Jase Bernhardt | Hofstra University (original) (raw)
Papers by Jase Bernhardt
Theoretical and Applied Climatology, 2018
The two main methods for determining the average daily near-surface air temperature, twice-daily ... more The two main methods for determining the average daily near-surface air temperature, twice-daily averaging (i.e., [Tmax+Tmin]/2) and hourly averaging (i.e., the average of 24 hourly temperature measurements), typically show differences associated with the asymmetry of the daily temperature curve. To quantify the relative influence of several land surface and atmosphere variables on the two temperature averaging methods, we correlate data for 215 weather stations across the Contiguous United States (CONUS) for the period 1981–2010 with the differences between the two temperature-averaging methods. The variables are land use-land cover (LULC) type, soil moisture, snow cover, cloud cover, atmospheric moisture (i.e., specific humidity, dew point temperature), and precipitation. Multiple linear regression models explain the spatial and monthly variations in the difference between the two temperature-averaging methods. We find statistically significant correlations between both the land surface and atmosphere variables studied with the difference between temperature-averaging methods, especially for the extreme (i.e., summer, winter) seasons (adjusted R2 > 0.50). Models considering stations with certain LULC types, particularly forest and developed land, have adjusted R2 values > 0.70, indicating that both surface and atmosphere variables control the daily temperature curve and its asymmetry. This study improves our understanding of the role of surface and near-surface conditions in modifying thermal climates of the CONUS for a wide range of environments, and their likely importance as anthropogenic forcings—notably LULC changes and greenhouse gas emissions—continues.
Journal of Climate, 2018
Traditionally, the daily average air temperature at a weather station is computed by taking the m... more Traditionally, the daily average air temperature at a weather station is computed by taking the mean of two values, the maximum temperature (Tmax) and the minimum temperature (Tmin) over a 24-hour period. These values form the basis for numerous studies of long-term climatologies (e.g., 30-year normals) and recent temperature trends and changes. However, many first-order weather stations-- such as those at airports-- also record hourly temperature data. Using an average of the 24 hourly temperature readings to compute daily average temperature has been shown to provide a more precise and representative estimate of a given day’s temperature. This study assesses the spatial variability of the differences in these two methods of daily temperature averaging (i.e., [Tmax + Tmin]/2, average of 24 hourly temperature values) for 215 first-order weather stations across the conterminous United States (CONUS) the 30-year period 1981-2010. A statistically significant difference is shown between the two methods, as well as consistent overestimation of temperature by the traditional method ([Tmax + Tmin]/2), particularly in southern and coastal portions of the CONUS. The explanation for the long-term difference between the two methods is the underlying assumption for the twice- daily method that the diurnal curve of temperature is symmetrical. Moreover, this paper demonstrates a spatially-coherent pattern in the difference compared to the most recent part of the temperature record (2001-2015). The spatial and temporal differences shown have implications for assessments of the physical factors influencing the diurnal temperature curve, as well as the exact magnitude of contemporary climate change.
Prior to the twentieth century, there was a dearth of official local weather and climate observat... more Prior to the twentieth century, there was a dearth of official local weather and climate observations for much of the United States outside of major cities. Useful information can be gleaned, however, from primary accounts, such as historical diaries kept by farmers and others whose interests were tied to the land. Herman Smith, a farmer in west-central New York State, kept a detailed record of daily life, including weather characteristics such as temperature, precipitation, and wind, for his farm near Covert. Two full years of his diary, 1884 and 1886, were recently published and selected for study. Although typically not numeric data, the lexicon used in the diary to describe relative heat and cold allow Smith's observations to be analyzed semiquantitatively in order to determine the weather experienced that year including factors affecting the growing season, as well as significant weather and climatic events. The analysis demonstrates that for Covert-located in an area of topographic variability and proximal to the Finger Lakes-microclimatic effects occasionally dominated over the synoptic circulation. This finding was further reinforced by comparison of Smith's 1886 records with those of a nearby farmer. Meanwhile, Smith's accounts also establish an inextricable link between his agricultural practices and the weather and climate patterns he observed. These findings underscore the value of acquiring climatic data from nonconventional sources for places and times when reliable data may be nonexistent in order to better understand how climate, and its impacts on the environment, have varied over time, across multiple scales.
