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- Author or Editor: Mohan K. Ramamurthy x
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Abstract
On 14–15 February 1992 a long-lived mesoscale gravity wave was observed over the Storm-scale Operational and Research Meteorology-Fronts Experiment Systems Test observational network. A precipitation band formed and intensified as the wave moved across Kansas and Missouri. The disturbance was tracked for 14 h. Surface, dual-Doppler radar, and wind profiler analyses, as well as isentropic analyses derived from a special rawinsonde network, are used to examine the origin and evolution of this wave.
The wave originated at the leading edge of a dry air mass associated with downslope flow in the lee of the Rockies. The earliest surface pressure signatures of wave motion began as a dry air mass, associated with the downslope flow, ascended a warm front east of a lee cyclone. A weak rainband developed simultaneously with the wave at the leading edge of the dry air mass. The mesoscale gravity wave, and convection, remained tied to the leading edge of the advancing dry air mass during the first 8–10 h of evolution, suggesting that both convection and dynamical processes near the leading edge of the dry air mass were instrumental in maintaining the wave during this phase of its evolution. These processes are investigated in Part II. The orientation of the wave front, determined from isochrone analyses of minimum pressure occurrence, and the orientation of the rainband, determined from radar analyses, corresponded closely to the leading edge of the advancing dry air mass for the first 8–10 h. In central Missouri, the convection and wave decoupled from the leading edge of the dry air mass. Barograms downstream in eastern Missouri and Illinois suggest a change in wave structure to a wave of elevation occurred during this latter stage of evolution.
Abstract
On 14–15 February 1992 a long-lived mesoscale gravity wave was observed over the Storm-scale Operational and Research Meteorology-Fronts Experiment Systems Test observational network. A precipitation band formed and intensified as the wave moved across Kansas and Missouri. The disturbance was tracked for 14 h. Surface, dual-Doppler radar, and wind profiler analyses, as well as isentropic analyses derived from a special rawinsonde network, are used to examine the origin and evolution of this wave.
The wave originated at the leading edge of a dry air mass associated with downslope flow in the lee of the Rockies. The earliest surface pressure signatures of wave motion began as a dry air mass, associated with the downslope flow, ascended a warm front east of a lee cyclone. A weak rainband developed simultaneously with the wave at the leading edge of the dry air mass. The mesoscale gravity wave, and convection, remained tied to the leading edge of the advancing dry air mass during the first 8–10 h of evolution, suggesting that both convection and dynamical processes near the leading edge of the dry air mass were instrumental in maintaining the wave during this phase of its evolution. These processes are investigated in Part II. The orientation of the wave front, determined from isochrone analyses of minimum pressure occurrence, and the orientation of the rainband, determined from radar analyses, corresponded closely to the leading edge of the advancing dry air mass for the first 8–10 h. In central Missouri, the convection and wave decoupled from the leading edge of the dry air mass. Barograms downstream in eastern Missouri and Illinois suggest a change in wave structure to a wave of elevation occurred during this latter stage of evolution.
Abstract
The importance of warm rain and melting processes in freezing precipitation events is investigated by analyzing 972 rawinsonde soundings taken during freezing precipitation. The soundings cover regions of the United States east of the Rocky Mountain states for the period 1970–94. The warm rain process was found to be unambiguously responsible for freezing precipitation in 47% of the soundings. In these soundings, the clouds had temperatures entirely below freezing, or had top temperatures that were above freezing. Another 28% of the soundings had cloud top temperatures between 0° and −10°C. Clouds with top temperatures >−10°C also can support an active warm rain process. Considered together, the warm rain process was potentially important in about 75% of the freezing precipitation soundings. This estimate is significantly higher than the estimate of 30% in a previous study by Huffman and Norman. The temperature, moisture, and wind profiles of the soundings, their geographic distribution, and the common occurrence of freezing drizzle at the sounding sites suggest that most of these events were associated with shallow cloud decks forming over arctic cold air masses.
The “classic” freezing rain sounding, with a deep moist layer and a midlevel warm (>0°C) layer, was observed in only 25% of the sample. In these soundings, the depth of the cloud layer implies that melting processes were important to precipitation production. From the geographic distribution, the common occurrence of freezing rain, and the sounding profile, these cases appear to be associated primarily with cold air damming and overrunning along the U.S. East Coast, and with warm-frontal overrunning in the midwestern United States.
