongue of moist air extending as far north as Illinois. Strengthening of the southwesterly wind ahead of the deepening trough over New Mexico contributes to an overnight temperature increase at Fort Worth of 7°C (0000–1200 UTC), while higher dewpoints remain far to the east. Although not directly connected to the moisture boundary developing farther east, this is an indicator of the increasing contribution of advection by both the southerly and westerly components of the lower-tropospheric flow. [It should be noted that analyzed isotherms west of FWD and north of Del Rio, Texas (DRT), are misleading due to intersection of the 925-hPa surface with the topography.] Overnight warming actually extends over a layer from 950 to 600 hPa at FWD and is also noted farther to the west at Midland, Texas (MAF). In a scenario reminiscent of more classical Gulf return flow (Crisp and Lewis 1992), the dewpoint gradient intensifies by 1200 UTC 11 January along the Texas coast and lower Mississippi River. A weak thermal gradient remains between Shreveport, Louisiana, and Birmingham, Alabama, but practically disappears by 0000 UTC 12 January. Nevertheless, the dewpoint gradient retains its intensity and advances northward into Texas and Louisiana. Later the moist air meets with a rapidly strengthening cold front associated with the upper-tropospheric long-wave trough at 1200 UTC 12 January.

Fig. 3.
Fig. 3.

The 925-hPa charts with geopotential height contours (black), temperatures (dashed), and dewpoint (green; above 12°C) at (a) 0000 UTC 10 Jan, (b) 0000 UTC 11 Jan, (c) 1200 UTC 11 Jan, (d) 0000 UTC 12 Jan, and (e) 1200 UTC 12 Jan 2013. Station model as in Fig. 1.

Citation: Weather and Forecasting 29, 6; 10.1175/WAF-D-14-00009.1

Surface frontal analyses from the National Oceanic and Atmospheric Administration’s (NOAA) Hydrometeorological Prediction Center (HPC, now known as the Weather Prediction Center) (Fig. 4) indicated a weak occluding frontal system over Louisiana at 0000 UTC 11 January, with evidence of regional confluence in the generally southerly flow. A remnant stationary front was still analyzed at 1200 UTC, extending approximately from Nashville, Tennessee, to Baton Rouge, Louisiana. The winds had mostly weakened overnight, and there was little evidence of a thermal or kinematic contrast near the frontal remnant in Louisiana–Mississippi by 1800 UTC. However, the dewpoint gradients remained relatively unchanged.

Fig. 4.
Fig. 4.

NOAA/HPC surface frontal analyses with standard station model [temperature and dewpoint (°F); sea level pressure (hPa)] at (a) 0000 UTC 11 Jan, (b) 1200 UTC 11 Jan, (c) 1800 UTC 11 Jan, and (d) 0000 UTC 12 Jan 2013.

Citation: Weather and Forecasting 29, 6; 10.1175/WAF-D-14-00009.1

In summary, the upper-level baroclinic Rossby wave forcing quickly exits the south-central states during this 2-day period, leaving behind a lower-tropospheric thermal contrast that experiences almost complete frontolysis. However, the lower-tropospheric dewpoint gradient in the lower Mississippi River valley intensifies in the southerly flow. This intensification is greater than would be expected from the weak horizontal confluence, apparently due to translation of a moisture gradient that developed as the decaying front reached the western Gulf of Mexico. During this stage of a dissipating lower-tropospheric thermal gradient and intensifying/advecting dewpoint gradient, a surface mobile transect was made between 2256 UTC 11 January and 0237 UTC 12 January from Shreveport to Jackson.

3. Collection and processing of mobile observations

The most well-documented surface mobile observing protocol is the Mobile Mesonet of NSSL. As described by Straka et al. (1996), the parameters normally measured include air temperature, humidity, barometric pressure, and wind speed and direction. Time and location are determined by global positioning system (GPS), and multiple instrumented vehicles are typically used in tandem. Due to low surface wind speeds in the current case [about 2 m s−1, based on nearby Automated Surface Observing System (ASOS) stations] and obstruction by trees, we made no attempt to mount wind sensors for this case. Similarly, there seemed little value to measuring pressure, especially in light of uncertainties likely to be introduced while removing topographic and aerodynamic effects. Measuring only temperature and humidity also greatly simplifies the mounting of sensors, since there should be relatively little affect by the vehicle while moving at highway speed, even without the use of large booms or masts. It is suggested that these observational considerations are widely applicable to most nonsevere weather environments in the southeastern United States.

The primary concerns for possible degradation of temperature and humidity measurements are radiative heating and the possibility of evaporative cooling if the sensor becomes wetted by precipitation. No precipitation was encountered during the transect. However, there was some fog/mist along the last part of the transect, with little opportunity for any subsequent evaporation from wetted sensor surfaces. The contribution of radiative heating was alleviated by mounting the sensor within a standard radiation shield and by collecting most observations while at a fairly constant highway speed of about 30 m s−1. Radiative effects should also be relatively negligible under full cloud cover or darkness, which characterized most of the transect. Sunset occurred at 2327 UTC, while at 93.55°W, near Bossier City, Louisiana.

