The windstorm that evolved along and ahead of a cold front over the Mid-Atlantic region during the morning of 1 December 2010 culminated in the development of a squall line that produced downburst winds over the Tidal Potomac region. Between 1400 and 1430 UTC, a line of convective storms developed along the cold front over northern Virginia and then tracked east and northeastward into central and southern Maryland between 1430 and 1500 UTC. Between 1430 and 1445 UTC, several strong downbursts occurred over the Tidal Potomac River and were recorded by WeatherFlow observing stations. Similar to the 17 November downburst event, the channeling of mid-tropospheric dry air into the core of convective storms provided significant downdraft energy that resulted in high winds.
Figure 1. GOES-13 VW-IR brightness temperature difference (BTD) image at 1445 UTC 1 December 2010.
Figure 1 shows a GOES-13 BTD microburst risk image over Maryland and Virginia near the time of downburst occurrence over the Tidal Potomac River. At 1430 UTC, a wind gust of 41 knots was recorded by Potomac Light 33 WeatherFlow station associated with a squall line downburst. Five minutes later, at 1435 UTC, a stronger downburst wind gust of 43 knots was recorded by Cobb Point station. Note that the dry-air notch identified in Figure 1 on the southwestern flank of the squall line was pointing toward Cobb Point, indicating that the injection of dry air into the convective storm was resulting in evaporational cooling and the generation of negative buoyancy that would result in the downburst winds over the Tidal Potomac.
Figure 2. Radiosonde observation (RAOB) from Dulles Airport, VA at 1200 UTC 1 December 2010.
Figure 2 displays favorable conditions for strong convective winds over the Tidal Potomac and Chesapeake Bay region. The sounding profile identifies a dry-air layer near the 800-mb level with winds near 45 knots, very close to the wind gust speed observed at Cobb Point. This provides evidence that in addition to precipitation loading and evaporational cooling, the downward transfer of horizontal momentum from the dry-air layer to the surface by heavy rainfall was also an important forcing factor in the strong downburst winds. As the squall line continued to track eastward, strong downburst winds were observed over the Chesapeake Bay between 1500 and 1530 UTC. At 1517 UTC, Thomas Point Lighthouse Coastal-Marine Automated Network (C-MAN) station, displayed in Figure 3, recorded a wind gust of 50 knots with the passage of the squall line.
Figure 3. Thomas Point Lighthouse, Maryland.
By this time, the dry-air notch on the southwestern flank of the line had become more pronounced in both satellite and radar imagery with a southwest to northeast orientation toward Thomas Point Light (white cross in Figure 4). The peak wind gust recorded at the lighthouse was from a southwesterly direction with a local maximum in radar reflectivity (> 40 dBZ) overhead. Nearby WeatherFlow observing stations, Tolly Point and Greenbury Point, recorded wind gusts of 44 knots and 46 knots, respectively, at 1520 UTC. The concurrent peak wind gust and passage of the heaviest rainfall core over the observing stations suggest that higher momentum from the mid-tropospheric dry air layer was being transported to the surface by heavy precipitation within the squall line. The physical process in downburst generation described with this event exemplifies the highly dynamic environment that is typical for cold-season severe convective storms.
Figure 4. GOES-13 WV-IR BTD image at 1515 UTC 1 December 2010 with overlying radar reflectivity from Dover, DE NEXRAD.
During the late evening of 16 November and early morning of 17 November 2010, lines of strong convective storms developed and tracked eastward over Pennsylvania, Maryland and Virginia. The line of convective storms that developed along a cold front boundary produced strong downburst winds over north-central Maryland, especially eastern Frederick County, as well as over the Tidal Potomac region. Several downbursts occurred over eastern Frederick County between 0545 and 0625 UTC. Sterling, Virginia NEXRAD indicated winds between 35 and 40 knots associated with the downbursts.
Figure 1. GOES-13 WV-IR BTD image at 0615 UTC with overlying radar reflectivity from Sterling, VA NEXRAD.
Figure 2. GOES-13 WV-IR BTD image at 0615 UTC with overlying radar velocity from Sterling, Virginia NEXRAD.
