Search Results

You are looking at 1 - 10 of 29 items for

  • Author or Editor: David Parker x
  • All content x
Clear All Modify Search
David E. Parker

Abstract

Daily anomalies of mean central England temperature (CET), relative to daily 1961–90 climatology, are analyzed in terms of the source of the air estimated from fields of mean sea level pressure. The average CET anomaly for a given source and calendar month during 1961–90 is taken as an estimate of the influence of atmospheric circulation for that source and calendar month, and the uncertainty in this influence is provided by the associated standard error. The atmospheric circulation influences are subtracted from the daily CET anomalies since the late nineteenth century to yield “residual anomalies,” which represent the influence of forcings other than atmospheric circulation. The use of air sources captures more circulation-related daily CET variance than the airflow indices used in previous studies. The warming in central England since the 1970s is not predominantly a result of atmospheric circulation changes, and the long-term changes of CET for air from major source regions are on the whole very similar to each other and to the overall long-term changes.

Full access
David E. Parker

Abstract

On the premise that urban heat islands are strongest in calm conditions but are largely absent in windy weather, daily minimum and maximum air temperatures for the period 1950–2000 at a worldwide selection of land stations are analyzed separately for windy and calm conditions, and the global and regional trends are compared. The trends in temperature are almost unaffected by this subsampling, indicating that urban development and other local or instrumental influences have contributed little overall to the observed warming trends. The trends of temperature averaged over the selected land stations worldwide are in close agreement with published trends based on much more complete networks, indicating that the smaller selection used here is sufficient for reliable sampling of global trends as well as interannual variations. A small tendency for windy days to have warmed more than other days in winter over Eurasia is the opposite of that expected from urbanization and is likely to be a consequence of atmospheric circulation changes.

Full access
Matthew D. Parker and David A. Ahijevych

Abstract

Nine years of composited radar data are investigated to assess the presence of organized convective episodes in the east-central United States. In the eastern United States, the afternoon maximum in thunderstorms is ubiquitous over land. However, after removing this principal diurnal peak from the radar data, the presence and motion of organized convective systems becomes apparent in both temporally averaged fields and in the statistics of convective episodes identified by an objective algorithm. Convective echoes are diurnally maximized over the Appalachian chain, and are repeatedly observed to move toward the east. Partly as a result of this, the daily maximum in storms is delayed over the Piedmont and coastal plain relative to the Appalachian Mountains and the Atlantic coast. During the 9 yr studied, the objective algorithm identified 2128 total convective episodes (236 yr−1), with several recurring behaviors. Many systems developed over the elevated terrain during the afternoon and moved eastward, often to the coastline and even offshore. In addition, numerous systems formed to the west of the Appalachian Mountains and moved into and across the eastern U.S. study domain. In particular, many nocturnal convective systems from the central United States entered the western side of the study domain, frequently arriving at the eastern mountains around the next day’s afternoon maximum in storm frequency. A fraction of such well-timed systems succeeded in crossing the Appalachians and continuing across the Piedmont and coastal plain. Convective episodes were most frequent during the high-instability, low-shear months of summer, which dominate the year-round statistics. Even so, an important result is that the episodes still occurred almost exclusively in above-average vertical wind shear. Despite the overall dominance of the diurnal cycle, the data show that adequate shear in the region frequently leads to long-lived convective episodes with mesoscale organization.