As shown in Fig. 5, a Campbell Scientific HMP45-C temperature–relative humidity sensor was mounted with a 41003 radiation shield about 0.4 m above the cabin of the vehicle. To enable quick setup, the mount is attached using the suction cup hardware of Sticky Pods (http://www.stickypod.com), with a tether looped through a seat for additional security. The data cable is fed through the passenger window to a Campbell Scientific CR23 datalogger. For time and location, a Garmin GPS16-HVS with magnetic mount is positioned on the roof. The datalogger is powered by plugging a 12-V power supply unit into an AC/DC inverter, which uses the standard vehicle outlet. Data are logged at a frequency of once every 10 s, corresponding to a typical horizontal resolution of about 300 m at highway speed. Quality control procedures, primarily with regard to relative humidity and GPS position, are described fully in the appendix.

Fig. 5.
Fig. 5.

Mounting of HMP45C with Sticky Pods and GPS16-HVS on vehicle roof.

Citation: Weather and Forecasting 29, 6; 10.1175/WAF-D-14-00009.1

To facilitate comparison with the mobile data, the surface synoptic reports were linearly interpolated to the time of closest proximity by the mobile platform. The time-matched ASOS and mobile observations after correction of the relative humidity and subsequent calculation of dewpoint are summarized in Fig. 6. The mean difference [mobile minus ASOS/Automated Weather Observing System (AWOS)] is 0.7°C for temperature and 0.2°C for dewpoint, which is well within the expected mesoscale variation between the observing sites and the highway. Most of the difference in temperature was from sites in the Shreveport metropolitan area around sunset.

Fig. 6.
Fig. 6.

(a) Position of surface synoptic stations relative to vehicle track, superimposed onto a digital elevation map (DEM). Along-track elevation ranges from about 20 to 130 m. (b) Comparison of mobile and synoptic station temperature and dewpoint (°C).

Citation: Weather and Forecasting 29, 6; 10.1175/WAF-D-14-00009.1

4. Data analysis

a. Time shifting to radiosonde launch time

The time series of data from the mobile platform spanned the period from 2256 UTC 11 January through 0237 UTC 12 January, going eastward approximately from the Shreveport (SHV) to Jackson (JAN) radiosonde launch sites. To remove diurnal and other temporal trends during this period, observations from the synoptic observing stations are used in order to time shift all observations to 2307 UTC (the mean of the radiosonde launch times at Shreveport and Jackson). At each of the synoptic stations, the temperature change between the time of closest proximity and 2307 UTC was plotted relative to the vehicle track in the geographic information system (GIS) software package QGIS. Contours were manually drawn in QGIS of these temporal temperature adjustments, using an interval of 0.5°C. The mobile data points closest to each of the contours (and at approximated maxima–minima) were then identified, and temperature adjustments at all points in between were linearly interpolated in time to give a smoothly varying adjustment (Fig. 7). A similar procedure was used for temporal variations of the dewpoint data (following bias corrections described in the appendix). These adjustments are then added to the mobile data in order to produce a time-shifted dataset applicable at the time of radiosonde launch (Fig. 8).

Fig. 7.
Fig. 7.

Time-shift adjustments of mobile temperature data (°C) to 2307 UTC (shown with all data points, including stops). Actual UTC observation time is shown on the horizontal axis.

Citation: Weather and Forecasting 29, 6; 10.1175/WAF-D-14-00009.1

Fig. 8.
Fig. 8.

Mobile temperature (blue) and dewpoint (red) data (°C) after time-shift adjustment to 2307 UTC. Actual UTC observation time is shown on the horizontal axis.

Citation: Weather and Forecasting 29, 6; 10.1175/WAF-D-14-00009.1

b. One-dimensional analysis of mobile transect

At each mobile data point, the altimeter setting is linearly interpolated in time and space from nearby synoptic stations. The altimeter setting and elevation are then used to determine station pressure, for use in determining potential temperature θ, equivalent potential temperature θe, and mixing ratio w (Fig. 9). Although several mesoscale θ variations on the order of 1–2 K are evident, there is little if any larger-scale pattern indicative of the earlier synoptic-scale air-mass contrast (consistent with Fig. 4). However, a much more robust transition between air masses is observed in θe and w at around 91.5°W, corresponding to a gradient over 40 km of about 8 K for θe and 2.8 g kg−1 for mixing ratio. Within this mesoscale transition zone, a more abrupt change of 6 K in θe and 1.8 g kg−1 in w occurs over a distance of only 15 km near the town of Delhi, Louisiana (91.5°W). Once this potential remnant frontal zone is identified, a consistent potential temperature jump is also detectable in the same location, though it is only about 1.8 K in magnitude (similar to the variability within the air mass to the west). For comparison, the 10 June 1999 dryline reported by Pietrycha and Rasmussen (2004) had a mixing ratio gradient of about 3 g kg−1 over 2 km. The geographic position of the strong mixing ratio gradient is displayed using Unidata’s Integrated Data Viewer (IDV) in Fig. 10. Relation to data from the broader regional surface observations is shown by the contour analysis of the mixing ratio (Fig. 11).

Fig. 9.
Fig. 9.

(a) Potential temperature (K), (b) equivalent potential temperature (K), and (c) mixing ratio (g kg−1) from mobile observations. Blue line is mobile transect data; red line is from nearby synoptic ASOS/AWOS stations.