Figures 1 and 2 show a downburst in progress over eastern Frederick County near the location of the white cross. Strong winds near 39 knots were measured by Doppler radar at the time of downburst occurrence between 0605 and 0610 UTC. The strong downburst occurred immediately downstream of a mid-tropospheric dry-air channel, marked by the white line pointing toward eastern Frederick County. The dry-air channel, pointing into the rear flank of the convective storm line, fostered downburst generation by injecting drier (unsaturated) air into the precipitation core of the convective storm. The resulting evaporational cooling and generation of negative buoyancy accelerated the convective storm downdraft toward the surface, producing strong winds on impact. The Tidal Potomac Region was also effected by the squall line between 0545 and 0630 UTC, as evidenced by high winds recorded by NOAA data buoys and WeatherFlow observing stations. During this time, several dry-air channels became apparent in BTD imagery on the western flank of the convective line. Figures 3 and 4 show that three channels over western Virginia were coming in line with rear-inflow notches as identified in radar imagery.
Figure 3. GOES-13 WV-IR BTD image at 0532 UTC with overlying radar reflectivity from Sterling, VA NEXRAD.
Figure 4. GOES-13 WV-IR BTD image at 0632 UTC with overlying radar reflectivity from Sterling, VA NEXRAD.
Figure 3 shows the squall line moving into the Washington, DC metropolitan area with the apex of a bow echo moving into southern Maryland. A severe wind gust of 50 knots was recorded by the Upper Potomac River buoy (white cross in Figure 3) between 0540 and 0550 UTC. Note that a dry-air notch, as apparent in the GOES BTD image, was apparent on the western flank of the squall line pointing east-southeastward toward the Upper Potomac buoy. At this time, the dry-air notch was in phase with rear-inflow notch (RIN) as evident in NEXRAD imagery. By 0602 UTC, as shown in Figure 4, two dry-air notches were pointing toward the location of downburst occurrence at Potomac Light 33 station where a wind gust of 43 knots was recorded. About 20 minutes later, near 0625 UTC, a stronger downburst wind gust of 45 knots was recorded by Cobb Point station. At this time, as displayed in Figure 5, a dry-air notch had come in phase with a bow echo moving over the lower Potomac River. It is evident that the injection of mid-tropospheric dry air into the heavy precipitation core within the convective storm line was providing a large amount of downdraft energy to realize strong downburst winds at the surface.
Figure 5. GOES-13 WV-IR BTD image at 0632 UTC with overlying radar reflectivity from Sterling, VA NEXRAD.
Near this time, as a bow echo was tracking over the Chesapeake Bay, a wind gust of 46 knots was recorded at Thomas Point Lighthouse Coastal-Marine Automated Network (C-MAN) station. The downburst wind gust was associated with the passage of the apex of the bow echo that tracked directly over Thomas Point Light. Figure 6 shows the passage of the bow echo over Thomas Point Light.
Figure 6. GOES-13 WV-IR BTD image at 0632 UTC with overlying radar reflectivity from Dover, DE NEXRAD.
The GOES BTD image in Figure 6 shows that a dry-channel (white line) was pointing toward Thomas Point Lighthouse (white cross) in phase with a (RIN) as apparent in radar imagery. Again, the channeling of dry air into the rear flank of the bow echo was a major forcing factor for downburst winds that were observed at the C-MAN station. It is noteworthy that one of the strongest wind gusts of the event occurred as the dry-air notch, RIN, and bow echo apex were in phase as the convective storm line passed over the Chesapeake Bay.
A cross-comparison of the Geostationary Operational Environmental Satellite (GOES) Microburst Windspeed Potential Index (MWPI) to the Haines Index that characterizes the potential impact of dry, unstable air on wildfire behavior and growth is currently in progress. During the afternoon of 23 June 2010, strong convective storms developed ahead of an upper-level disturbance over New Mexico and then tracked eastward into the western Texas Panhandle region. Convective storms produced numerous downbursts over western Texas during the evening hours of 23 June. Both the 2300 UTC GOES Microburst Windspeed Potential Index (MWPI) and Rapid Update Cycle (RUC) model-derived Haines Index products effectively indicated the potential for strong downbursts about three to four hours prior to each event. Large convective available potential energy (CAPE) and a steep temperature lapse rate, especially below the 700-mb level, were forcing factors for strong convective downdrafts.
Figure 1. Geostationary Operational Environmental Satellite (GOES) Microburst Windspeed Potential Index (MWPI) product image at 2300 UTC 23 June 2010.
Figure 2. Haines Index product based on the Rapid Update Cycle (RUC) model analysis valid at 2300 UTC with overlying radar reflectivity at 0224 UTC 24 June (bottom). White crosses mark the location of Friona (FAS) and Dimmitt (DMS) West Texas Mesonet stations.
Figure 3. Geostationary Operational Environmental Satellite (GOES) sounding profile from Eunice, Texas (near Dimmitt) at 2300 UTC 23 June 2010.