Full access
David M. Gaffin and Stephen S. Parker

Abstract

A 54-yr climatology (1950–2003) of synoptic conditions associated with significant (F2 or greater) tornado events in the southern Appalachian region was compiled to 1) investigate the observed relative minimum of tornadoes in the Great Tennessee Valley, 2) test a hypothesis concerning northwest versus southwest 500-hPa flow events across the Great Tennessee Valley and Cumberland Plateau, 3) examine common operational forecasting techniques often used with synoptic-scale data to determine potentially tornadic environments, and 4) compare the patterns associated with significant, outbreak, and weak tornado events. Individual surface and upper-air charts along with composite charts closest to the time of significant tornado occurrences were used in this investigation. It was found that significant tornado events that occurred with prefrontal troughs (the most common surface boundary in the study) produced significant tornadoes almost exclusively over the Cumberland Plateau and southern Appalachian Mountains, with very few prefrontal trough events producing significant tornadoes in the Great Tennessee Valley of eastern Tennessee. Only four northwest 500-hPa flow events (which produced significant tornadoes almost exclusively in the southern Appalachian Mountains and during the summer) were found in this study, which refuted the initial hypothesis that northwest 500-hPa flow may produce tornadoes mainly in the Great Tennessee Valley with southwest 500-hPa flow producing tornadoes mainly across the Cumberland Plateau. Most significant tornado events in this study were associated with southwest 500-hPa flow ahead of a neutral-tilted trough, which revealed that the particular tilt of a 500-hPa trough does not necessarily enhance the formation of significant tornadoes in the southern Appalachian region. However, outbreak events (with five or more significant tornadoes) in the southern Appalachian region were typically associated with positive-tilted troughs. At 300 or 250 hPa, the southern Appalachian region was frequently located on the right side of a jet streak, with an even split between the entrance and exit regions. This finding indicated that significant tornado events in the southern Appalachian region did not necessarily favor the right-entrance or left-exit regions of a jet streak where rising motion is expected to be most intense (with straight jet streaks). A comparison of the composites of weak, significant, and outbreak tornado events revealed that wind dynamics were more important than instability in the distinction between weak and significant tornado events across the southern Appalachian region.

Full access
Brandon A. Storm, Matthew D. Parker, and David P. Jorgensen

Abstract

On 31 May 2003, a front-fed convective line with leading stratiform precipitation (FFLS) was observed during the Bow Echo and Mesoscale Convective Vortex Experiment (BAMEX). The high-resolution BAMEX measurements provided one of the first opportunities to thoroughly observe the characteristics of an FFLS system. The 31 May system had an overturning updraft during its early stages, and produced leading stratiform precipitation. As the system matured, a jump updraft developed and the system began to produce trailing stratiform precipitation. It appears that this transition was facilitated by a local decrease in the low-level line-perpendicular vertical wind shear over time, as well as an increase in the surface cold pool’s strength. The BAMEX data further help to address the question of how FFLS systems can be long lived when their inflow passes through the line-leading precipitation: preline soundings suggest a destabilization mechanism resulting from the vertical profile of cooling within the leading stratiform precipitation. This destabilization also helps to explain the 31 May convective system’s persistence in an environment with very low CAPE.

Full access
David M. Gaffin, Stephen S. Parker, and Paul D. Kirkwood

Abstract

On 26 March 1999, an unexpectedly heavy and complex snowfall event occurred across the southern Appalachian region. This event produced 20–30 cm (8–12 in.) of snow across the Smoky Mountains and 10–15 cm (4–6 in.) across other portions of southwest North Carolina, northeast Tennessee, and southwest Virginia. This snowfall event was complex in that several different lifting mechanisms combined to produce unexpectedly heavy amounts, especially in a narrow band across the Great Tennessee Valley. Lift from frontogenesis, orography, cold air damming, and mesoscale waves contributed to the snowfall amounts across the entire region. An interesting aspect of this snowfall was the banded enhancements observed during the initial stage of the event. These banded enhancements, observed by both satellite and radar, were determined to be the result of mesoscale waves. These waves developed around 0900 UTC in the lee of the Smoky Mountains as a strengthening southerly flow above 850 hPa became nearly perpendicular to the Smokies. A moist stable layer just above the mountain ridges (between 850 and 650 hPa) provided a sufficient duct for mountain waves to form across northeast Tennessee. Convective activity later developed around 1200 UTC across northeast Georgia along an inverted surface trough. This convective activity appeared to have helped trigger additional waves across western North Carolina. It appeared that the waves contributed to the heavy snowfall amounts by providing additional lift to the larger-scale lift present, which together maximized the release of the conditional instability across the region. After 1400 UTC, wave activity appeared to diminish across the southern Appalachian region as the larger-scale lift overwhelmed the waves.

Full access
David A. Sutherland, Parker MacCready, Neil S. Banas, and Lucy F. Smedstad

Abstract

A realistic hindcast simulation of the Salish Sea, which encompasses the estuarine systems of Puget Sound, the Strait of Juan de Fuca, and the Strait of Georgia, is described for the year 2006. The model shows moderate skill when compared against hydrographic, velocity, and sea surface height observations over tidal and subtidal time scales. Analysis of the velocity and salinity fields allows the structure and variability of the exchange flow to be estimated for the first time from the shelf into the farthest reaches of Puget Sound. This study utilizes the total exchange flow formalism that calculates volume transports and salt fluxes in an isohaline framework, which is then compared to previous estimates of exchange flow in the region. From this analysis, residence time distributions are estimated for Puget Sound and its major basins and are found to be markedly shorter than previous estimates. The difference arises from the ability of the model and the isohaline method for flux calculations to more accurately estimate the exchange flow. In addition, evidence is found to support the previously observed spring–neap modulation of stratification at the Admiralty Inlet sill. However, the exchange flow calculated increases at spring tides, exactly opposite to the conclusion reached from an Eulerian average of observations.