Citation: Weather and Forecasting 29, 6; 10.1175/WAF-D-14-00009.1

Fig. 10.
Fig. 10.

Geographic portrayal of mixing ratio (g kg−1) from mobile observations using IDV.

Citation: Weather and Forecasting 29, 6; 10.1175/WAF-D-14-00009.1

Fig. 11.
Fig. 11.

Surface mixing ratio analysis (g kg−1) at 2307 UTC 11 Jan 2013 from surface station reports and mobile transect.

Citation: Weather and Forecasting 29, 6; 10.1175/WAF-D-14-00009.1

An alternate perspective of the airmass contrast is shown by scatterplots of mixing ratio and potential temperature relative to elevation, binned into 0.5°-longitude segments (Fig. 12). Elevations below 35 m correspond to the very flat Mississippi River alluvial plain east of Monroe, Louisiana. In spite of small-scale variability, mixing ratios are consistently much higher east of 91°W and lower west of 92°W. The temperature relationship is more complicated. Over the hilly terrain between 93.5° and 92.5°W, there is a weak positive correlation (0.44) between elevation and potential temperature. Sloping downward toward the Ouachita River, there is a great deal of local variability between 92.5° and 92°W. The flat terrain between 92° and 91°W simply shows warmer temperatures to the east. East of 91°W there is again a positive correlation (0.39) between elevation and potential temperature. The appearance of two different regression slopes in this easternmost segment seems to be related to greater static stability in an area of rain-cooled air extending across Mississippi and western Alabama. This rain-cooled area was even more evident in the National Centers for Environmental Prediction (NCEP) North American Mesoscale (NAM) model (12-km resolution) at 2100 UTC (Fig. 13). Approximate potential temperature lapse rates implied are 10 K km−1 between 91° and 90.8°W and 14 K km−1 between 90.8° and 90.5°W, based on linear regression (R2 of 0.87 and 0.82, respectively). Although only crude estimates, comparison with 10 K km−1 for an isothermal atmosphere would indicate the existence of a near-surface inversion layer east of 90.8°W. Corresponding values from layers in the lowest 1000 m of the JAN sounding range from −4 to 10 K km−1.

Fig. 12.
Fig. 12.

(a) Mixing ratio (g kg−1) vs elevation (m) according to 0.5°-longitude bins. (b) Potential temperature (K) vs elevation.

Citation: Weather and Forecasting 29, 6; 10.1175/WAF-D-14-00009.1

Fig. 13.
Fig. 13.

The NAM-12 operational model data from NOAA/ARL’s READY website (http://www.ready.noaa.gov; Rolph 2013) at 2100 UTC 11 Jan 2013. (a) Accumulated precipitation over 3 h (mm) and (b) 2-m air temperature (°C).

Citation: Weather and Forecasting 29, 6; 10.1175/WAF-D-14-00009.1

c. Vertical cross sections

To examine the moisture boundary with respect to the lower-tropospheric environment, vertical cross sections have been analyzed centered on the mobile track, extending from FWD to Birmingham (BMX). Although data above the surface are only available at the four radiosonde sites, details at the surface are incorporated from the mobile transect, ASOS/AWOS sites, and the Newton, Mississippi (89.1°W), mesonet site (White 2009b). The time period considered is from 0000 UTC 11 January through 1200 UTC 12 January. All surface data (including ASOS/AWOS) are interpolated or adjusted to the mean radiosonde launch time. However, the cross sections will be referred to according to the nominal radiosonde observation times of 0000 and 1200 UTC.

The potential temperature cross section at 0000 UTC 11 January (Fig. 14a) shows at least two zones of concentrated surface thermal gradient, in Texas (94°W) and in Alabama (88°W). Neither corresponds to the HPC-analyzed surface front position. Stable layers slope up from both surface features to around 2000 m over FWD. For convenience we will refer to these layers of thermal gradient as TA (eastern/upper) and TB (western/lower). Relative humidity above 90% and synoptic station ceilometer observations indicate a thin layer of clouds beneath the 293-K isentrope from Meridian, Mississippi (88.75°W), westward. Light to heavy rain extends eastward of TA from Meridian to BMX. There are also two surface concentrations of mixing ratio gradient (Fig. 14b), although not quite collocated with TA and TB. To the west of the surface extension of TA, there is a weak moisture boundary (designated MA) near Meridian, while a stronger gradient (MB) is to the east of SHV. TA appears to be the most robust feature in terms of temporal continuity (since at least the previous 24 h; not shown), although TB and MB would appear to be the most significant in terms of surface airmass changes. The air mass between Newton and Ruston, Louisiana (92.6°W), is quite uniform. In an environment of modest southerly flow and little wind shift or convergence, the upper short-wave trough (north of SHV) seems to be the only dynamical forcing of significance at the time.

Fig. 14.
Fig. 14.

Cross-section analysis at 0000 UTC 11 Jan of (a) potential temperature (K) and (b) mixing ratio (g kg−1). Relative humidity >90% (or >80% in rain) is shaded in (a). Vertical axis is height above sea level (m); horizontal axis is longitude. TA and TB refer to layers/zones of potential temperature gradient, and MA and MB to moisture gradients.