Multiple persistent jet aviation contrails -contrail 'outbreaks' -occur frequently over certain p... more Multiple persistent jet aviation contrails -contrail 'outbreaks' -occur frequently over certain portions of the Continental United States (CONUS). The artificial cloudiness generated by contrail outbreaks alters the atmospheric radiation budget, potentially impacting the surface air temperature, particularly the diurnal temperature range (DTR), or difference between daytime maximum and nighttime minimum temperatures. This study evaluates the hypothesis that contrail outbreaks reduce the DTR relative to clear-sky conditions. We utilize a database of longer-lived (>4 h duration) jet contrail outbreaks for the CONUS previously determined from interpretation of high-resolution satellite imagery, for the January and April months of 2008 and 2009. The outbreak impact on DTR was determined by comparing maximum and minimum temperatures at pairs of surface weather stations (one outbreak and one non-outbreak) across two regions of climatologically high outbreak frequency; the South in January, and Midwest in April. We ensured that each station pair selected had broadly similar land use-land cover, soil moisture, and synoptic air mass conditions. For outbreaks in the South (January), there was a statistically significant reduction of DTR at the outbreak versus non-outbreak stations. This result was similar to that obtained for a smaller subset of outbreaks for which lower-level clouds could be confirmed as being absent (from North American Regional Reanalysis (NARR) output). For the Midwest (April), the results are mixed; statistically different for satellite-retrieved outbreaks, but not significantly different for the NARR-validated dataset. These results suggest that persistent jet contrails should be considered in short-term weather forecasting, and for their potential influence on the climatology of more frequently impacted areas.
Natural Hazards, 2012
The speeds of historical cool-season extratropical cyclones along the U.S. east coast, hereafter ... more The speeds of historical cool-season extratropical cyclones along the U.S. east coast, hereafter East Coast Winter Storms (ECWS), occurring during the period from 1951 to 2006 were computed. Average storm speed was 13.8 ms -1 with stronger storms generally moving faster than weaker storms and faster storms forming during the midwinter months (December-March). There was no clear trend in ECWS speed during the time period, although considerable season-to-season variability was present. The monthly and seasonal variations in storm speed could not be attributed to the El Niño-Southern Oscillation or North Atlantic Oscillation (NAO) alone. However, the speed of ECWS was considerably slower when both El Niño and the negative phase of NAO occurred simultaneously. Characteristic patterns in the upper levels of the atmosphere, specifically 300 hPa zonal winds and 500 hPa geopotential heights, were present during periods when ECWS speeds were among the slowest (and separately fastest). For slow storm speed, these patterns also prevailed during months in which El Niño and negative NAO phase occurred. These patterns were also present during months with extended runs of high oceanic storm surge. This provides a qualitative link between the atmospheric conditions associated with slow storms and potentially high coastal storm surge impacts. Among the prime consequences of ECWS speed are extended periods of high storm surge, mainly due to slow-moving storms. The sustained high tidal levels often lead to substantial damage caused by coastal flooding, overwash, and beach erosion.
Journal of Applied Meteorology and Climatology, 2015
1 Contrail statistical prediction methods are often location specific. To take advantage of 2 the... more 1 Contrail statistical prediction methods are often location specific. To take advantage of 2 the fact that the upper-troposphere (UT) meteorological conditions favoring "clear-sky 3 outbreaks" of persisting contrails, or Contrail Favored Areas (CFAs), tend to be synoptic 4 in scale, we develop and test a visual UT map technique to hindcast CFAs for sub-regions 5 of the contiguous United States (CONUS) having high outbreak frequencies in mid-6 season months (January, April, July, October) of 2000-02. The method compares daily 7 maps with the composite fields for outbreak (CON) versus non-outbreak (NON) days, 8 and those assessments are evaluated using standard skill measures. Binary logistic 9 regression determines which UT variables are significant predictors, individually and in 10 combination. We test reproducibility of the outbreak hindcast results on the same sub-11 regions for corresponding months of 2008-09. The results confirm the importance of UT 12 relative humidity and vertical motion (omega) map patterns in regional clear-sky 13 outbreaks. Although the hindcast skill is modest, sensitivity tests suggest the method 14 would be substantially improved when a longer-term outbreaks climatology becomes 15 available (to increase sample sizes), and with explicit consideration of the synoptic types 16 on CON days. The latter is demonstrated specifically for the South CONUS in January, 17 where improving hindcast success should also consider the vertical wind shear in the UT, 18 given the importance of the subtropical jet stream in contrail outbreaks there. Further 19 development of the method to improve its skill ultimately should permit its use in 20 combination with existing objective (statistical, physical model) methods of contrail 21 prediction. 22