Full access
Kieran M. R. Hunt, Andrew G. Turner, Peter M. Inness, David E. Parker, and Richard C. Levine

Abstract

ERA-Interim reanalysis data from the past 35 years have been used with a newly developed feature tracking algorithm to identify Indian monsoon depressions originating in or near the Bay of Bengal. These were then rotated, centralized, and combined to give a fully three-dimensional 106-depression composite structure—a considerably larger sample than any previous detailed study on monsoon depressions and their structure. Many known features of depression structure are confirmed, particularly the existence of a maximum to the southwest of the center in rainfall and other fields and a westward axial tilt in others. Additionally, the depressions are found to have significant asymmetry owing to the presence of the Himalayas, a bimodal midtropospheric potential vorticity core, a separation into thermally cold (~−1.5 K) and neutral (~0 K) cores near the surface with distinct properties, and the center has very large CAPE and very small CIN. Variability as a function of background state has also been explored, with land–coast–sea, diurnal, ENSO, active–break, and Indian Ocean dipole contrasts considered. Depressions are found to be markedly stronger during the active phase of the monsoon, as well as during La Niña. Depressions on land are shown to be more intense and more tightly constrained to the central axis. A detailed schematic diagram of a vertical cross section through a composite depression is also presented, showing its inherent asymmetric structure.

Full access
Aiguo Dai, Junhong Wang, Peter W. Thorne, David E. Parker, Leopold Haimberger, and Xiaolan L. Wang

Abstract

Radiosonde humidity records represent the only in situ observations of tropospheric water vapor content with multidecadal length and quasi-global coverage. However, their use has been hampered by ubiquitous and large discontinuities resulting from changes to instrumentation and observing practices. Here a new approach is developed to homogenize historical records of tropospheric (up to 100 hPa) dewpoint depression (DPD), the archived radiosonde humidity parameter. Two statistical tests are used to detect changepoints, which are most apparent in histograms and occurrence frequencies of the daily DPD: a variant of the Kolmogorov–Smirnov (K–S) test for changes in distributions and the penalized maximal F test (PMFred) for mean shifts in the occurrence frequency for different bins of DPD. These tests capture most of the apparent discontinuities in the daily DPD data, with an average of 8.6 changepoints (∼1 changepoint per 5 yr) in each of the analyzed radiosonde records, which begin as early as the 1950s and ended in March 2009. Before applying breakpoint adjustments, artificial sampling effects are first adjusted by estimating missing DPD reports for cold (T < −30°C) and dry (DPD artificially set to 30°C) conditions using empirical relationships at each station between the anomalies of air temperature and vapor pressure derived from recent observations when DPD reports are available under these conditions. Next, the sampling-adjusted DPD is detrended separately for each of the 4–10 quantile categories and then adjusted using a quantile-matching algorithm so that the earlier segments have histograms comparable to that of the latest segment. Neither the changepoint detection nor the adjustment uses a reference series given the stability of the DPD series.

Using this new approach, a homogenized global, twice-daily DPD dataset (available online at www.cgd.ucar.edu/cas/catalog/) is created for climate and other applications based on the Integrated Global Radiosonde Archive (IGRA) and two other data sources. The adjusted-daily DPD has much smaller and spatially more coherent trends during 1973–2008 than the raw data. It implies only small changes in relative humidity in the lower and middle troposphere. When combined with homogenized radiosonde temperature, other atmospheric humidity variables can be calculated, and these exhibit spatially more coherent trends than without the DPD homogenization. The DPD adjustment yields a different pattern of change in humidity parameters compared to the apparent trends from the raw data. The adjusted estimates show an increase in tropospheric water vapor globally.

Full access
Loran Carleton Parker, Gerald H. Krockover, Sonia Lasher-Trapp, and David C. Eichinger
Full access