Figure 1 shows that the 2300 UTC Geostationary Operational Environmental Satellite (GOES) Microburst Windspeed Potential Index (MWPI) product image indicated a local maximum in values downstream of developing convective storms over New Mexico that would eventually track into western Texas after 0000 UTC 24 June. The corresponding GOES sounding profile in Figure 3 at Eunice, Texas displays a favorable classic “inverted V” profile that prevailed over western Texas near the maximum in MWPI values. A deep dry adiabatic lapse rate (DALR) layer below the 700-mb level and a dry subcloud layer fostered intense downdraft development due to evaporational cooling and the resulting downburst winds. The Rapid Update Cycle (RUC) model-derived Haines Index product in Figure 2 was generated and visualized by Man computer Interactive Data Access System (McIDAS)-V. A vertical temperature difference (between 850 and 700-mb levels) greater than 15°C (dark orange shading) and a dewpoint depression greater than 25°C yielded a Haines Index value of six (6) in the vicinity of Dimmitt and Friona, Texas, the highest for the index. Consequently, this index echoed favorable conditions for strong downbursts as presented in the MWPI image. Downburst wind gusts of 40 knots and 54 knots were recorded at Dimmitt and Friona West Texas Mesonet station, respectively, between 0200 and 0300 UTC 24 June. This case shows tremendous potential for the Haines Index, originally conceived as a wildfire threat product, in downburst potential assessment.
During the afternoon of 5 August 2010, strong convective storms developed over western Maryland and Virginia ahead of a cold front and produced a severe downburst that was observed at Washington, DC National Airport. The 1900 UTC Geostationary Operational Environmental Satellite (GOES) Microburst Windspeed Potential Index (MWPI) product on 5 August effectively indicated the potential for strong downbursts over the greater Washington, DC metropolitan area during the late afternoon.The MWPI product indicated wind gust potential of 35 to 49 knots where wind gusts of 42 to 51 knots were recorded in the Washington, DC area between 1940 and 2000 UTC.The pre-convective environment over the Washington area was characterized by an "inverted-V" profile with strong lower atmospheric instability that resulted from intense solar heating during the afternoon hours. Large convective available potential energy (CAPE) and a steep temperature lapse rate, especially below the 850-mb level, were forcing factors for strong convective downdrafts.
It has been found recently that the Haines Index (Haines 1988) can provide beneficial information for assessing downburst potential. The 5 August downburst event very effectively highlighted the application of the Haines Index especially when compared to corresponding MWPI product imagery and RUC sounding profiles. The Haines Index characterizes the potential impact of dry, unstable air on wildfire behavior and growth (Haines 1988) and is driven by classic atmospheric stability indicators such as temperature lapse rate (DT) and near-surface dewpoint depression (DD). Similar to the MWPI, the Haines Index is composed of both a stability (A) and moisture (B) component. The A component represents the environmental lapse rate, while the B component is the dewpoint depression for a specific pressure level. For both components, the calculated temperature and dewpoint depression are categorized into three groups that are assigned an ordinal value of 1, 2, or 3, and then summed. The resulting index has a range from 2 (very low risk) to 6 (high risk). A Haines Index product that consists of composite of the vertical temperature difference and dewpoint depression has been implemented in Man computer Interactive Data Access System (McIDAS)-V.
Figure 1. RUC sounding at 1900 UTC 5 August 2010 (top) compared to Haines Index product based on the RUC one-hour forecast valid at 1900 UTC (bottom). White cross marks the location of Washington National Airport and the above sounding. Note that the severe downburst, indicated by the spearhead echo apparent in radar imagery, occurred in a region of maximum temperature lapse rate and dewpoint depression.
The Haines Index product in Figure 1 was generated and visualized by McIDAS-V. Figure 1 shows that the severe downburst, associated with a wind gust of 51 knots, recorded at Washington National Airport at 1952 UTC occurred in a region characterized by strong low-level instability with maxima in temperature lapse rate and dewpoint depression:
DT (950-850mb): 9C
DD (950mb): 13C
A value: 3
B value: 3
Haines Index: 6 (high)
The index value of six is the highest for the Haines Index. Consequently, this index value indicated that sub-cloud evaporational cooling in a highly unstable lower atmospheric layer fostered intense downdraft generation that resulted in the observed downburst winds. The high index value corresponded well with the unstable, "inverted-V" sounding profile displayed in Figure 1. This case shows tremendous potential for the Haines Index, originally conceived as a wildfire threat product, in downburst potential assessment.
