Analysis of RUC model data has provided new results from the study of the March 2004 Chesapeake Bay downburst event. This event was associated with the Baltimore water taxi accident that occurred during the afternoon of 6 March 2004, discussed in a previous entry. Computation and analysis of RUC-derived downdraft instability parameters, including temperature lapse rate, vertical relative humidity difference, and precipitable water, revealed local maxima in proximity to downburst occurrence about one hour prior.
Figure 1. RUC derived temperature lapse rate, vertical humidity difference (dRH, middle), and precipitable water (PW, bottom) at 2000 UTC 6 March 2004 with overlying NEXRAD radar reflectivity .
The table below outlines two strong downbursts that occurred in the upper Chesapeake Bay region between 2050 and 2120 UTC 6 March 2004 and associated RUC-derived microburst parameters from 2000 UTC: Time (UTC)/Location/Wind Gust (kt)/Lapse Rate (K/km)/dRH (%)/PW (mm) 2050/Baltimore Harbor/35 to 45/8.6/16/25 2118/Tolchester Beach/48/8.9/17/27
The first downburst resulted in the capsize of the "Lady D" in the Baltimore Harbor. As displayed in Figure 1, and noted in the table above, the stronger downburst recorded at Tolchester Beach was associated with higher values of all the listed parameters. In general, the stronger downburst was associated with a steeper sub-cloud temperature lapse rate and a larger vertical humidity difference below 850mb, and a higher storm precipitable water content. In accordance with findings of Srivastava (1985), downbursts were associated with sub-cloud lapse rates greater than 8.5 K/km. This suggests that sub-cloud evaporational cooling in a more well-mixed boundary layer and precipitation loading were factors in the generation of downdraft instability and resulting strong downbursts. These conditions, more typically found over the Great Plains during the warm season, were effectively indicated by RUC analysis-derived parameters about one hour prior to the first downburst occurrence near the Baltimore Harbor.
References
Srivastava, R.C., 1985: A simple model of evaporatively driven downdraft: Application to microburst downdraft. J. Atmos. Sci., 42, 1004-1023.
During the afternoon of 26 August 2009, strong convective storms developed along a weak, slow-moving cold front as it was tracking eastward over Oklahoma. Although there was very little temperature contrast across the front, the front acted as a convergence zone and a trigger for deep, moist convection. The pre-convective environment downstream of the cold front over western Oklahoma was dominated by vertical mixing that fostered the development and evolution of a convective boundary layer. Strong downbursts that were recorded by Oklahoma Mesonet stations between 0000 and 0100 UTC 27 August resulted from a combination of precipitation loading and sub-cloud evaporation of precipitation as described in a previous entry. New RUC model graphical guidance effectively indicated the potential for strong downbursts near and west of Oklahoma City. Parameters calculated include 850-1000mb temperature lapse rate (LR), 850-1000mb relative humidity difference (dRH), precipitable water (PW), and surface dewpoint depression (DD). Figure 1. RUC derived temperature lapse rate and radar reflectivity with overlying surface dewpoint depression (DD,top), precipitable water (PW, middle), and vertical humidity difference (dRH, bottom) at 2200 UTC 26 August.
The table below lists three strong downbursts that occurred over central and western Oklahoma between 0020 and 0040 UTC 27 August and associated RUC-derived microburst parameters from 2200 UTC 26 August: Time (UTC)-Station-Wind Gust (kt)-Lapse Rate (K/km)-dRH (%)-DD (K)-PW (mm) 0020-Kingfisher (K)-43-8.2-9-13-40 0030-Weatherford (W)-41-8.6-13-18-35 0040-El Reno (E)-50-8.5-17-16-40
As displayed in Figure 1, and noted in the table above, the strongest downburst recorded at El Reno, overall, was associated with local maxima in all of the listed parameters. In general, stronger downbursts were associated with steeper sub-cloud temperature lapse rates, higher storm precipitable water content, and larger surface dewpoint depressions. This suggests that a combination of sub-cloud evaporational cooling in a more well-mixed boundary layer and precipitation loading was a factor in the generation of downdraft instability and resulting strong downbursts. These conditions were effectively indicated by RUC analysis-derived parameters over two hours prior to downburst occurrence.
A new microburst graphical guidance product has been developed that employs data from the Rapid Update Cycle (RUC) model.Prototypical conditions for microbursts include a steep temperature lapse rate and decreasing humidity with decreasing height in the boundary layer.Thus, the graphical guidance product incorporates boundary layer temperature lapse rate and vertical relative humidity difference, important factors in initiating and sustaining a convective downdraft.The new guidance product demonstrated effectiveness in indicating favorable conditions for downbursts over the northern Chesapeake Bay region during the afternoon of 5 November 2009 in which a convective storm produced a strong wind gust of 38 knots at Tolchester Beach, Maryland (See 6 November blog entry) .The 1800 UTC RUC graphical microburst product indicated high downburst risk in proximity to TolchesterBeach.
Figure 1. RUC graphical microburst product at 1800 UTC November 5, 2009 with radar reflectivity from Dover Air Force Base NEXRAD at 2237 UTC overlying the image (top) and RUC model analysis sounding over Tolchester Beach, Maryland at 1800 UTC (bottom).
