1. Introduction
A significant fraction of warm-season (Mar–Sep) precipitation in the central United States comes from mesoscale convective systems (MCSs), so that periods of anomalous MCS activity may result in abnormal precipitation patterns (Fritsch et al. 1986; Kunkel et al. 1994; Anderson and Arritt 1998). MCSs develop in regions of concentrated latent heat release as clusters of localized, convective storms merge and self-organize into precipitation systems of larger spatial dimension. Although the convective storms are local phenomena, the processes that control MCS development are observed over meso- to continental scales and from hourly to monthly periods (see, e.g., Mo et al. 1997; Maddox 1983). Thus, the distribution of MCSs varies with the large-scale condition (Augustine and Howard 1988, 1991; Anderson and Arritt 1998).
A process external to the United States that is capable of influencing the large-scale circulation over the United States is the El Niño–Southern Oscillation (ENSO). El Niño and the Southern Oscillation are quasi-regular changes in tropical Pacific sea surface temperature (SST) and surface pressure, respectively. The two fields are anticorrelated so that during an El Niño event an extensive pool of warm SST anomalies is overlaid by below average surface pressure (Montroy 1997). ENSO is the driving process for much of the interannual variability of precipitation in the tropical Pacific, because the location of warm SST determines regions of tropical convection [for a more complete summary of ENSO see Trenberth (1997)]. Precipitation in the extratropics is influenced by ENSO through a less direct mechanism. Persistent upper-level outflow from tropical convection in the central and eastern equatorial Pacific during an El Niño event can initiate a chain of alternating regions of upper-level divergence and convergence that extend into mid- and high latitudes (Horel and Wallace 1981; Sardeshmukh and Hoskins 1988; Rasmussen and Mo 1993) and may affect the position of the time-mean extratropical jet stream and storm tracks. The position of the time-mean extratropical jet stream and storm tracks, in turn, affects the distribution of precipitation in the central United States.
Studies that have examined linkages between anomalies of equatorial Pacific SST and of central U.S. precipitation have focused mostly on time-mean quantities. They have found that, on average, an increase in rainfall in an area extending from northern Texas into the Dakotas is observed during April through October as warm equatorial Pacific SST anomalies increase in coverage (Ropelewski and Halpert 1986, 1987, 1989, 1996; Montroy 1997; Ting and Wang 1997; Bunkers et al. 1996). Precipitation is sometimes above average in the western Great Plains the following spring (Ropelewski and Halpert 1996; Montroy 1997). These studies were designed for the purpose of discovering repetitive patterns in both accumulated precipitation and the large-scale atmospheric fields. This strategy is unable to examine the episodic nature of precipitation and its dependence on regional weather systems. MCS summaries contain such information and are a natural complement to the aforementioned body of statistical research. However, because MCS summaries rely on satellite information, the length of the data record is much smaller than that of the statistical studies. Here we contribute a summary of MCS activity during the warm seasons of the extraordinary El Niño event in 1997–98.
2. Data sources
a. GOES-8 IR
Cloud-top characteristics of large, long-lived MCSs in GOES-8 infrared (IR) digital satellite imagery were cataloged for March–September of 1997–98 with a modified version of the cloud-top documentation procedure developed by Augustine (1985). A more complete description of the routine is provided in Anderson and Arritt (1998). Hourly satellite data were obtained from the archive maintained by the Space Science and Engineering Center at the University of Wisconsin. From March through May of 1997, hourly GOES-8 IR images were missing from 0400 to 0700 UTC on many days. This has influenced our tabulation by creating some uncertainty in the time of maximum cloud-top area of events that occurred in this time frame. We viewed satellite imagery for each case to avoid including nonconvective cloud shields, such as cirrus cloud streaks, and consulted surface station reports for cases that were ambiguous.