Citation: Weather and Forecasting 29, 6; 10.1175/WAF-D-14-00009.1

Twelve hours later radiational cooling has occurred overnight in the lowest 600 m within the drier air of Texas and Louisiana (Fig. 15a). Concurrently, most of the column at FWD and SHV has been modified by a combination of advection and adiabatic subsidence warming, with Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) backward trajectories (Draxler and Rolph 2013; Rolph 2013) (not shown) indicating that the air at 1000 m had descended from about 400 m higher over western and central Texas. On the other hand, air from the central Gulf of Mexico rose 600 m to the 1000-m level at JAN. This differential vertical motion and confluent advection partly explain the descent of TA by about 500 m at FWD while rising about 2000 m in the eastern half of the cross section, effectively removing (or reversing) the horizontal thermal gradient. There is some evidence to at least partially identify the surface inversion layer with the previous TB air mass, and its eastern edge with the 1200 UTC HPC-analyzed surface front. Again, there are two moisture boundaries (Fig. 15b): one remaining near SHV and one near the analyzed surface front at Tallulah, Louisiana (KTVR; 91°W). Widespread fog, low clouds, and rain extend to the east from Ruston.

Fig. 15.
Fig. 15.

Cross-section analysis at 1200 UTC 11 Jan of (a) potential temperature and (b) mixing ratio. Relative humidity >90% (or >80% in rain) is shaded in (a). Dashed black line separates areas that cooled or warmed over previous 12 h.

Citation: Weather and Forecasting 29, 6; 10.1175/WAF-D-14-00009.1

Diurnal heating has eliminated large-scale near-surface thermal contrasts by the time of the 0000 UTC 12 January mobile transect and radiosonde launches (Fig. 16a), although the TA stable layer is largely unchanged. By careful incorporation of the mobile observations, a weak signature of the TB air mass is identified over Louisiana and a rain-cooled surface layer to the east of Vicksburg, Mississippi (90.9°W) (TC). A traditional portrayal of SHV and JAN radiosonde data by skew T thermodynamic diagrams is shown in Fig. 17. Indicated both by the 90% relative humidity and by the mixing ratio (Fig. 16b), a tongue of moist air has spread westward between 1000 and 1500 m within the Gulf of Mexico return flow. The single relatively intense mixing ratio gradient identified in the surface mobile data at 91.5°W is consistent with the return-flow lower boundary near 800 m over SHV. In spite of minimal frontogenetical forcing, it seems that the MB moisture boundary has not only been maintained, but possibly even intensified. A 50-kt (26 m s−1) low-level jet (Fig. 16c) has developed in the moist layer above SHV (with only slight directional shear), which would facilitate the more rapid arrival of Gulf moisture relative to the surface layer. Blowing from 225°, the low-level jet closely corresponds to the westernmost straight trajectory from the Gulf of Mexico to SHV (210°). The differential origins of the near-surface air at Monroe and Jackson are demonstrated by HYSPLIT backward trajectories (Draxler and Rolph 2013; Rolph 2013) (Fig. 18) over the 24 h prior to 0000 UTC 12 January. The trajectories indicate weak ascent in Mississippi (consistent with broad areas of rain) and descent in north Louisiana (consistent with relatively nonprecipitating broken cloud cover).

Fig. 16.
Fig. 16.

Cross-section analysis at 0000 UTC 12 Jan of (a) potential temperature, (b) mixing ratio, and (c) wind speed (kt). Relative humidity and warming–cooling as in Fig. 15.

Citation: Weather and Forecasting 29, 6; 10.1175/WAF-D-14-00009.1

Fig. 17.
Fig. 17.

Skew T thermodynamic diagrams for (a) SHV and (b) JAN radiosonde data at 0000 UTC 12 Jan (http://weather.rap.ucar.edu).

Citation: Weather and Forecasting 29, 6; 10.1175/WAF-D-14-00009.1

Fig. 18.
Fig. 18.

Ensemble backward trajectories over 24 h from HYSPLIT (using NAM-12 model vertical velocity) ending at 0000 UTC 12 Jan 2013 at 500 m MSL for (a) Monroe and (b) Jackson.

Citation: Weather and Forecasting 29, 6; 10.1175/WAF-D-14-00009.1

Finally, by 1200 UTC on 12 January both the potential temperature and mixing ratio are greatly homogenized in a deep moist return flow air mass (Fig. 19). The environment of the next approaching cold front and an upper trough are now dominant within the region.

Fig. 19.
Fig. 19.

Cross-section analysis of mixing ratio at 1200 UTC 12 Jan.

Citation: Weather and Forecasting 29, 6; 10.1175/WAF-D-14-00009.1

5. Summary discussion

Very high-resolution surface observations from a mobile platform complement standard surface and radiosonde observations in order to better determine the conditions between observing sites for specific events. The complexities of the highlighted case in terms of dissipation of thermal boundaries coincident with intensification of moisture gradients precluded thorough analysis using only standard data sources. At times there appear to have been multiple boundaries (or enhanced gradients) within the vicinity of the operationally analyzed fronts. While the existence of a moisture gradient between Monroe and Tallulah is discernible using surface synoptic data, the mobile data are necessary in order to determine the actual spatial scale of the transition (less than half of the distance between these stations). Additionally, reliance on only the surface synoptic data reduces the number of directly relevant observational samples to only two, so that there is little redundancy to assure quality and representativeness of the data. The surface synoptic observations are also typically measured in the United States to a precision of only 1.0°F. All of these factors make it relatively difficult to determine detailed patterns with much certainty using only the synoptic stations.