References
Haines, D.A. 1988. A lower atmospheric severity index for wildland fire. National Weather Digest, 13, 23-27.
The Virtual Institute for Satellite Integration Training (VISIT) Lesson titled “Forecasting Convective Downburst Potential Using GOES Sounder Derived Products” has been revised by K. Pryor to include new case studies and new instructional material pertaining to the Graphyte Toolkit and the Geostationary Operational Environmental Satellite (GOES) channel 3 – channel 4 brightness temperature difference (BTD) product.The objective of the lesson is to provide better understanding of techniques for predicting the risk of convective downbursts utilizing Geostationary Operational Environmental Satellite (GOES) sounder derived data.The guide for the lesson as well as the revised version of the lesson are available on the VISIT web site.
Figure 1. Page 29 from VISIT lesson “Forecasting Convective Downburst Potential Using GOES Sounder Derived Products” displaying the new Geostationary Operational Environmental Satellite (GOES) imager channel 3 – channel 4 brightness temperature difference (BTD) microburst product.
Figure 1 shows new instructional material from the VISIT lesson “Forecasting Convective Downburst Potential Using GOES Sounder Derived Products”. This page compares a Geostationary Operational Environmental Satellite (GOES) imager channel 3 – channel 4 brightness temperature difference (BTD) product to a pre-convective GOES sounding profile during the afternoon of 5 August 2010. Note that the dry-air notch in the product image on the left is pointing southeastward toward a convective storm moving through the Washington, DC area. The presence of mid-tropospheric dry air was established by the GOES sounding profile over Washington, DC about two hours prior to strong downburst occurrence. The channeling of dry air into the rear of the convective storm most likely played a significant role in generating downdraft energy. Downburst wind gusts of 42 knots were recorded by the Washington Tide Station and the Upper Potomac River (white cross) buoy during the afternoon of 5 August 2010.
The 2010 assessment of the GOES microburst products has been featured in a research paper titled “Recent developments in microburst nowcasting using GOES” published in the electronic journal “ArXiv.org” and the preprints of the 17th Conference on Satellite Meteorology and Oceanography.The paper provides an updated assessment of the Geostationary Operational Environmental Satellite (GOES) Microburst Windspeed Potential Index (MWPI) and new GOES brightness temperature difference (BTD) algorithm, presents case studies demonstrating effective operational use of the microburst products, and presents validation results for the 2010 convective season over the United States Great Plains and Atlantic Coast regions.Favorable results presented in the paper include a correlation between MWPI values and measured wind gusts of 0.62 that was found to be statistically significant near the 100% confidence level.The very high confidence level indicates a high likelihood that the correlation represents a physical relationship between MWPI values and downburst magnitude and is not an artifact of the sampling process.
Figure 1.Poster titled “Recent developments in microburst nowcasting using GOES” presented by K. Pryor at the 17th Conference on Satellite Meteorology and Oceanography in Annapolis, Maryland on 30 September 2010.
Figure 1 displays the poster presented by K. Pryor at the 17th Conference on Satellite Meteorology and Oceanography.The poster presents a new visualization of the MWPI product as generated by the Graphyte Toolkit, two case studies of significant downburst events, and validation results.Favorable validation results include a statistically significant correlation between index values and measured downburst wind gust speeds with a confidence level near 100%.The poster also introduces a new GOES-imager derived brightness temperature difference (BTD) product that highlights the physical process of dry air entrainment into deep, moist convective storms and its role in the generation of downbursts.
The most significant tornado outbreak in Oklahoma since 3 May 1999 was observed during the afternoon and evening of 10 May 2010. Associated with the tornado activity were numerous downbursts, especially over central Oklahoma. The period between 2218 and 2238 UTC 10 May was especially active with two tornado touchdowns and reports of wind gusts between 51 and 56 knots in the Norman area. The GOES Microburst Windspeed Potential Index (MWPI) and GOES imager band 3-4 brightness temperature difference (BTD) products effectively indicated the potential for severe downbursts from three hours to 10 minutes prior to the observation of high winds in Norman. Figure 1. Surface analysis of Oklahoma Mesonet observations at 2000 UTC (top) and GOES MWPI product at 1919 UTC (bottom).