Each MCS documented by the automated routine was categorized as either quasi-circular or quasi-linear. Following previous summaries, we labeled a quasi-circular MCS as a mesoscale convective complex (MCC) and a quasi-linear MCS as a persistent elongated convective system (PECS). The formal criteria for these definitions are provided in Table 1. Note, first, that the duration and size criteria were devised by Maddox (1980) to indicate that these MCSs are likely to be organized on scales greater than those of convective storms and, second, that the shape criterion is applied only at the time of maximum extent.
b. Seasonal precipitation
Climate division monthly precipitation was obtained from the National Climate Data Center (NCDC). There are at most 10 climate divisions within each state. Monthly averages of precipitation are computed by NCDC for each division by equally weighting the monthly total precipitation at stations that consistently report both precipitation and temperature. NCDC requires the inclusion of both surface variables over a long time period as a quality control measure. We have averaged the monthly values for each division to obtain seasonal values for March–April–May and June–July–August. For presentation purposes we have interpolated these data from irregular-spaced positions to a latitude–longitude grid with nodes equally spaced 2° apart. Our analysis scheme is based on Barnes (1964) and is performed with a single pass and an e-folding distance set equal to one grid length. These parameters were selected because our main interest is broad regional trends rather than local details. We computed anomalies from the 20-yr period 1979–98.
c. Microwave wind profilers
Hourly wind data from the the National Oceanic and Atmospheric Administration Wind Profiler Network (NPN) were categorized according to a wind speed threshold applied at low levels. NPN wind speed profiles that contained maximum speed ≥12 m s−1 in the lowest 1500 m were labeled wind events (Mitchell et al. 1995). In order to ensure adequate sampling of the low-level wind, profiles were included only if four out of five levels below 1500 m were available. Daniel et al. (1999) and Arritt et al. (1997) found that NPN wind speed errors (Stensrud 1996) influenced the frequency of detection mainly for higher speed thresholds, so that such errors are less likely to influence the results herein.
3. Results and discussion
a. Overview of 1997–98 El Niño
The 1997–98 El Niño was unique in a number of ways. The onset of this event occurred in March 1997, well before the typical onset for El Niño events (Wolter and Timlin 1998). The event intensified unusually rapidly. It reached the highest intensity at the earliest date yet to be recorded and maintained that level of intensity through December 1997, as the warm SST anomaly moved and expanded eastward into the central equatorial Pacific (McPhaden 1999; Wolter and Timlin 1998). The demise of the event occurred in mid-May 1998 and was as sudden as its onset, ending with a dramatic reversal of SST anomalies from positive to negative values along the coastline of Ecuador and Peru. McPhaden and Xu (1999) noted that the cooling rate at one location during the demise of this event was 10 times the normal cooling rate. Since 1950, only the El Niño event of 1982–83 was comparable in intensity to that of 1997–98 (Wolter and Timlin 1998).
Upper-tropospheric conditions over the extratropics were influenced by tropical convection in the central equatorial Pacific earlier than is usual during El Niño events (Barnston et al. 1999). Anomalous seasonal troughs were established off the United States west coast and over the southeastern United States from the fall of 1997 through the spring of 1998 (Bell and Halpert 1998; Bell et al. 1999). During this time the subtropical jet stream was abnormally strong and active (Bell et al. 1999). Above average precipitation was observed in the western and southern United States during November 1997–April 1998 and in the Midwest during spring of 1998. A sudden northward shift of the subtropical ridge in mid-May 1998 caused an abrupt northward displacement of the jet stream over the central United States that persisted through the summer months, resulting in record drought and heat over the southern United States (Bell et al. 1999). This evolution was unexpected and highly atypical; in fact Barnston et al. (1999) note that previous examples of such behavior are nonexistent. However atypical it may have been, the final stages of this El Niño were likely to have contributed to the displacement of the jet stream as equatorial convection moved eastward with the SST anomalies, creating a shift in the upper-level convergence that may have led to ridging in the upper troposphere over the central United States (Bell et al. 1999; Barnston et al. 1999).