Aside from lack of intensity, the structure of the 0000 UTC 12 January moisture boundary differs from that of a dryline by the overrunning of moist return-flow air between 1000 and 1500 m to the west of the surface boundary. Occurrence of such a “moist layer aloft” within a return-flow environment was noted by Merrill (1992). The presence of a low-level jet in this layer and very limited directional shear throughout the lower troposphere is another important difference from drylines, relative to the Parsons et al. (2000) description of dryline structure. However, there are some elements of similarity during the previous 24 h to the conceptual interaction between a front and dryline proposed by Neiman and Wakimoto (1999), as the weakening frontal features encounter the developing return flow. Some structural similarity may also be noted with the shallow (but intense) coastal fronts examined by Riordan (1990) in North Carolina and by Nielsen and Neilley (1990) in New England. The transient nature seems to be influenced both by the diurnal cycle of diabatic effects in the planetary boundary layer and the significant changes in broader-scale upper-tropospheric forcing (exit of one short-wave trough and approach of a more intense long-wave trough).

It appears that the rapid thermal frontolysis over the previous 24 h was linked to the removal of favorable upper-tropospheric forcing, compounded by development of a rain-cooled mesoscale air mass to the east and radiative surface heating to the west during the daylight hours of 11 January. The rapid exit of upper-tropospheric forcing might also be viewed in terms of similarity to the split-front scenario of Browning and Monk (1982). The subtle transition from southwesterly to southerly predominant boundary layer flow then enabled rapid moisture transport from the Gulf of Mexico. Such a boundary layer interaction between weak thermal and moisture boundaries has been described in the presence of deep convection by Koch and McCarthy (1982), while the difficulties of nonclassical, weakening, and diabatically modified surface boundaries have been discussed by Kessler (2008).

Although some aspects of this case may be unusual, it is fairly common for the arrival of Gulf of Mexico return flow in the region to be more strongly characterized by moisture than by thermal contrasts. The interaction of these airmass transitions with preexisting synoptic fronts and with the dryline has yet to be systematically investigated. In contrast to the current case, the leading edge of the return flow is often analyzed operationally as a weak warm front, particularly when accompanied by precipitation. A schematic conceptualization of the 0000 UTC 12 January 2013 case in comparison to the more typical return-flow scenario is shown in Fig. 20. Normally, a strong moisture gradient is seen on the northeast edge of the moist return flow, following the passage of a cold air mass through the southeast United States. White (2009a) has previously referred to these features as moist lines. In this case however, the return flow evolved differently due to the dissipation of the surface front in the Mississippi valley. At the time of the transect, dry air remained to the west (instead of east), before the advance of the broader return flow plume northward through Texas. Hence, the observed moisture boundary may best be conceptualized as a transitional form between the dissipation of the cold front and the development of the full return-flow moisture surge. The transient nature and subtleties of scale in such phenomena make them especially well suited to the application of the mobile observing strategies. With practice, it is feasible to identify at least the strongest such return-flow moisture boundaries by careful dewpoint analyses from surface synoptic data, although there may be variations in vertical structure and spatial scale that are more difficult to determine. It can be presumed that the strength and position of the downstream surface anticyclone would be important contributing factors to the evolution of the return flow moisture gradient.

Fig. 20.
Fig. 20.

Conceptual schematic illustrating differences between (a) a typical return-flow moisture surge and (b) the situation up to and during the 0000 UTC 12 Jan 2013 transect.

Citation: Weather and Forecasting 29, 6; 10.1175/WAF-D-14-00009.1

Acknowledgments

Suggestions by Steve Lyons (NWS San Angelo) and the reviewers were helpful in refining the manuscript. The author is supported by the NOAA Educational Partnership Program under Agreement NA11SEC4810003, as part of the NOAA Center for Atmospheric Sciences (NCAS). The contents are solely the responsibility of the author and do not necessarily represent the official views of the U.S. Department of Commerce/NOAA. The author gratefully acknowledges the NOAA/Air Resources Laboratory (ARL) for the provision of the HYSPLIT transport and dispersion model and the READY website (http://www.ready.noaa.gov) used in this publication.

APPENDIX

Quality Control Procedures

During postanalysis of the mobile transect data, two problems were identified: 1) incomplete GPS location data for some observations and 2) the appearance of a low bias of relative humidity in comparison to nearby synoptic weather reports.

The incomplete position data tended to only occur for one or two observations and then resume normal behavior, with usually only the latter portion of the observation missing (e.g., arc seconds of longitude). After consideration of possible problems due to obstructions, or loose wiring to the datalogger, or a fault with the particular GPS receiver, testing eventually revealed the issue to be a data-buffering problem from the datalogger programming. Since the route was known and the vehicle velocity was fairly constant, it was possible to interpolate position values with very high certainty using a combination of interpolation and aerial imagery mapping software. To facilitate these procedures, a Perl script was written to take position data from the datalogger file and create a file of latitude–longitude positions in .gpx mapping format. These were later converted into ESRI shapefiles for geographic analysis of the meteorological data. It is now standard protocol to verify the locations of every data point in our mobile measurements since small random errors are possible with GPS measurements even when they appear complete. Data from the first 5 min following stops have been removed from most analyses.