Figure 1 shows the favorable environmental set-up for severe convective storms and downbursts during the afternoon of 10 May. At 2000 UTC, the dryline extended from near Freedom to Grandfield. The dryline served as a trigger for supercell convective storms as well as primed the atmosphere for downbursts over central Oklahoma. Figure 2, a radiosonde observation (RAOB) from Norman at 2000 UTC, displayed a classic "loaded gun" profile with large CAPE, a shallow, well-developed mixed layer, and a significant dry-air layer between the 500 and 700-mb levels. The dry-air layer, in conjunction with heavy precipitation resulting from large CAPE, played a major role in forcing intense convective downdrafts in the supercell storms.
Figure 2. RAOB from Norman, OK at 2000 UTC.
Figure 3. GOES MWPI product at 2121 UTC and BTD image product at 2215 UTC with overlying radar reflectivity image from Oklahoma City TDWR.
The MWPI image in Figure 3 indicated values between 60 and 70 southeast of Norman, corresponding to wind gust potential near 55 knots. The BTD image displayed overshooting tops associated with the supercell storms moving through Cleveland County as well as a dry-air notch on the southwestern flank. The overshooting tops signified the presence of intense storm updrafts that were generating heavy precipitation. The dry air notch most likely indicated the presence of drier air that was channeled into the rear of the storm and provided the energy for intense downdrafts due to evaporation within the intense precipitation core. The Norman RAOB showed southwesterly winds greater than 50 knots in the dry-air layer.
Severe weather occurred in the Norman area with the following time line: 2218-Tornado touches down near Moore associated with supercell #1 2225-51 knot wind gust recorded by Norman OK mesonet station 2232- 56 knot wind gust recorded at National Weather Center 2233-2238- Tornado touches town in southeast Norman, associated with supercell #2, with wind gust of 90 knots recorded
During the evening of 25 April 2010, an intense linear mesoscale convective system (MCS) (or squall line) tracked through central Maryland producing strong outflow winds. The squall line produced strong downburst winds in the Baltimore area between 0200 and 0230 UTC 26 April. The data buoy at Francis Scott Key Bridge recorded a wind gust of 35 knots at 0224 UTC. Both GOES-13 channel 3-4 brightness temperature difference (BTD) imagery (Pryor 2010) and radar reflectivity imagery from BWI TDWR indicated the presence of a strong rear-inflow jet (RIJ)(Weisman 1992), illustrated in Figure 1, that enhanced downdraft strength due to entrainment of mid-tropospheric dry air into the leading convective storm line of the MCS. Figure 2 shows the dry-air notch, as indicated in the BTD image, in line with the RIN in the trailing stratiform region identified in radar imagery. The juxtaposition of these two features signified the presence of a strong RIJ that was enhanced by 50 knot-winds in the elevated dry-air layer near the 500-mb level. Figure 1. A schematic of a RIJ (from Weisman (1992)).
Figure 2. BTD image with overlying radar image from BWI TDWR (top) and RAOB from Dulles Airport (bottom). Location of FSK data buoy indicated by an "X".
Interaction of the RIJ with the leading convective storm cells in the MCS was an important forcing factor in generating intense downdrafts and subsequent downbursts in the vicinity of the Baltimore Harbor. The direction of the downburst wind gust recorded at FSK Bridge, from the west-northwest as illustrated in Figure 3, corresponded closely to the orientation of the RIJ shown in Figure 1. This provides evidence that downward momentum transport and evaporational cooling in the convective precipitation cores fostered downburst generation with this linear MCS.
Figure 3. Wind histogram from FSK Bridge data buoy.
Weisman, M. L., 1992: The Role of Convectively Generated Rear-Inflow Jets in the Evolution of Long-Lived Mesoconvective Systems. J. Atmos. Sci., 49, 1826–1847.
During the evening of 8 April 2010, a line of strong convective storms developed over northern Virginia and tracked northeastward over the Washington, DC and Baltimore, Maryland metropolitan areas between 2345 UTC 8 April and 0045 UTC 9 April 2010. The convective storm line evolved into a bow echo and produced widespread damaging winds in the Washington, DC area near 0000 UTC. Geostationary Operational Environmental Satellite (GOES)-12 imagery was very effective in identifying the evolution of the convective storm system and favorable pre-conditions for downburst activity at least fifteen minutes prior to the observation of severe winds in Washington, DC. Satellite and radar imagery indicated the presence of a dry-air channel on the rear flank of the bow echo (Przybylinski 1995) that most likely resulted in downburst generation. It has been found recently that the BTD between GOES infrared channel 3 (water vapor at 6.5μm) and channel 4 (thermal infrared at 11μm) can highlight regions where severe outflow wind generation (i.e. downbursts, microbursts) is likely due to the channeling of dry mid-tropospheric air into the precipitation core of a deep, moist convective storm.