Abstract
The importance of warm rain and melting processes in freezing precipitation events is investigated by analyzing 972 rawinsonde soundings taken during freezing precipitation. The soundings cover regions of the United States east of the Rocky Mountain states for the period 1970–94. The warm rain process was found to be unambiguously responsible for freezing precipitation in 47% of the soundings. In these soundings, the clouds had temperatures entirely below freezing, or had top temperatures that were above freezing. Another 28% of the soundings had cloud top temperatures between 0° and −10°C. Clouds with top temperatures >−10°C also can support an active warm rain process. Considered together, the warm rain process was potentially important in about 75% of the freezing precipitation soundings. This estimate is significantly higher than the estimate of 30% in a previous study by Huffman and Norman. The temperature, moisture, and wind profiles of the soundings, their geographic distribution, and the common occurrence of freezing drizzle at the sounding sites suggest that most of these events were associated with shallow cloud decks forming over arctic cold air masses.
The “classic” freezing rain sounding, with a deep moist layer and a midlevel warm (>0°C) layer, was observed in only 25% of the sample. In these soundings, the depth of the cloud layer implies that melting processes were important to precipitation production. From the geographic distribution, the common occurrence of freezing rain, and the sounding profile, these cases appear to be associated primarily with cold air damming and overrunning along the U.S. East Coast, and with warm-frontal overrunning in the midwestern United States.
Abstract
An analysis of 411 winter storms that produced freezing precipitation events in the United States east of the Rocky Mountains over the 25-yr period of 1970–94 is presented to identify specific weather patterns associated with freezing precipitation and to determine their frequency of occurrence. Seven archetypical weather patterns are identified associated with freezing precipitation. Four patterns (arctic fronts, the warm front–occlusion sector of cyclones, cyclone–anticyclone couplets, and the west quadrant of anticyclones) are not associated with specific topographic features. Three patterns (East Coast cold-air damming with an anticyclone, cold-air damming with a coastal cyclone, and cold-air trapping during approaching continental cyclones) are associated with freezing precipitation in and along the Appalachian Mountains. The frequency of occurrence and duration of each of these patterns are presented, and variability within patterns is discussed. In the second part of the paper, the vertical structure of the atmosphere during freezing precipitation events is investigated by analyzing 972 rawinsonde soundings taken during freezing precipitation. The soundings cover regions of the United States east of the Rocky Mountain states for the period of 1970–94. Statistical summaries of soundings from each archetypical weather pattern and from the entire dataset are presented for 1) the depth and minimum temperature of the cold surface layer, 2) the depth and maximum temperature of the warm layer aloft, 3) stability characteristics of air above the inversion, 4) layer thickness for the 1000–500-mb and 1000–850-mb layers, and 5) wind speed and direction at the surface, the 850-mb level, and the 700-mb level.
Abstract
An analysis of 411 winter storms that produced freezing precipitation events in the United States east of the Rocky Mountains over the 25-yr period of 1970–94 is presented to identify specific weather patterns associated with freezing precipitation and to determine their frequency of occurrence. Seven archetypical weather patterns are identified associated with freezing precipitation. Four patterns (arctic fronts, the warm front–occlusion sector of cyclones, cyclone–anticyclone couplets, and the west quadrant of anticyclones) are not associated with specific topographic features. Three patterns (East Coast cold-air damming with an anticyclone, cold-air damming with a coastal cyclone, and cold-air trapping during approaching continental cyclones) are associated with freezing precipitation in and along the Appalachian Mountains. The frequency of occurrence and duration of each of these patterns are presented, and variability within patterns is discussed. In the second part of the paper, the vertical structure of the atmosphere during freezing precipitation events is investigated by analyzing 972 rawinsonde soundings taken during freezing precipitation. The soundings cover regions of the United States east of the Rocky Mountain states for the period of 1970–94. Statistical summaries of soundings from each archetypical weather pattern and from the entire dataset are presented for 1) the depth and minimum temperature of the cold surface layer, 2) the depth and maximum temperature of the warm layer aloft, 3) stability characteristics of air above the inversion, 4) layer thickness for the 1000–500-mb and 1000–850-mb layers, and 5) wind speed and direction at the surface, the 850-mb level, and the 700-mb level.
