A Comparison of Inverted Troughs in the Central U.S.

Tyler J. Fleming and Mark R. Anderson
Department of Geosciences, University of Nebraska-Lincoln 

Introduction

        An inverted trough (IT) can be observed throughout the year in the mid-latitudes on leeward sides of north-south oriented mountain ranges.  An IT appears as a poleward bulge of low pressure on a surface pressure contour map and can act as a third front on a mid-latitude cyclone, separating two different polar air masses.  Previous work conducted on ITs has focused on cold-season cases and their potential to produce heavy snowfall, yet little work has been done during the warm-season.  However, ITs can also be associated with severe weather, and in particular, flash flooding during the warm-season. 

Background

        One of the earliest studies of cold-season ITs was done by Keshishian et al. (1994).  Weisman et al. (2002) expanded on that research by classifying ITs based on where precipitation fell in relation to the IT axis.  Weisman et al. (2002) divided a schematic of an IT into four quadrants using the warm front, cold front, and IT axis as dividers (figure 1).  Additionally, the fourth border was made by extending the warm front west through the low.  Weisman et al. (2002) found that certain ITs precipitation falls mostly in the “A” quadrant while in others, the precipitation is confined to the “B” quadrant.  These two types of ITs, referred to as “ahead” and “behind” cases respectively, were found to possess very different structures and processes.  Among other differences, “ahead” cases are much more common, although the “behind” cases were comprised of heavier precipitation events.  Additionally, “ahead” cases tend to have a progressive upper-level flow and are vertically stacked in the lower atmosphere.  “Behind” cases, on the other hand are usually associated with a more amplified upper-level pattern and are closely linked to jet streak circulations, resulting in stronger pressure and thermal gradients and a vertical slant over cold air.  Additional differences can be found in the location of areas of warm air advection (WAA), which is found just east of the IT in the “ahead” cases and along the IT axis for “behind” cases.  Perhaps the most important difference, however, is that “behind” cases generate precipitation from isentropic lift, while “ahead” cases were not found to produce much additional lift (Weisman et al. 2002).

Methodology

        A search of daily surface maps from 2000 to 2007 turned up numerous examples of ITs at all times of the year.  One representative warm-season and one cold-season IT were selected for study.  For the cold-season case, 1800 UTC 28 December 2006 through 1800 UTC 31 December 2006 was selected.  The warm-season case occurred from 0000 UTC 9 July 2002 to 0000 UTC 12 July 2002.  North American Regional Reanalysis (NARR) data were obtained from the National Climatic and Data Center (NCDC) at six hour time steps for each case study with an extra 24 hour period added on either end.  Each case was analyzed and compared to examples from previous work done by Keshishian et al. (1994) and Weisman et al. (2002).  At the lower and mid-levels, comparisons were performed at the surface, 850 hPa, 700 hPa, and 500 hPa.  Upper level comparisons were made at 300 hPa for the cold-season case and 200 hPa for the warm-season case.

Cold-Season Case

        A strong surface low passed over the Rockies into central New Mexico by 0600 UTC    28 December 2006, forming into an IT by 1800 UTC (figure 2).  The initial IT, which stretched from the panhandle region through the southern Great Lakes, was visible only up to 850 hPa with little vertical tilt in the lower atmosphere.  Above 850 hPa, a cutoff low was over Arizona, well west of the surface position forming a strongly amplified trough.  Low relative humidity around the IT as well as the absence of WAA prevented precipitation from occurring at this stage.  In the upper levels, a weak ridge was present over the eastern U.S. separating two jet streaks, with the entrance region of the northern branch extending just into quadrant “B”.  Although little WAA was present in the mid-levels, diffluent flow at 500 hPa and 300 hPa encouraged precipitation along the IT axis. 
        By 1800 UTC on 29 December, the surface low had progressed into southern Texas and the IT had become much more north-south oriented, reaching to Minnesota.  Behind the IT axis, the temperature and pressure gradients started to tighten.  As a result of this thermal gradient along the IT, the low and IT axis developed a stronger vertical tilt than 24 hours earlier, leaning back over cold air, similar to a front. At the upper levels difluence was still present along the IT axis, however the jet entrance region had moved into the “ahead” quadrant.  These factors combined to allow for heavy precipitation that began at this time.
        By 30 December, the surface low had moved poleward into Oklahoma and Kansas, shortening the length of the IT axis.  High relative humidity and WAA near the IT axis in the mid-levels along with strong upper-level difluence contributed to heavy precipitation during this time.  By 1800 UTC 31 December the IT had decayed, taking on the appearance of a cutoff low.  The IT first dissipated at the 850 hPa level before dying at the surface a few hours later.  The surface cutoff low deepened as it moved to the northeast and over the Atlantic. 