Figure 1. GOES-12 channel 3 (WV)-channel 4 (IR) brightness temperature difference (BTD) product at 2345 UTC April 8, 2010 with overlying radar reflectivity from Washington Terminal Doppler Weather Radar (TDWR) at 2355 UTC. White line represents the co-located dry-air notch and rear-inflow notch pointing to Washington, DC National Ocean Service observing station (location “X”).
Figure 2. GOES-12 channel 3 (WV)-channel 4 (IR) brightness temperature difference (BTD) product at 0045 UTC April 9, 2010 with overlying radar reflectivity from Baltimore-Washington International Airport TDWR at 0034 UTC. White line represents the co-located dry-air notch and rear-inflow notch pointing to Francis Scott Key Bridge, Maryland National Ocean Service observing station (location “X”).
The BTD images in Figures 1 and 2 mark the location of downbursts recorded at Washington, DC and Francis Scott Key (FSK) Bridge, Maryland National Ocean Service (NOS) observation stations. A well-defined southwest-to-northeast oriented dry-air notch appears on the southwestern flank of the bow echo that is co-located with a broad rear-inflow notch (RIN) as identified in radar imagery. This dry-air notch satellite feature, co-located with the RIN, most likely represents drier (lower relative humidity) air that was channeled into the rear of the storm and provided the energy for intense downdrafts and the resulting downburst winds at Washington (50 knots) near 0000 UTC and at FSK Bridge (42 knots) at 0036 UTC April 9, 2010. Note that the dry-air notch was pointing directly to the location of the observed downburst winds in both images.
Figure 3. GOES-12 channel 3 (WV)-channel 4 (IR) brightness temperature difference (BTD) product at 0015 UTC 9 April 2010 (top) and a plot of transmittance weighting functions for channels 3 and 4 compared to the temperature and mixing ratio curves of the 0000 UTC RAOB at Dulles Airport (bottom). Location of Dulles Airport is marked by an "X". Figure 4. Conceptual model of a strong bow echo evolution showing bookend vortices and development of a Rear-Inflow Notch (RIN) (top) and schematic of a vertical cross-section through a mature bow echo (bottom). Courtesy COMET (1999).
Comparison of product images to transmittance weighting functions and a radiosonde observation at Dulles International Airport, Virginia at 0000 UTC 9 April emphasizes the importance of the dry air in the rear inflow region in the generation of intense downdrafts. Figure 3 shows a steep, near dry-adiabatic temperature lapse rate in the mid-troposphere, between the 300 and 500-mb levels, and a co-located dry-air layer with a mixing ratio near zero. The channel 3 weighting function peak near the 400-mb level and channel 4 weighting function peak near the surface is associated with an overall steep temperature lapse rate between the 300-mb level and the surface. More information pertaining to transmittance weighting functions is available at this site. Thus, the maximum BTD value near 30K observed at 0015 UTC near Dulles Airport can be related to a convectively unstable environment with sufficient mid-level dry air to generate strong downdrafts as the dry air is channeled into the rear flank of the convective storm line and heavy precipitation core. A correlation has been found between maximum BTD in the dry air notch and corresponding downburst wind gust magnitude: 39K over northern Virginia at 2345 UTC associated with the 50-knot gust observed at Washingtion, DC at 0000 UTC; and 30K southwest of Baltimore associated with the 42-knot gust observed at FSK Bridge at 0036 UTC. This entrained dry air results in evaporation of descending precipitation and cooling within the downdraft, and the subsequent acceleration of the downdraft toward the surface. When this intense downdraft impacts the surface, air flows outward as a downburst. A graphical description of the physical process of downburst generation within a bow echo is presented in Figure 4.
References
COMET, 1999: Mesoscale Convective Systems: Squall Lines and Bow Echoes. Online training module, http://www.meted.ucar.edu/convectn/mcs/index.htm.
Przybylinski, R.W., 1995: The bow echo. Observations, numerical simulations, and severe weather detection methods. Wea. Forecasting, 10, 203-218.
During the afternoon of 17 February 2010, the Canadian Sailing Vessel (SV) Concordia sank off the coast of Brazil due to strong convective storm-generated winds. The SV Concordia was capsized in a downburst that occurred near 1722 UTC, about 290 nautical miles south-southeast of Rio deJaneiro (Capt. William Curry, SV Concordia, personal communication). Geostationary Operational Environmental Satellite (GOES)-12 Southern Hemisphere imagery was very effective in identifying the developing convective storm complex and favorable pre-conditions for downburst activity over one hour prior to the capsize of the Concordia. Satellite imagery indicated the presence of strong convective storm updrafts that resulted in heavy rainfall as well as the presence of a dry-air channel on the rear flank of the storm complex that most likely resulted in downburst generation.