The cirrus-level 'condensation trails' (contrails) produced by jet aircraft often occur as sub-re... more The cirrus-level 'condensation trails' (contrails) produced by jet aircraft often occur as sub-regional-scale 'outbreaks' of multiple contrails, suggested as contributing to post ~1965 climate trends in parts of the US and Europe. Several previously-developed, satellite-image based contrail spatial inventories for the conterminous US (CONUS) revealed regional-scale differences in frequency. However, the use of such geographically-fixed regions was not ideal for climate studies. As a first step towards determining the potential climate impacts of contrail outbreaks for the CONUS, we develop maps of overlapping (in time, space) outbreak occurrences -'overlaps'by applying GIS to a recent period (2000)(2001)(2002) satellite-image derived inventory for mid-season months. The higher-frequency outbreak overlap regions undergo substantial between-season variations in magnitude and extent that reflect an association with upper-tropospheric temperature gradients and winds. Overlap maps generated for additional mid-season months in 2008-2009 indicate the inter-annual variability of the outbreak regionalization. To clarify the role of uppertroposphere synoptic meteorological conditions in contrail outbreak occurrence, we form composites -multi-case averages -for the sub-region of maximum overlap frequency in each midseason month. Regional and seasonal variations in the relative roles of 'thermo-dynamic' (here, temperature, humidity) and 'dynamic' (vertical motion of air, horizontal wind) controls in outbreaks are identified. Last, we demonstrate potential utility of the spatial overlap method by deriving fallseason surface station trends of sky cover variables for contrasting high versus low contrail and overlap frequency grid cells in the eastern CONUS. These suggest a contrail contribution to recent high-cloud increases, notably for the Midwest.
Social Media and micro-blogging is being used during crisis events to provide live up-to-date inf... more Social Media and micro-blogging is being used during crisis events to provide live up-to-date information as events evolve (before, during and after). Messages are posted by citizens or public officials. To understand the effectiveness of these messages, we examined the content of geo-located Twitter messages ("tweets") sent during the Moore, Oklahoma tornado of May 20 th , 2013 (+/-1day) to explore the spatial and temporal relationships of real-time reactions of the general public. We found a clear transition of topics during each stage of the tornado event. Twitter was useful for posting and retrieving updates, reconstructing the sequence of events as well as capturing people's reactions leading up to, during and after the tornado. A long-term goal for the research reported here is to provide insights to forecasters and emergency response personnel concerning the impact of warnings and other advisory messages.
Theoretical and Applied Climatology, 2018
The two main methods for determining the average daily near-surface air temperature, twice-daily ... more The two main methods for determining the average daily near-surface air temperature, twice-daily averaging (i.e., [Tmax+Tmin]/2) and hourly averaging (i.e., the average of 24 hourly temperature measurements), typically show differences associated with the asymmetry of the daily temperature curve. To quantify the relative influence of several land surface and atmosphere variables on the two temperature averaging methods, we correlate data for 215 weather stations across the Contiguous United States (CONUS) for the period 1981–2010 with the differences between the two temperature-averaging methods. The variables are land use-land cover (LULC) type, soil moisture, snow cover, cloud cover, atmospheric moisture (i.e., specific humidity, dew point temperature), and precipitation. Multiple linear regression models explain the spatial and monthly variations in the difference between the two temperature-averaging methods. We find statistically significant correlations between both the land surface and atmosphere variables studied with the difference between temperature-averaging methods, especially for the extreme (i.e., summer, winter) seasons (adjusted R2 > 0.50). Models considering stations with certain LULC types, particularly forest and developed land, have adjusted R2 values > 0.70, indicating that both surface and atmosphere variables control the daily temperature curve and its asymmetry. This study improves our understanding of the role of surface and near-surface conditions in modifying thermal climates of the CONUS for a wide range of environments, and their likely importance as anthropogenic forcings—notably LULC changes and greenhouse gas emissions—continues.
Journal of Climate, 2018
Traditionally, the daily average air temperature at a weather station is computed by taking the m... more Traditionally, the daily average air temperature at a weather station is computed by taking the mean of two values, the maximum temperature (Tmax) and the minimum temperature (Tmin) over a 24-hour period. These values form the basis for numerous studies of long-term climatologies (e.g., 30-year normals) and recent temperature trends and changes. However, many first-order weather stations-- such as those at airports-- also record hourly temperature data. Using an average of the 24 hourly temperature readings to compute daily average temperature has been shown to provide a more precise and representative estimate of a given day’s temperature. This study assesses the spatial variability of the differences in these two methods of daily temperature averaging (i.e., [Tmax + Tmin]/2, average of 24 hourly temperature values) for 215 first-order weather stations across the conterminous United States (CONUS) the 30-year period 1981-2010. A statistically significant difference is shown between the two methods, as well as consistent overestimation of temperature by the traditional method ([Tmax + Tmin]/2), particularly in southern and coastal portions of the CONUS. The explanation for the long-term difference between the two methods is the underlying assumption for the twice- daily method that the diurnal curve of temperature is symmetrical. Moreover, this paper demonstrates a spatially-coherent pattern in the difference compared to the most recent part of the temperature record (2001-2015). The spatial and temporal differences shown have implications for assessments of the physical factors influencing the diurnal temperature curve, as well as the exact magnitude of contemporary climate change.