b. Latitude–time distribution of MCS centroid positions at initiation
1) PECS
The PECS populations for 1997 and 1998 are about the same size in each year and exhibit similar spatial patterns in summertime (Fig. 1; additional information for each PECS is provided in appendix Table A1). In particular, PECSs are most frequent during May and June (Julian days 121–181), when the average position of initiation is near 40°N. Similar seasonal characteristics are described in other MCS climatologies. In particular, Bartels et al. (1984) find that large cloud lines and hybrid systems, which correspond roughly to PECS, are most frequent from May into early June, and those in June are generally positioned north of those in May. Anderson and Arritt (1998) document a similar seasonal shift for PECS in 1992 and 1993. Noticeable differences between 1997 and 1998 are evident during March and April (Julian days 60–121). PECSs in March 1998 outnumber those in March 1997, while the reverse is true for April (Table A1). In addition, many events in March–April 1998 occur north of 40°N, whereas in 1997 nearly all events remain south of 40°N until late April (about Julian day 114), when a sudden increase in frequency is observed.
The uniqueness of the March 1998 PECS distribution is difficult to assess, since the number of satellite-based climatologies is small. Bartels et al. (1984) provides a relatively long record (1979–83) of MCS activity, and from this a comparison of MCS statistics in April is facilitated. Of the seven events in early April 1998 (Julian days 91–110), four PECS are positioned within the STORM-Central domain. For the years 1979–83 the frequency of large cloud lines and hybrid systems is similar only in April of 1981; however, large data gaps are present in April of 1979 and 1982 (Bartels et al. 1984). In neither 1992 nor 1993 was PECS frequency in March close to that of 1998, although early April of 1992 was nearly as active as early April of 1998 (Anderson and Arritt 1998). The suggestion from this small climatology is that a high incidence of large, quasi-linear MCS in the central United States during March and early April may be rare.
2) MCC
MCC distributions differ substantially between 1997 and 1998 (Fig. 2; more information about each MCC is provided in Table A2). The MCC distribution for 1997 contains a northward shift in the mean position of initiation and an increase in frequency during July and August. In contrast, the 1998 distribution is defined by two space–time clusters, a springtime cluster from mid-May (Julian day 140) through mid-June (Julian day 170) and a late summer cluster from late July (Julian day 205) through late August (Julian day 240). Both clusters contain about the same number of events and are spread about 37°N so that a seasonal shift is not apparent. Other MCC summaries have documented a northward shift in position and maximum frequency in July (e.g., Augustine and Howard 1991). The 1997 distribution is quite typical in this regard; even the total number of MCCs (32) is nearly equal to that of the annual average (33). While MCCs in 1998 are almost as frequent (29) as in 1997, the space–time distribution is atypical.
A persistent, large-scale ridge dominated the anomalous, large-scale circulation over the central United States at all levels from May through August of 1998 (see section 3a), along the periphery of which MCCs formed and propagated. Similar behavior is reported in Rodgers et al. (1985) and Augustine and Howard (1988, 1991) for MCCs in the central United States and Velasco and Fritsch (1987) and Laing and Fritsch (1997) for other regions of the world. It is curious that an anomalous northward shift of the mean position of MCCs occurred during the demise of El Niño events in both 1983 and 1998. While these two El Niño events are extraordinary and may not be representative of El Niño events in general, it is possible that they are cases in which equatorial convection influenced the distribution of summertime precipitation in the central United States.
c. Precipitation anomalies
We have related seasonal precipitation to episodes of heavy precipitation by superimposing MCS tracks onto seasonal precipitation anomaly maps. We have constructed PECS and MCC tracks from the positions of the −52°C cloud centroids at the time of initiation, maximum, and termination.
A precipitation maximum in the 20-yr mean for March–May is located in the southeastern United States (Fig. 3a). Mean precipitation decreases to the west and north of the maximum. Anomaly maps for 1997 and 1998 contain opposite north–south dipole patterns in the central United States (Figs. 3b and 3c). However, the maps share a common trait. That is, positive anomalies in the central United States are coincident with densely spaced PECS and MCC tracks, while negative anomalies occur where PECSs and MCCs are relatively infrequent.