A winter storm that crossed the continental United States in mid-February 1990 produced hazardous weather across a vast area of the nation. A wide range of severe weather was reported, including heavy snowfall; freezing rain and drizzle; thunderstorms with destructive winds, lightning, large hail, and tornadoes; prolonged heavy rain with subsequent flooding; frost damage to citrus orchards; and sustained destructive winds not associated with thunderstorms. Low-end preliminary estimates of impacts included 9 deaths, 27 injuries, and $120 million of property damage. At least 35 states and southeastern Canada were adversely affected. The storm occurred during the field operations of four independent atmospheric research projects that obtained special, detailed observations of it from the Rocky Mountains to the eastern Great Lakes.
A winter storm that crossed the continental United States in mid-February 1990 produced hazardous weather across a vast area of the nation. A wide range of severe weather was reported, including heavy snowfall; freezing rain and drizzle; thunderstorms with destructive winds, lightning, large hail, and tornadoes; prolonged heavy rain with subsequent flooding; frost damage to citrus orchards; and sustained destructive winds not associated with thunderstorms. Low-end preliminary estimates of impacts included 9 deaths, 27 injuries, and $120 million of property damage. At least 35 states and southeastern Canada were adversely affected. The storm occurred during the field operations of four independent atmospheric research projects that obtained special, detailed observations of it from the Rocky Mountains to the eastern Great Lakes.
Over the past several years, the Department of Atmospheric Sciences at the University of Illinois has developed a computerized weather laboratory that permits interactive access to real-time data from observing sites around the United States and to output from numerical weather prediction models at the operational centers. Such a setup, with timely access to observations and numerical model forecasts from any networked terminal, personal computer, or workstation, is a valuable tool for education and research in meteorology. The University of Illinois system acts as a real-time, on-line, in-class instructional meteorology laboratory for students. The data-display software is based on the X-windows protocol, which is network transparent and system independent.
In addition to software packages distributed by the University Data Project (UNIDATA), software tools developed by the National Center for Supercomputing Applications and the University of Illinois are used to display, animate, and manipulate conventional maps, satellite images, and radar summaries. The underlying idea is to bring every product from a traditional synoptic laboratory to any desktop computer residing on the network.
An overview of the University of Illinois prototype for a paperless, desktop synoptic lab, together with the details of the hardware and software involved, is presented here, along with some examples of its use in teaching and research.
Over the past several years, the Department of Atmospheric Sciences at the University of Illinois has developed a computerized weather laboratory that permits interactive access to real-time data from observing sites around the United States and to output from numerical weather prediction models at the operational centers. Such a setup, with timely access to observations and numerical model forecasts from any networked terminal, personal computer, or workstation, is a valuable tool for education and research in meteorology. The University of Illinois system acts as a real-time, on-line, in-class instructional meteorology laboratory for students. The data-display software is based on the X-windows protocol, which is network transparent and system independent.
In addition to software packages distributed by the University Data Project (UNIDATA), software tools developed by the National Center for Supercomputing Applications and the University of Illinois are used to display, animate, and manipulate conventional maps, satellite images, and radar summaries. The underlying idea is to bring every product from a traditional synoptic laboratory to any desktop computer residing on the network.
An overview of the University of Illinois prototype for a paperless, desktop synoptic lab, together with the details of the hardware and software involved, is presented here, along with some examples of its use in teaching and research.
No Abstract available.
No Abstract available.
Abstract
The general applicability of an isonomogram developed by Czys and coauthors to diagnose the position of the geographic boundary between freezing precipitation (freezing rain or freezing drizzle) and ice pellets (sleet or snow grains) was tested using a 25-yr sounding database consisting of 1051 soundings, 581 where stations were reporting freezing drizzle, 391 reporting freezing rain, and 79 reporting ice pellets. Of the 1051 soundings, only 306 clearly had an environmental temperature and moisture profile corresponding to that assumed for the isonomogram. This profile consisted of a three-layer atmosphere with 1) a cold cloud layer aloft that is a source of ice particles, 2) a midlevel layer where the temperature exceeds 0°C and ice particles melt, and 3) a surface layer where T < 0°C. The remaining soundings did not conform to the profile either because 1) the freezing precipitation was associated with the warm rain process or 2) the ice pellets formed due to riming rather than melting and refreezing. For soundings conforming to the profile, the isonomogram showed little diagnostic skill. Freezing rain or freezing drizzle occurred about 50% of the time that ice pellets were expected. Ice pellets occurred in nearly a third of the cases where freezing precipitation was diagnosed. Possible reasons for the poor diagnostic skill of the method are suggested.