Prior to the twentieth century, there was a dearth of official local weather and climate observat... more Prior to the twentieth century, there was a dearth of official local weather and climate observations for much of the United States outside of major cities. Useful information can be gleaned, however, from primary accounts, such as historical diaries kept by farmers and others whose interests were tied to the land. Herman Smith, a farmer in west-central New York State, kept a detailed record of daily life, including weather characteristics such as temperature, precipitation, and wind, for his farm near Covert. Two full years of his diary, 1884 and 1886, were recently published and selected for study. Although typically not numeric data, the lexicon used in the diary to describe relative heat and cold allow Smith's observations to be analyzed semiquantitatively in order to determine the weather experienced that year including factors affecting the growing season, as well as significant weather and climatic events. The analysis demonstrates that for Covert-located in an area of topographic variability and proximal to the Finger Lakes-microclimatic effects occasionally dominated over the synoptic circulation. This finding was further reinforced by comparison of Smith's 1886 records with those of a nearby farmer. Meanwhile, Smith's accounts also establish an inextricable link between his agricultural practices and the weather and climate patterns he observed. These findings underscore the value of acquiring climatic data from nonconventional sources for places and times when reliable data may be nonexistent in order to better understand how climate, and its impacts on the environment, have varied over time, across multiple scales.
Multiple persistent jet aviation contrails -contrail 'outbreaks' -occur frequently over certain p... more Multiple persistent jet aviation contrails -contrail 'outbreaks' -occur frequently over certain portions of the Continental United States (CONUS). The artificial cloudiness generated by contrail outbreaks alters the atmospheric radiation budget, potentially impacting the surface air temperature, particularly the diurnal temperature range (DTR), or difference between daytime maximum and nighttime minimum temperatures. This study evaluates the hypothesis that contrail outbreaks reduce the DTR relative to clear-sky conditions. We utilize a database of longer-lived (>4 h duration) jet contrail outbreaks for the CONUS previously determined from interpretation of high-resolution satellite imagery, for the January and April months of 2008 and 2009. The outbreak impact on DTR was determined by comparing maximum and minimum temperatures at pairs of surface weather stations (one outbreak and one non-outbreak) across two regions of climatologically high outbreak frequency; the South in January, and Midwest in April. We ensured that each station pair selected had broadly similar land use-land cover, soil moisture, and synoptic air mass conditions. For outbreaks in the South (January), there was a statistically significant reduction of DTR at the outbreak versus non-outbreak stations. This result was similar to that obtained for a smaller subset of outbreaks for which lower-level clouds could be confirmed as being absent (from North American Regional Reanalysis (NARR) output). For the Midwest (April), the results are mixed; statistically different for satellite-retrieved outbreaks, but not significantly different for the NARR-validated dataset. These results suggest that persistent jet contrails should be considered in short-term weather forecasting, and for their potential influence on the climatology of more frequently impacted areas.
Natural Hazards, 2012
The speeds of historical cool-season extratropical cyclones along the U.S. east coast, hereafter ... more The speeds of historical cool-season extratropical cyclones along the U.S. east coast, hereafter East Coast Winter Storms (ECWS), occurring during the period from 1951 to 2006 were computed. Average storm speed was 13.8 ms -1 with stronger storms generally moving faster than weaker storms and faster storms forming during the midwinter months (December-March). There was no clear trend in ECWS speed during the time period, although considerable season-to-season variability was present. The monthly and seasonal variations in storm speed could not be attributed to the El Niño-Southern Oscillation or North Atlantic Oscillation (NAO) alone. However, the speed of ECWS was considerably slower when both El Niño and the negative phase of NAO occurred simultaneously. Characteristic patterns in the upper levels of the atmosphere, specifically 300 hPa zonal winds and 500 hPa geopotential heights, were present during periods when ECWS speeds were among the slowest (and separately fastest). For slow storm speed, these patterns also prevailed during months in which El Niño and negative NAO phase occurred. These patterns were also present during months with extended runs of high oceanic storm surge. This provides a qualitative link between the atmospheric conditions associated with slow storms and potentially high coastal storm surge impacts. Among the prime consequences of ECWS speed are extended periods of high storm surge, mainly due to slow-moving storms. The sustained high tidal levels often lead to substantial damage caused by coastal flooding, overwash, and beach erosion.