The inconsistency of relative humidity measurements with nearby synoptic reports was in spite of temperature measurements that typically agreed within 1°C. By intercomparison with new calibrated HMP45C sensors afterward under laboratory conditions, a recalibration equation was determined by regression, which provided much better agreement with nearby ASOS/AWOS reports. Upon disassembly it was discovered that the sensor had a significant accumulation of dirt from its previous long-term use in a mesonet station.

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Save
  • Anderson, A. R. S., Chapman M. , Drobot S. D. , Tadesse A. , Lambi B. , Wiener G. , and Pisano P. , 2012: Quality of mobile air temperature and atmospheric pressure observations from the 2010 Development Test Environment Experiment. J. Appl. Meteor. Climatol., 51, 691701, doi:10.1175/JAMC-D-11-0126.1.

    • Search Google Scholar
    • Export Citation
  • Arnott, N. R., Richardson Y. P. , Wurman J. M. , and Rasmussen E. M. , 2006: Relationship between a weakening cold front, misocyclones, and cloud development on 10 June 2002 during IHOP. Mon. Wea. Rev., 134, 311335, doi:10.1175/MWR3065.1.

    • Search Google Scholar
    • Export Citation
  • Bergeron, T., 1937: On the physics of fronts. Bull. Amer. Meteor. Soc., 18, 265275.

  • Bougeault, P., and Coauthors, 2001: The MAP Special Observing Period. Bull. Amer. Meteor. Soc., 82, 433462, doi:10.1175/1520-0477(2001)082<0433:TMSOP>2.3.CO;2.

    • Search Google Scholar
    • Export Citation
  • Brock, F. V., Crawford K. , Elliot R. , Cupernus G. , Stadler S. , Johnson H. , and Eilts M. , 1995: The Oklahoma Mesonet: A technical overview. J. Atmos. Oceanic Technol., 12, 519, doi:10.1175/1520-0426(1995)012<0005:TOMATO>2.0.CO;2.

    • Search Google Scholar
    • Export Citation
  • Browning, K. A., and Monk G. A. , 1982: A simple model for the synoptic analysis of cold fronts. Quart. J. Roy. Meteor. Soc., 108, 435452, doi:10.1002/qj.49710845609.

    • Search Google Scholar
    • Export Citation
  • Brundidge, K. C., 1965: The wind and temperature structure of nocturnal cold fronts in the first 1,420 feet. Mon. Wea. Rev., 93, 587603, doi:10.1175/1520-0493(1965)093<0587:TWATSO>2.3.CO;2.

    • Search Google Scholar
    • Export Citation
  • Businger, S., Bauman W. H. III, and Watson G. F. , 1991: The development of the Piedmont front and associated outbreak of severe weather on 13 March 1986. Mon. Wea. Rev., 119, 22242251, doi:10.1175/1520-0493(1991)119<2224:TDOTPF>2.0.CO;2.

    • Search Google Scholar
    • Export Citation
  • Chow, W. T. L., Pope R. L. , Martin C. A. , and Brazel A. J. , 2011: Observing and modeling the nocturnal park cool island of an arid city: Horizontal and vertical impacts. Theor. Appl. Climatol., 103, 197211, doi:10.1007/s00704-010-0293-8.

    • Search Google Scholar
    • Export Citation
  • Colle, B. A., Mass C. F. , and Smull B. F. , 1999: An observational and numerical study of a cold front interacting with the Olympic Mountains during COAST IOP5. Mon. Wea. Rev., 127, 13101334, doi:10.1175/1520-0493(1999)127<1310:AOANSO>2.0.CO;2.

    • Search Google Scholar
    • Export Citation
  • Crisp, C. A., and Lewis J. M. , 1992: Return flow in the Gulf of Mexico. Part I: A classificatory approach with a global historical perspective. J. Appl. Meteor., 31, 868881, doi:10.1175/1520-0450(1992)031<0868:RFITGO>2.0.CO;2.

    • Search Google Scholar
    • Export Citation
  • Demoz, B. B., and Coauthors, 2005: The cold front of 15 April 1994 over the central United States. Part I: Observations. Mon. Wea. Rev., 133, 15251543, doi:10.1175/MWR2932.1.

    • Search Google Scholar
    • Export Citation
  • Draxler, R. R., and Rolph G. D. , cited 2013: HYSPLIT - Hybrid Single-Particle Lagrangian Integrated Trajectory model. NOAA/Air Resources Laboratory. [Available online at http://www.arl.noaa.gov/HYSPLIT.php.]

  • Geerts, B., Damiani R. , and Haimov S. , 2006: Finescale vertical structure of a cold front as revealed by an airborne Doppler radar. Mon. Wea. Rev., 134, 251271, doi:10.1175/MWR3056.1.

    • Search Google Scholar
    • Export Citation
  • Hill, C. J., 2013: Concept and operations for road weather connected vehicle applications. Federal Highway Administration Tech. Rep. FHWA-JPO-13-047, 87 pp.