The primary maximum in mean precipitation for June–August is located in Florida, and mean precipitation decreases northwestward until a secondary maximum in the Great Plains (Fig. 4a). As in March–May positive (negative) anomalies in the central United States for both 1997 and 1998 occur where PECSs and MCCs are frequent (infrequent). The abrupt onset and northward shift of the PECS distribution during May and June of 1997 is evident in the MCS tracks (Figs. 1 and 2), which display a minimum in MCS activity that is collocated with a negative precipitation anomaly over much of the Midwest. In 1998, areas of positive and negative anomalies are more coherent and extensive (Fig. 4c), due to the persistent upper-level ridge. The position of the mean ridge periphery is reflected in the PECS and MCC tracks as a preponderance of tracks north of 35°N (Figs. 1 and 2).
A correspondence between central U.S. precipitation and MCS activity has been noted in other MCS summaries. During the summertime drought in the central United States in 1983, quasi-circular MCSs were infrequent and were displaced northward of the central United States into a region where atmospheric water vapor is less abundant, thereby reducing the precipitation efficiency of the quasi-circular MCSs (Rodgers et al. 1983, 1985; Fritsch et al. 1986). In contrast, the high incidence of large MCSs, both quasi-linear and quasi-circular, over the central United States was an important factor in the development of the Midwestern flood in the summer of 1993 (Kunkel et al. 1995; Anderson and Arritt 1998). Compared with previous MCS summaries, the correspondence between positive precipitation anomalies and large, quasi-linear MCSs in March through mid-April of 1998 is unique. However, Montroy (1997) has demonstrated that a statistically significant positive correlation exists between equatorial Pacific SSTs and precipitation in the south-central United States during the late stages of El Niño events. It is possible that in previous El Niño events large, quasi-linear MCS were connected to precipitation anomalies as in March through mid-April of 1998. We suggest as an area of future exploratory research an emphasis on heavy precipitation events during the warm seasons of El Niño, which might further connect the results of statistical studies to those of MCS summaries.
d. Episodic low-level wind features for March and April
A linkage between anomalies in equatorial Pacific SST and central U.S. precipitation in March through mid-April 1998 appears plausible, since El Niño influenced the extratropical jet stream, the jet stream affected the development of PECS, and the PECS distribution affected the precipitation anomalies. However, PECSs are dependent on transient weather conditions, so that analysis of the time-mean circulation provides only partial insight into how persistent features shaped the immediate environment of PECS. In order to build a meaningful measure of the transient flow we note that Anderson and Arritt (1998) found that PECS develop in regions of strong southerly flow, similar to MCCs for which low-level warm advection contributes mesoscale ascent and thermodynamic support (Cotton et al. 1989). In accordance with this finding, we have determined the frequency of northerly (NWE) and southerly wind events (SWE) from NPN observations and have created a time series of normalized coverage for both wind categories (see section 2c for definition of wind event). NWE (SWE) normalized coverage is defined as the percentage of reporting NPN stations that are categorized as NWE (SWE). We have smoothed the time series with a 5-h running average centered on the plotted hour. Half or more of the NPN sites must report in order for that hour to be included in the time series.
Time series of normalized coverage contain irregular oscillations between extensive SWE and NWE coverage through April of both years (Fig. 5). In 1997, these oscillations are more frequent in March to mid-April, when at least seven episodes of greater than 0.7 NWE coverage are observed, compared to about three in 1998. We have viewed 0000 and 1200 UTC 850-hPa and surface maps and have found that periods of NWE coverage of 0.7 or greater are associated with southward moving cold fronts as a synoptic-scale low pressure system moves over the northeastern United States.