Figure 1 compares the new Rapid Update Cycle (RUC) graphical guidance microburst product to a corresponding RUC sounding over Tolchester Beach, Maryland at 1800 UTC, 5 November 2009. At 1800 UTC, the image product showed a large area with steep boundary layer lapse rates, greater than 8.5 K/km, extending from eastern Pennsylvania and New Jersey to Virginia.The product image also displayed a local maximum in vertical humidity difference over northern Maryland and southeastern Pennsylvania.The highest microburst risk was indicated where the highest vertical humidity difference was co-located with steep lapse rates in the 850-1000mb layer.Srivastava (1985) noted that microbursts are likely with lapse rates greater than 8.5 K/km. Overlying radar reflectivity imagery from Dover Air Force Base NEXRAD at 2237 UTC displayed a downburst-producing convective storm as a spearhead echo over TolchesterBeach. At 2242 UTC, the Tolchester Beach PORTS station recorded a wind gust of 38 knots.The corresponding RUC sounding profile echoed favorable conditions for downbursts in the TolchesterBeach area with the presence of a 5000-foot deep mixed layer and steep temperature lapse rate below 850mb. Although radar reflectivities with this storm were not impressive (25-40 dBZ), the steep lapse rate was a major contributor to downdraft instability. As demonstrated in this case study, the RUC graphical guidance product, visualized by McIDAS-V software, effectively highlighted a region favored for strong convective winds. The ability to overly radar reflectivity on the microburst product shows the utility of the product in the downburst nowcasting process.
During the early morning of 3 December 2009, strong convective storms developed over the lower Chesapeake Bay region ahead of a cold front. A supercell storm produced a strong downburst over the lower Chesapeake Bay, with a wind gust of 44 knots recorded at York River East Rear Range Light PORTS station at 0636 UTC as shown in Figure 1.
Figure 1. Wind histogram from York River East Rear Range Light PORTS station.
Although radar reflectivity displayed in Figure 2 was modest with the supercell (35-40 dBZ), downward horizontal momentum transport within a shallow mixed layer still resulted in the generation of strong surface winds.
Figure 2. Radar reflectivity image from Wakefield, Virginia NEXRAD.
The 0600 UTC RUC analysis sounding near the mouth of the York River, displayed in Figure 3, indicated wind speeds between 40 and 45 knots about 1000 feet AGL. This supercell downburst was a typical cold-season low CAPE, strong shear-forced event that is not well-anticipated by the GOES microburst products. In this case, downburst generation was driven by a combination of precipitation loading and downward momentum transport processes. Figure 3. RUC analysis sounding at 0600 UTC 3 December 2009.
Pryor (2009) presented validation results for the GOES Microburst Windspeed Potential Index (MWPI) for the 2009 convective season over the U.S. southern Great Plains. Further statistical analysis of a dataset built by comparing wind gust speeds recorded by Oklahoma Mesonet stations to adjacent MWPI values for 35 downburst events has yielded some favorable results. Correlation has been computed between key parameters in downburst process, including temperature lapse rate (LR) and dewpoint depression difference (DDD) between two levels (670mb/850mb), CAPE, and radar reflectivity (Z). The first important finding is a statistically significant negative correlation (r=-.34) between lapse rate and radar reflectivity, as shown in Figure 1. Similar to the findings of Srivastava (1985), for lapse rates greater than 8 K/km, downburst occurrence is nearly independent of radar reflectivity. For lapse rates less than 8 K/km, downburst occurrence was associated with high reflectivity (> 50 dBZ) storms. The majority of downbursts occurred in sub-cloud environments with lapse rates greater than 8.5 K/km. Adding the dewpoint depression difference to lapse rate yielded an even greater negative correlation (r=- .42) when compared to radar reflectivity, as demonstrated in Figure 2. Finally, comparing the sum of LR and DDD (the former hybrid microburst index (HMI)) to CAPE resulted in the strongest negative correlation (r=-.82), with a confidence level above 99%. This emphasizes the complementary nature of the HMI and CAPE in generating a robust and physically meaningful MWPI value. This result also shows that CAPE can serve as an adequate proxy variable for precipitation loading (expressed as radar reflectivity) in the MWPI . The strong negative correlation, or negative functional relationship, between HMI and CAPE terms in the MWPI algorithm indicates that the MWPI should be effective in capturing both negative buoyancy generation and precipitation loading as downburst forcing mechanisms. Figure 1. Scatterplot of lapse rate versus radar reflectivity. Figure 2.Scatterplot of the sum of lapse rate and DDD versus radar reflectivity. Figure 3. Scatterplot of the sum of lapse rate and DDD versus CAPE. References
During the afternoon of 6 March 2004, the "Lady D", a water taxi servicing the Baltimore Harbor, was capsized in a strong convective windstorm, resulting in five deaths. A cluster of convective storms developed over north-central Maryland during the afternoon, and then tracked rapidly eastward through the Baltimore Harbor and upper Chesapeake Bay between 2050 and 2120 UTC. Radar wind velocity measurements from Dover AFB NEXRAD between 35 and 45 knots were associated with a cluster of downbursts as the convective storm complex approached the Baltimore Harbor. About two hours prior to the water taxi accident, the GOES imager microburst product, derived from 1847 UTC sounder data, indicated output BTD 35 to 36K over western Baltimore City in proximity to the location of the generation of the downbursts. Previous validation has identified that BTD greater than 35K corresponds to wind gust potential greater than 35 knots. According to news reports, the Lady D capsized between 2050 and 2100 UTC (see Baltimore Sun article).