  • Kessler, E., 2008: An empirical perspective on cold fronts. Synoptic–Dynamic Meteorology and Weather Analysis and Forecasting: A Tribute to Fred Sanders, Meteor. Monogr., No. 55, Amer. Meteor. Soc., 97–108.

  • Klink, K., 1999: Climatological mean and interannual variance of United States surface wind speed, direction and velocity. Int. J. Climatol., 19, 471488, doi:10.1002/(SICI)1097-0088(199904)19:5<471::AID-JOC367>3.0.CO;2-X.

    • Search Google Scholar
    • Export Citation
  • Koch, S. E., and McCarthy J. , 1982: The evolution of an Oklahoma dryline. Part II: Boundary-layer forcing of mesoconvective systems. J. Atmos. Sci., 39, 237257, doi:10.1175/1520-0469(1982)039<0237:TEOAOD>2.0.CO;2.

    • Search Google Scholar
    • Export Citation
  • Mahoney, B., Drobot S. , Pisano P. , McKeever B. , and O’Sullivan J. , 2010: Vehicles as mobile weather observation systems. Bull. Amer. Meteor. Soc., 91, 11791182, doi:10.1175/2010BAMS2954.1.

    • Search Google Scholar
    • Export Citation
  • Markowski, P. M., 2002: Mobile mesonet observations on 3 May 1999. Wea. Forecasting, 17, 430444, doi:10.1175/1520-0434(2002)017<0430:MMOOM>2.0.CO;2.

    • Search Google Scholar
    • Export Citation
  • Mayr, G. J., Vergeiner J. , and Gohm A. , 2002: An automobile platform for the measurement of foehn and gap flows. J. Atmos. Oceanic Technol., 19, 15451556, doi:10.1175/1520-0426(2002)019<1545:AAPFTM>2.0.CO;2.

    • Search Google Scholar
    • Export Citation
  • Merrill, R. T., 1992: Synoptic analysis of the GUFMEX return-flow event of 10–12 March 1988. J. Appl. Meteor., 31, 849867, doi:10.1175/1520-0450(1992)031<0849:SAOTGR>2.0.CO;2.

    • Search Google Scholar
    • Export Citation
  • Neiman, P. J., and Wakimoto R. M. , 1999: The interaction of a Pacific cold front with shallow air masses east of the Rocky Mountains. Mon. Wea. Rev., 127, 21022127, doi:10.1175/1520-0493(1999)127<2102:TIOAPC>2.0.CO;2.

    • Search Google Scholar
    • Export Citation
  • Neiman, P. J., Ralph F. M. , Shapiro M. A. , Smull B. F. , and Johnson D. , 1998: An observational study of fronts and frontal mergers over the continental United States. Mon. Wea. Rev., 126, 25212554, doi:10.1175/1520-0493(1998)126<2521:AOSOFA>2.0.CO;2.

    • Search Google Scholar
    • Export Citation
  • Nielsen, J. W., and Neilley P. P. , 1990: The vertical structure of New England coastal fronts. Mon. Wea. Rev., 118, 17931807, doi:10.1175/1520-0493(1990)118<1793:TVSONE>2.0.CO;2.

    • Search Google Scholar
    • Export Citation
  • Orlanski, I., 1975: A rational subdivision of scales for atmospheric processes. Bull. Amer. Meteor. Soc., 56, 527530.

  • Parsons, D. B., Shapiro M. A. , and Miller E. R. , 2000: The mesoscale structure of a nocturnal dryline and of a frontal dryline merger. Mon. Wea. Rev., 128, 38243838, doi:10.1175/1520-0493(2001)129<3824:TMSOAN>2.0.CO;2.

    • Search Google Scholar
    • Export Citation
  • Pietrycha, A. E., and Rasmussen E. N. , 2004: Finescale surface observations of the dryline: A mobile mesonet perspective. Wea. Forecasting, 19, 10751088, doi:10.1175/819.1.

    • Search Google Scholar
    • Export Citation
  • Rasmussen, E. N., Straka J. M. , Davies-Jones R. P. , Doswell C. A. , Carr F. H. , Eilts M. D. , and MacGorman D. R. , 1994: Verification of the Origins of Rotation in Tornadoes Experiment: VORTEX. Bull. Amer. Meteor. Soc., 75, 9951006, doi:10.1175/1520-0477(1994)075<0995:VOTOOR>2.0.CO;2.

    • Search Google Scholar
    • Export Citation
  • Reed, R. J., and Albright M. D. , 1997: Frontal structure in the interior of an intense mature ocean cyclone. Wea. Forecasting, 12, 866876, doi:10.1175/1520-0434(1997)012<0866:FSITIO>2.0.CO;2.

    • Search Google Scholar
    • Export Citation
  • Riordan, A. J., 1990: Examination of the mesoscale features of the GALE coastal front of 24–25 January 1986. Mon. Wea. Rev., 118, 258282, doi:10.1175/1520-0493(1990)118[258:EOTMFO]2.0.CO;2.

    • Search Google Scholar
    • Export Citation
  • Rolph, G. D., cited 2013: Real-time Environmental Applications and Display System (READY). NOAA/Air Resources Laboratory. [Available online at http://www.ready.noaa.gov.]