It is clear from Fig. 5 that PECS frequency is inversely related to NWE coverage. In particular, PECSs are infrequent when NWE coverage exceeds 0.7. Those that do occur when this threshold is exceeded are situated near the Gulf coast. As cold air spreads over the central United States during NWE outbreaks, the thermodynamic energy within the boundary layer becomes limited and vertical stratification becomes strongly stable. Favorable conditions return either by warming and moistening of boundary layer air from the surface or by replacement by advection. The latter scenario is more likely during spring months, since the solar zenith angle is relatively low, thereby limiting the amount of evapotranspiration that may occur. This inference has support in Fig. 5 as PECS development usually is delayed after NWE outbreaks until SWE coverage exceeds NWE coverage. Frequent peaks above the 0.7 NWE threshold in March 1997 indicate that the large-scale circulation guided synoptic-scale pressure systems across the United States in such a way as to cause bursts of cold air to move southward and suppress PECS development. In contrast, the active PECS period in March to early April of 1998 is characterized by persistent southerly low-level flow (Figs. 5c and 5d). As in March 1997, peaks of NWE coverage in 1998 are associated with the passage of synoptic-scale pressure systems through the central United States, but are less frequent even though the frequency of synoptic-scale systems is about the same or slightly greater than in 1997.
4. Conclusions
MCC and PECS were documented during the 1997–98 El Niño. Their time–latitude distributions followed climatological trends during the incipient stages of El Niño, while deviations from climatology were evident late in the El Niño event. PECSs were abnormally frequent in the Midwest in March and early April of 1998, when cold air outbreaks were infrequent despite the passage of several synoptic-scale pressure systems. MCCs during the summer of 1998 did not follow a typical northward migration but instead were clustered near 37°N, along the periphery of a tropospheric ridge that persisted from mid-May through the summer.
Positive precipitation anomalies and densely spaced MCS tracks were coincident over the central United States in every season, while negative precipitation anomalies occurred where MCS tracks were sparse. Of particular interest were the positive precipitation anomalies in summer 1997 and spring 1998, since these anomalies are similar to statistically significant patterns associated with ENSO found in other studies. The additional documentation provided herein is offered as a possible starting point for more detailed study of dynamical linkages between ENSO, MCSs, and precipitation.
Acknowledgments
This research was sponsored by National Science Foundation Grants ATM-9616728 and ATM-9909650. This is Journal Paper Number J-19018 of the Iowa Agriculture and Home Economics Experiment Station, Project Number 3803, supported by Hatch Act and State of Iowa funds.
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APPENDIX
Detailed information on mesoscale convective complexes for 1997 and 1998.
Table A1a. PECS events for 1997
Table A1a. (Continued)
Table A1b. PECS events for 1998
Table A1b. (Continued)
Table A2a. MCC events for 1997
Table A2b. MCC event for 1998
Latitude–time distribution of −52°C cloud shield centroid at initiation for PECS in 1997 (stars) and 1998 (open squares)
Citation: Monthly Weather Review 129, 9; 10.1175/1520-0493(2001)129<2443:MCSOTU>2.0.CO;2
Latitude–time distribution of −52°C cloud shield centroid at initiation for PECS in 1997 (stars) and 1998 (open squares)
Citation: Monthly Weather Review 129, 9; 10.1175/1520-0493(2001)129<2443:MCSOTU>2.0.CO;2
Latitude–time distribution of −52°C cloud shield centroid at initiation for PECS in 1997 (stars) and 1998 (open squares)
Citation: Monthly Weather Review 129, 9; 10.1175/1520-0493(2001)129<2443:MCSOTU>2.0.CO;2