Abstract
The general applicability of an isonomogram developed by Czys and coauthors to diagnose the position of the geographic boundary between freezing precipitation (freezing rain or freezing drizzle) and ice pellets (sleet or snow grains) was tested using a 25-yr sounding database consisting of 1051 soundings, 581 where stations were reporting freezing drizzle, 391 reporting freezing rain, and 79 reporting ice pellets. Of the 1051 soundings, only 306 clearly had an environmental temperature and moisture profile corresponding to that assumed for the isonomogram. This profile consisted of a three-layer atmosphere with 1) a cold cloud layer aloft that is a source of ice particles, 2) a midlevel layer where the temperature exceeds 0°C and ice particles melt, and 3) a surface layer where T < 0°C. The remaining soundings did not conform to the profile either because 1) the freezing precipitation was associated with the warm rain process or 2) the ice pellets formed due to riming rather than melting and refreezing. For soundings conforming to the profile, the isonomogram showed little diagnostic skill. Freezing rain or freezing drizzle occurred about 50% of the time that ice pellets were expected. Ice pellets occurred in nearly a third of the cases where freezing precipitation was diagnosed. Possible reasons for the poor diagnostic skill of the method are suggested.
To obtain a better understanding of the role of synoptic-scale disturbances in organizing mesoscale precipitation in the midwestern United States during the winter season, and to address scientific issues regarding mesoscale dynamics of winter storms, the University of Illinois Winter Precipitation Program was conducted over a period of three winters between 1988 and 1990. The observing systems included a 10-cm wavelength meteorological Doppler radar operated by the Illinois State Water Survey, the Flatland 6-m wind profiler operated by the NOAA Aeronomy Laboratory, and an NCAR Cross-chain Loran Atmospheric Sounding System. In all, 26 storms were observed during the 3-year period. The associated precipitation ranged from highly convective storms in the warm sector to stratified clouds containing organized banded structure within the occlusion. The principle dynamical mechanisms at work often varied widely from one storm to another and sometimes within a storm. This article describes the goals and objectives of the project, as well as a few selected observations and some preliminary findings from the data gathered.
To obtain a better understanding of the role of synoptic-scale disturbances in organizing mesoscale precipitation in the midwestern United States during the winter season, and to address scientific issues regarding mesoscale dynamics of winter storms, the University of Illinois Winter Precipitation Program was conducted over a period of three winters between 1988 and 1990. The observing systems included a 10-cm wavelength meteorological Doppler radar operated by the Illinois State Water Survey, the Flatland 6-m wind profiler operated by the NOAA Aeronomy Laboratory, and an NCAR Cross-chain Loran Atmospheric Sounding System. In all, 26 storms were observed during the 3-year period. The associated precipitation ranged from highly convective storms in the warm sector to stratified clouds containing organized banded structure within the occlusion. The principle dynamical mechanisms at work often varied widely from one storm to another and sometimes within a storm. This article describes the goals and objectives of the project, as well as a few selected observations and some preliminary findings from the data gathered.
Abstract
No Abstract available.
Abstract
No Abstract available.
The American Meteorological Society (AMS) held its Fifth Symposium on Education in conjunction with the 76th Annual Meeting in Atlanta, Georgia. The theme of this year's symposium was “Atmospheric and Oceanic Sciences: Building the Future on a Solid Foundation.” Thirty-four oral presentations and 41 poster presentations summarized a variety of educational programs or examined issues of importance for both the precollege and university levels. There was also a joint session with the 12th International Conference on Interactive Information and Processing Systems for Meteorology, Oceanography, and Hydrography on new technologies for the classroom. Over 200 people representing a wide spectrum of the Society attended one or more of the sessions in this two-day conference, where they increased their awareness of educational initiatives of members and institutions associated with AMS.
The American Meteorological Society (AMS) held its Fifth Symposium on Education in conjunction with the 76th Annual Meeting in Atlanta, Georgia. The theme of this year's symposium was “Atmospheric and Oceanic Sciences: Building the Future on a Solid Foundation.” Thirty-four oral presentations and 41 poster presentations summarized a variety of educational programs or examined issues of importance for both the precollege and university levels. There was also a joint session with the 12th International Conference on Interactive Information and Processing Systems for Meteorology, Oceanography, and Hydrography on new technologies for the classroom. Over 200 people representing a wide spectrum of the Society attended one or more of the sessions in this two-day conference, where they increased their awareness of educational initiatives of members and institutions associated with AMS.