  • Saarikivi, P., and Puhakka T. , 1990: The structure and evolution of a wintertime occluded front. Tellus, 42A, 122139, doi:10.1034/j.1600-0870.1990.00012.x.

    • Search Google Scholar
    • Export Citation
  • Sanders, F., 1955: An investigation of the structure and dynamics of an intense surface frontal zone. J. Meteor., 12, 542552, doi:10.1175/1520-0469(1955)012<0542:AIOTSA>2.0.CO;2.

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  • Fig. 1.

    The 250-hPa charts from NOAA/Storm Prediction Center (http://www.spc.noaa.gov) with isotachs [25-knot (kt; 1 kt = 0.51 m s−1) interval above 75 kt] and streamlines at (a) 0000 UTC 10 Jan, (b) 0000 UTC 11 Jan, (c) 1200 UTC 11 Jan, and (d) 0000 UTC 12 Jan 2013. Station model shows temperature (red) and dewpoint (green) (°C).

  • Fig. 2.

    The 500-hPa charts with geopotential height contours (solid) and temperatures (dashed) at (a) 0000 UTC 10 Jan, (b) 0000 UTC 11 Jan (c) 1200 UTC 11 Jan, and (d) 0000 UTC 12 Jan 2013. Station model as in Fig. 1.

  • Fig. 3.

    The 925-hPa charts with geopotential height contours (black), temperatures (dashed), and dewpoint (green; above 12°C) at (a) 0000 UTC 10 Jan, (b) 0000 UTC 11 Jan, (c) 1200 UTC 11 Jan, (d) 0000 UTC 12 Jan, and (e) 1200 UTC 12 Jan 2013. Station model as in Fig. 1.

  • Fig. 4.

    NOAA/HPC surface frontal analyses with standard station model [temperature and dewpoint (°F); sea level pressure (hPa)] at (a) 0000 UTC 11 Jan, (b) 1200 UTC 11 Jan, (c) 1800 UTC 11 Jan, and (d) 0000 UTC 12 Jan 2013.

  • Fig. 5.

    Mounting of HMP45C with Sticky Pods and GPS16-HVS on vehicle roof.

  • Fig. 6.

    (a) Position of surface synoptic stations relative to vehicle track, superimposed onto a digital elevation map (DEM). Along-track elevation ranges from about 20 to 130 m. (b) Comparison of mobile and synoptic station temperature and dewpoint (°C).

  • Fig. 7.

    Time-shift adjustments of mobile temperature data (°C) to 2307 UTC (shown with all data points, including stops). Actual UTC observation time is shown on the horizontal axis.

  • Fig. 8.

    Mobile temperature (blue) and dewpoint (red) data (°C) after time-shift adjustment to 2307 UTC. Actual UTC observation time is shown on the horizontal axis.

  • Fig. 9.

    (a) Potential temperature (K), (b) equivalent potential temperature (K), and (c) mixing ratio (g kg−1) from mobile observations. Blue line is mobile transect data; red line is from nearby synoptic ASOS/AWOS stations.

  • Fig. 10.

    Geographic portrayal of mixing ratio (g kg−1) from mobile observations using IDV.

  • Fig. 11.

    Surface mixing ratio analysis (g kg−1) at 2307 UTC 11 Jan 2013 from surface station reports and mobile transect.

  • Fig. 12.

    (a) Mixing ratio (g kg−1) vs elevation (m) according to 0.5°-longitude bins. (b) Potential temperature (K) vs elevation.

  • Fig. 13.

    The NAM-12 operational model data from NOAA/ARL’s READY website (http://www.ready.noaa.gov; Rolph 2013) at 2100 UTC 11 Jan 2013. (a) Accumulated precipitation over 3 h (mm) and (b) 2-m air temperature (°C).

  • Fig. 14.

    Cross-section analysis at 0000 UTC 11 Jan of (a) potential temperature (K) and (b) mixing ratio (g kg−1). Relative humidity >90% (or >80% in rain) is shaded in (a). Vertical axis is height above sea level (m); horizontal axis is longitude. TA and TB refer to layers/zones of potential temperature gradient, and MA and MB to moisture gradients.

  • Fig. 15.

    Cross-section analysis at 1200 UTC 11 Jan of (a) potential temperature and (b) mixing ratio. Relative humidity >90% (or >80% in rain) is shaded in (a). Dashed black line separates areas that cooled or warmed over previous 12 h.

  • Fig. 16.

    Cross-section analysis at 0000 UTC 12 Jan of (a) potential temperature, (b) mixing ratio, and (c) wind speed (kt). Relative humidity and warming–cooling as in Fig. 15.

  • Fig. 17.

    Skew T thermodynamic diagrams for (a) SHV and (b) JAN radiosonde data at 0000 UTC 12 Jan (http://weather.rap.ucar.edu).

  • Fig. 18.

    Ensemble backward trajectories over 24 h from HYSPLIT (using NAM-12 model vertical velocity) ending at 0000 UTC 12 Jan 2013 at 500 m MSL for (a) Monroe and (b) Jackson.

  • Fig. 19.

    Cross-section analysis of mixing ratio at 1200 UTC 12 Jan.

  • Fig. 20.

    Conceptual schematic illustrating differences between (a) a typical return-flow moisture surge and (b) the situation up to and during the 0000 UTC 12 Jan 2013 transect.

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