Figure 1. GOES-12 infrared (IR, top) and water vapor (WV, bottom) imagery at 1709 UTC 17 February 2010. Location of the SV Concordia is indicated by an "X".
Soden and Bretherton (1996) (SB96), in their study of the relationship of water vapor radiance and layer-average relative humidity, found a strong negative correlation between 6.5μm (channel 3) brightness temperature (BT) and layer-averaged relative humidity (RH) between the 200 and 500-mb levels.Thus, in the middle to upper troposphere, decreases in BT are associated with increases in RH as illustrated in Figure 4 of SB96. In the WV image in Figure 1, a notch of warmer brightness temperatures, indicated by the "V" pattern with light green shading on the southwestern flank of the storm complex, signified the presence of lower 500-mb humidity air being channeled into the rear of the storm.
Figure 2. GOES-12 channel 3 (WV)-channel 4 (IR) brightness temperature difference (BTD) product at 1709 UTC.
The BTD image in Figure 2 at 1709 UTC 17 February marks the location of the Concordia and Rio deJaneiro. Also shown in the image is the thunderstorm complex that produced the severe downburst. The purple shading in the thunderstorm complex indicates the presence of intense convection and associated strong updrafts that generated heavy rainfall. At the same time, a well-defined dry-air notch appears on the southwestern flank of the storm complex. This dry-air notch most likely represents the drier (lower relative humidity) air that was channeled into the rear of the storm and provided the energy for intense downdrafts and the resulting downburst winds near 1720 UTC. Entrainment of drier mid-tropospheric air into the precipitation core of the convective storm resulted in evaporation of precipitation, the subsequent cooling and generation of negative buoyancy (sinking air), and resultant acceleration of the downdraft. When this intense localized downdraft reached the ocean surface, air flowed outward as a downburst. The resulting strong winds then capsized the SV Concordia. Note that the dry-air notch was pointing directly to the location of the Concordia, and thus, the vessel was in the direct path of downburst winds. Ellrod (1989) noted the importance of low mid-tropospheric (500 mb) relative humidity air in the generation of the severe Dallas-Fort Worth, Texas microburst in August 1985.
Figure 3. GOES-12 BTD product animation between 1609 and 1739 UTC 17 February 2009.
The BTD image animation in Figure 3 shows that the dry-air notch was visible as early as 1609 UTC and moved east-southeastward along the rear flank of the storm between 1609 and 1739 UTC. The dry-air notch, readily apparent in both water vapor (channel 3) and BTD imagery, was an effective indicator of the immanent occurrence of a downburst. Thus, with further case studies of this phenomena to be conducted, the juxtaposition of a dry-air notch and overshooting convective storm tops, may prove to be useful in the microburst detection and forecasting process.
References
Ellrod, G. P., 1989: Environmental conditions associated with the Dallasmicroburst storm determined from satellite soundings. Wea. Forecasting, 4, 469-484.
Soden, B.J. and F.P. Bretherton, 1996:Interpretation of TOVS water vapor radiances in terms of layer-average relative humidities: Method and climatology for the upper, middle, and lower troposphere.J. Geophys. Res., 101, 9333-9343.
During the afternoon of 10 March 2010, strong convective storms, triggered by an upper-level disturbance, developed over southwestern Oklahoma and tracked northeastward. The area of convective storms produced strong downbursts in the Oklahoma City area while tracking northeastward in the wake of a cold front. Although the temperature contrast across the front was weak, a sharp decrease in boundary layer moisture and increase in low-level wind speed were apparent over central and southern Oklahoma in the wake of the cold front. Strong solar heating and gradient winds behind the cold front resulted in the development and evolution of a deep convective mixed layer, especially in the Oklahoma City area. This favorable environment, characterized by steep sub-cloud temperature lapse rate, super-adiabatic surface layer lapse rate, and large vertical humidity difference, fostered intense convective storm downdrafts. This environment was effectively illustrated as a classic "inverted V" profile in the late afternoon GOES sounding in Figure 1. Figure 1. 2200 UTC GOES sounding profile over Oklahoma City.
This sounding profile translated into a significant risk of downbursts as shown in the GOES imager microburst products in Figure 2. About one hour later, strong downbursts were observed north of Oklahoma City, near the center of the upper-level disturbance.
Figure 2. GOES imager microburst products at 2200 UTC.