Warm-Season Case

        A surface low on the lee side of the Rockies moved into western Montana by 0000 UTC 9 July 2002, initially exhibiting only a short IT axis (figure 3).  The IT was visible up to 850 hPa and was vertically stacked throughout its life.  Although areas of WAA were located along and ahead of the IT axis, the WAA was substantially weaker at later time periods.  Upper level flow was relatively slow during this time period, resulting in weak difluence, however the only jet streak in the vicinity remained in the “A” quadrant.  On 10 July, the low had moved southeast and into northern Nebraska and the IT had taken on a curved shape, particularly at 850 hPa.  By 11 July, the low had dipped into eastern Kansas and had returned to a north-south orientation.  Though the IT had weakened noticeably, the previously dry IT had moved into a region of high relative humidity, setting up an environment that allowed for precipitation to occur.  By 0000 UTC 12 July 2002, the surface low had largely dissipated along with the IT.

Conclusions and Summary

        The cold-season case presented fits the model of a “behind” case using the description of Weisman et al. (2002), exhibiting an amplified flow while fostering WAA along the IT axis, and in later stages, showing a strong vertical tilt over cold air similar to a front.  However, the jet entrance was in front of the IT through the latter half of its life and the precipitation pattern crossed over the IT, showing traits similar to an “ahead” case, as well.  The warm-season case, on the other hand, is closer to an “ahead” case.  The IT remained vertically stacked throughout its lifecycle and was associated with a zonal upper-level flow.  The entrance region of the jet streak also stayed well east of the IT axis preventing a strengthening of the pressure and temperature gradient near the IT.  However, the WAA displayed along the IT axis is more common in “behind” ITs, although climate records confirm that the precipitation developed ahead of the IT axis.  Although the cold and warm-season ITs appear to be examples of “behind” and “ahead” cases, respectively, neither case fits the mold perfectly.  This may in fact be normal with just under half of the 247 cases Weisman et al. (2002) examined falling into the categories of “ahead” or “behind”, with the rest either being completely dry, or some hybrid of the two.  Nevertheless, the warm-season case is similar to the “ahead” cases presented by Weisman et al. (2002), ignoring typical seasonal differences (e.g., warmer surface temperatures and weaker pressure gradients).  However, a larger number of cases need to be done before firm conclusions are drawn.

Acknowledgements               

        The authors would like to thank Clinton Rowe for invaluable help with creating maps with GEMPAK.  Financial support was provided by the University of Nebraska-Lincoln Department of Geosciences.

Figures     

Fig. 1.  Schematic drawing of an inverted trough (dashed line) with a mid-latitude cyclone.  Dotted line separating quadrants “B” and “C” is a continuation of the warm front and does not represent a baroclinic boundary (adapted for Weisman et al., 2002).  


Fig. 2.  Selected fields for 1800 UTC 28 December 2006 through 1800 UTC 31 December 2006 at the surface,  850, 700, 500, and 300 hPa.   Mean sea level pressure (solid lines, every 4 hPa) is shown for the surface, while heights are drawn for 850, 700, 500 and 300 hPa at intervals of 30, 30, 60, and 120 m, respectively.  Isotherms are drawn as dashed lines for 850, 700, and 500 hPa every 5°C.  Relative humidity of 60-80% is shaded in yellow and >80% is shaded in green at 850 and 700 hPa.  Shading at 300 hPa represents isotachs for speeds of 40-60 m s-1 (yellow) and >60 m s-1 (green). 


Fig. 3. Selected fields for 0000 UTC 9 July 2002 through 0000 UTC 12 July 2002 at the surface,  850, 700, 500, and 200 hPa.   Mean sea level pressure (solid lines, every 4 hPa) is shown for the surface, while heights are drawn for 850, 700, 500 and 200 hPa at intervals of 30, 30, 60, and 120 m, respectively.  Isotherms are drawn as dashed lines for 850, 700, and 500 hPa every 5°C.  Relative humidity of 60-80% is shaded in yellow and >80% is shaded in green at 850 and 700 hPa.  Shading at 200 hPa represents isotachs for speeds of 40-60 m s-1 (yellow) and >60 m s-1 (green).



References

Weisman, R. A., K. G. McGregor, D. R. Novak, J. L. Selzler, M. L. Spinar, and B. C. Thomas, 2002:  Precipitation regimes during cold-season central U.S. inverted trough cases.  Part 1: Synoptic climatology and composite study.  Wea. Forecasting, 17, 1173-1193.