Latitude–time distribution of −52°C cloud shield centroid at initiation for MCC in 1997 (stars) and 1998 (open squares)
Citation: Monthly Weather Review 129, 9; 10.1175/1520-0493(2001)129<2443:MCSOTU>2.0.CO;2
Latitude–time distribution of −52°C cloud shield centroid at initiation for MCC in 1997 (stars) and 1998 (open squares)
Citation: Monthly Weather Review 129, 9; 10.1175/1520-0493(2001)129<2443:MCSOTU>2.0.CO;2
Latitude–time distribution of −52°C cloud shield centroid at initiation for MCC in 1997 (stars) and 1998 (open squares)
Citation: Monthly Weather Review 129, 9; 10.1175/1520-0493(2001)129<2443:MCSOTU>2.0.CO;2
Climate division precipitation (cm) for Mar–Apr–May (a) 20-yr mean (1979–98), (b) anomaly in 1997, and (c) anomaly in 1998. Overlaid on the anomaly maps are tracks of −52°C cloud shield centroid based on the position at initiation, maximum, and termination for each MCC (solid) and PECS (dashed)
Citation: Monthly Weather Review 129, 9; 10.1175/1520-0493(2001)129<2443:MCSOTU>2.0.CO;2
Climate division precipitation (cm) for Mar–Apr–May (a) 20-yr mean (1979–98), (b) anomaly in 1997, and (c) anomaly in 1998. Overlaid on the anomaly maps are tracks of −52°C cloud shield centroid based on the position at initiation, maximum, and termination for each MCC (solid) and PECS (dashed)
Citation: Monthly Weather Review 129, 9; 10.1175/1520-0493(2001)129<2443:MCSOTU>2.0.CO;2
Climate division precipitation (cm) for Mar–Apr–May (a) 20-yr mean (1979–98), (b) anomaly in 1997, and (c) anomaly in 1998. Overlaid on the anomaly maps are tracks of −52°C cloud shield centroid based on the position at initiation, maximum, and termination for each MCC (solid) and PECS (dashed)
Citation: Monthly Weather Review 129, 9; 10.1175/1520-0493(2001)129<2443:MCSOTU>2.0.CO;2
Climate division precipitation (cm) for Jun–Jul–Aug (a) 20-yr mean (1979–98), (b) anomaly in 1997, and (c) anomaly in 1998. Overlaid on the anomaly maps are tracks of −52°C cloud shield centroid based on the position at initiation, maximum, and termination for each MCC (solid) and PECS (dashed)
Citation: Monthly Weather Review 129, 9; 10.1175/1520-0493(2001)129<2443:MCSOTU>2.0.CO;2
Climate division precipitation (cm) for Jun–Jul–Aug (a) 20-yr mean (1979–98), (b) anomaly in 1997, and (c) anomaly in 1998. Overlaid on the anomaly maps are tracks of −52°C cloud shield centroid based on the position at initiation, maximum, and termination for each MCC (solid) and PECS (dashed)
Citation: Monthly Weather Review 129, 9; 10.1175/1520-0493(2001)129<2443:MCSOTU>2.0.CO;2
Climate division precipitation (cm) for Jun–Jul–Aug (a) 20-yr mean (1979–98), (b) anomaly in 1997, and (c) anomaly in 1998. Overlaid on the anomaly maps are tracks of −52°C cloud shield centroid based on the position at initiation, maximum, and termination for each MCC (solid) and PECS (dashed)
Citation: Monthly Weather Review 129, 9; 10.1175/1520-0493(2001)129<2443:MCSOTU>2.0.CO;2
Time series of northerly and southerly wind event normalized coverage for (a) Mar 1997, (b) Apr 1997, (c) Mar 1998, and (d) Apr 1998. Gaps in the time series indicate periods when less than half of the stations reported. Diamonds above the time series mark ongoing PECSs
Citation: Monthly Weather Review 129, 9; 10.1175/1520-0493(2001)129<2443:MCSOTU>2.0.CO;2
Time series of northerly and southerly wind event normalized coverage for (a) Mar 1997, (b) Apr 1997, (c) Mar 1998, and (d) Apr 1998. Gaps in the time series indicate periods when less than half of the stations reported. Diamonds above the time series mark ongoing PECSs
Citation: Monthly Weather Review 129, 9; 10.1175/1520-0493(2001)129<2443:MCSOTU>2.0.CO;2
Time series of northerly and southerly wind event normalized coverage for (a) Mar 1997, (b) Apr 1997, (c) Mar 1998, and (d) Apr 1998. Gaps in the time series indicate periods when less than half of the stations reported. Diamonds above the time series mark ongoing PECSs
Citation: Monthly Weather Review 129, 9; 10.1175/1520-0493(2001)129<2443:MCSOTU>2.0.CO;2
MCC and PECS definitions