Figure 2 shows two versions of the GOES microburst product. The McIDAS-V version (bottom), displays overlying NEXRAD reflectivity from Oklahoma City (TLX) and the downburst-producing convective storms north of Oklahoma City near 2300 UTC. The microburst product indicates moderate risk (yellow to orange shading, top, green to yellow shading, bottom) associated with a wind gust potential of 35 to 40 knots. About one hour later, the two strongest downbursts of the event were recorded at Guthrie (45 knots) and Marshall (44 knots) Oklahoma Mesonet stations. This event demonstrated the effectiveness of the GOES microbust products, especially in this environment more typical of the summer season over Oklahoma.
During the morning of 17 February 2010, the Canadian Sailing Vessel (SV) Concordia sank off the coast of Brazil, near Rio de Janeiro, most likely due to strong convective storm-generated winds. The SV Concordia was capsized in a downburst that occurred around 1500 UTC about 500 km off the coast. Graphical microburst potential guidance derived from a three-hour forecast of the Global Forecast System (GFS) model, valid at 1500 UTC, indicated a high risk of downbursts in the approximate location of the sinking of the Concordia. A conditionally unstable temperature lapse rate, large atmospheric precipitable water content, and the presence of mid-tropospheric dry air, as indicated by low relative humidity at the 500-mb level, contributed to strong downdraft instability that resulted in the downburst and hazardous winds on the ocean surface.
Figure 1. GFS model-derived graphical downburst potential product at 1500 UTC 17 February 2010.
A favorable downburst environment is illustrated in Figure 1, with the highest risk area outlined in red. The juxtaposition of a conditionally unstable temperature lapse rate (> 7K/km), large atmospheric precipitable water values near 50 mm (2 inches), and low mid-tropospheric (500 mb) relative humidity near the location of the sinking of the SV Concordia suggests that precipitation loading and mid-level dry air entrainment were major forcing mechanisms for strong convective winds. The presence of mid-tropospheric dry air fostered convective instability and the generation of negative buoyancy due to evaporational cooling as the drier air was entrained into the precipitation core of the convective storm. The combination of these forcing mechanisms would result in the acceleration of a downdraft to the ocean surface and the generation of a downburst that capsized the SV Concordia. For more information, please read:
During the early morning of 3 December 2009, a line of convective storms developed over Frederick County, Maryland and then tracked eastward over north-central Maryland between 0800 and 1000 UTC before dissipating over the upper Chesapeake Bay. The convective storm line produced strong winds over eastern Frederick County between 0845 and 0855 UTC. Doppler radar-derived winds between 23 and 34 knots were associated with the storm line as it tracked through eastern Frederick County. National Weather Service spotter estimated winds between 30 and 34 knots over eastern Frederick County reflected wind speeds near the top of the boundary layer at the 925-mb level (about 1500 feet above ground level). Near the time of the strong wind observation, radar reflectivity imagery indicated the presence of a well-defined weak echo channel over central Frederick County, signifying the immenent occurrence of downburst winds. RUC model-derived graphical guidance indicated high precipitable water values near 30 mm (> 1 inch) and 925 mb level wind speeds near 32 knots (16 m/s). These parameter values indicated that the combination of precipitation loading and downward horizontal momentum transfer was an important forcing factor in the strong convective winds observed over Frederick County. These conditions prompted the NWS to issue a Special Marine Warning for the Baltimore Harbor at 0915 UTC.
Figure 1. RUC model-derived graphical microburst guidance product at 0900 UTC 3 December 2009 with radar reflectivity image from BWI TDWR at 0850 UTC (top) and corresponding RUC sounding profile over eastern Frederick County.
Figure 2. RUC graphical microburst guidance product at 0900 UTC with radar reflectivity image from BWI TDWR at 0958 UTC (top) and wind histogram from FSK Bridge PORTS station (location "X" in RUC image).
A peak wind gust of 24 knots was observed at Francis Scott Key Bridge PORTS station at 0954 UTC as the southwestern end of the convective storm line tracked over the Baltimore Harbor. RUC model graphical guidance indicated 925 mb winds near 24 knots (12 m/s) in the vicinity of the Baltimore Harbor, comparable to the wind gust speed observed by the FSK Bridge PORTS station. Although a wind gust of this magnitude did not satisfy special marine warning criteria, a 24-knot wind gust would still pose a threat to small craft over the open water of the Baltimore Harbor. This event demonstrated the effectiveness of boundary layer wind speed data in the nowcasting of convective wind gust potential.