Journal of Applied Meteorology and Climatology, 2015
1 Contrail statistical prediction methods are often location specific. To take advantage of 2 the... more 1 Contrail statistical prediction methods are often location specific. To take advantage of 2 the fact that the upper-troposphere (UT) meteorological conditions favoring "clear-sky 3 outbreaks" of persisting contrails, or Contrail Favored Areas (CFAs), tend to be synoptic 4 in scale, we develop and test a visual UT map technique to hindcast CFAs for sub-regions 5 of the contiguous United States (CONUS) having high outbreak frequencies in mid-6 season months (January, April, July, October) of 2000-02. The method compares daily 7 maps with the composite fields for outbreak (CON) versus non-outbreak (NON) days, 8 and those assessments are evaluated using standard skill measures. Binary logistic 9 regression determines which UT variables are significant predictors, individually and in 10 combination. We test reproducibility of the outbreak hindcast results on the same sub-11 regions for corresponding months of 2008-09. The results confirm the importance of UT 12 relative humidity and vertical motion (omega) map patterns in regional clear-sky 13 outbreaks. Although the hindcast skill is modest, sensitivity tests suggest the method 14 would be substantially improved when a longer-term outbreaks climatology becomes 15 available (to increase sample sizes), and with explicit consideration of the synoptic types 16 on CON days. The latter is demonstrated specifically for the South CONUS in January, 17 where improving hindcast success should also consider the vertical wind shear in the UT, 18 given the importance of the subtropical jet stream in contrail outbreaks there. Further 19 development of the method to improve its skill ultimately should permit its use in 20 combination with existing objective (statistical, physical model) methods of contrail 21 prediction. 22
The cirrus-level 'condensation trails' (contrails) produced by jet aircraft often occur as sub-re... more The cirrus-level 'condensation trails' (contrails) produced by jet aircraft often occur as sub-regional-scale 'outbreaks' of multiple contrails, suggested as contributing to post ~1965 climate trends in parts of the US and Europe. Several previously-developed, satellite-image based contrail spatial inventories for the conterminous US (CONUS) revealed regional-scale differences in frequency. However, the use of such geographically-fixed regions was not ideal for climate studies. As a first step towards determining the potential climate impacts of contrail outbreaks for the CONUS, we develop maps of overlapping (in time, space) outbreak occurrences -'overlaps'by applying GIS to a recent period (2000)(2001)(2002) satellite-image derived inventory for mid-season months. The higher-frequency outbreak overlap regions undergo substantial between-season variations in magnitude and extent that reflect an association with upper-tropospheric temperature gradients and winds. Overlap maps generated for additional mid-season months in 2008-2009 indicate the inter-annual variability of the outbreak regionalization. To clarify the role of uppertroposphere synoptic meteorological conditions in contrail outbreak occurrence, we form composites -multi-case averages -for the sub-region of maximum overlap frequency in each midseason month. Regional and seasonal variations in the relative roles of 'thermo-dynamic' (here, temperature, humidity) and 'dynamic' (vertical motion of air, horizontal wind) controls in outbreaks are identified. Last, we demonstrate potential utility of the spatial overlap method by deriving fallseason surface station trends of sky cover variables for contrasting high versus low contrail and overlap frequency grid cells in the eastern CONUS. These suggest a contrail contribution to recent high-cloud increases, notably for the Midwest.
Social Media and micro-blogging is being used during crisis events to provide live up-to-date inf... more Social Media and micro-blogging is being used during crisis events to provide live up-to-date information as events evolve (before, during and after). Messages are posted by citizens or public officials. To understand the effectiveness of these messages, we examined the content of geo-located Twitter messages ("tweets") sent during the Moore, Oklahoma tornado of May 20 th , 2013 (+/-1day) to explore the spatial and temporal relationships of real-time reactions of the general public. We found a clear transition of topics during each stage of the tornado event. Twitter was useful for posting and retrieving updates, reconstructing the sequence of events as well as capturing people's reactions leading up to, during and after the tornado. A long-term goal for the research reported here is to provide insights to forecasters and emergency response personnel concerning the impact of warnings and other advisory messages.