Figure 1. GOES imager microburst product derived from 1847 UTC 6 March 2004 sounder data. Radar reflectivity and velocity imagery from DOVER AFB NEXRAD at 2048 UTC are overlying the microburst product image.
Figure 1 shows the 1847 UTC GOES microburst product with overlying radar imagery from Dover AFB NEXRAD. Figure 1 displays several features associated with high downburst wind gust potential: Output BTD greater than 35K (orange shading) over western Baltimore City, and radar wind velocity of 35 to 45 knots (green shading) surrounding a high reflectivity (red shading) convective storm over downtown Baltimore near the inner harbor. Between 2050 and 2100 UTC, the Lady D capsized in the Baltimore Harbor due to the strong convective winds. The cluster of convective storms then continued to track rapidly southeastward over the Chesapeake Bay, moving over the Eastern Shore at Tolchester Beach, Maryland. Although the high winds were not verified in surface observations in proximity to the Baltimore Harbor, an NOS PORTS station recorded a wind gust of 48 knots at Tolchester Beach about 20 minutes later at 2118 UTC. This high wind report was the result of a downburst, clearly marked in a wind histogram from Tolchester Beach PORTS station in Figure 2.
Figure 2. Wind histogram from Tolchester Beach PORTS station on 6 March 2004 (top) and GOES imager microburst product at 1946 UTC with overlying radar reflectivity from Dover AFB NEXRAD at 2117 UTC.
The 1946 UTC microburst product indicated high risk values downstream of the convective storm over Tolchester at 2117 UTC. Maximum output BTD over the Delmarva Peninsula of 35 to 37K indicated wind gust potential of 35 to 37 knots. The measured wind gust of 48 knots at Tolchester signifies that the rapid forward motion of the storm as well as precipitation loading, with radar reflectivity greater than 55 dBZ, were also factors in the magnitude of the wind gust associated with the downburst at Tolchester Beach. Based on radar velocity and the measured wind gust at Tolchester, wind gusts of 35 to 45 knots were likely with the convective storm complex as it moved over the Baltimore Harbor. The GOES imager microburst product two hours prior to the event would have been useful in assessing convective wind gust potential and, perhaps, may have provided guidance in issuing more timely warnings.
During the afternoon of 5 November 2009, an upper-level disturbance triggered the development of scattered convective storms over the mid-Atlantic region.Solar heating of the lower atmosphere during the early afternoon fostered conditions for strong convective storm downdrafts that were more typical of the Great Plains region.A convective storm that developed west of Baltimore, Maryland during the late afternoon produced a strong downburst as it tracked from the Chesapeake Bay eastward into the Delmarva Peninsula.An associated wind gust of 38 knots was recorded at Tolchester Beach, Maryland at 2242 UTC. The 1800 UTC Geostationary Operational Environmental Satellite (GOES)imager microburst product, derived from brightness temperature differences obtained from the GOES-11 imager (in full disk mode), indicated high downburst risk in proximity to TolchesterBeach.
Figure 1. GOES-11 imager microburst product at 1800 UTC 5 November 2009 with radar reflectivity from Dover Air Force Base NEXRAD at 2237 UTC overlying the image (top) and RUC model analysis sounding over Tolchester Beach, Maryland at 1800 UTC (bottom).
Figure 1 compares the Geostationary Operational Environmental Satellite (GOES)-11 imager microburst product to a corresponding Rapid Update Cycle (RUC) model sounding over Tolchester Beach, Maryland at 1800 UTC, 5 November 2009. At 1800 UTC, the GOES-11 image product showed a large area of high downburst risk (red shading) over southern New Jersey and the Delmarva Peninsula where strong solar heating was destabilizing the lower atmosphere.Output brightness temperature difference (BTD) in this region was greater than 40°K, indicating wind gust potential greater than 40 knots based on a previously established statistical relationship. Overlying radar reflectivity imagery from Dover Air Force Base NEXRAD at 2237 UTC displayed the downburst-producing convective storm as a spearhead echo over TolchesterBeach. At 2242 UTC, the Tolchester Beach Physical Oceanographic Real-Time System (PORTS) station recorded a wind gust of 38 knots.The corresponding RUC sounding profile echoed favorable conditions for downbursts in the TolchesterBeach area with the presence of a 5000-foot deep mixed layer and steep temperature lapse rate in the lower atmosphere. These boundary layer conditions promoted strong downdraft generation due to the effects of precipitation loading, evaporational cooling, and subsequent generation of negative buoyancy.
A week after the passage of an intense coastal storm and associated winter-like conditions, a more spring-like regime over the Chesapeake Bay region resulted in a significant downburst event. During the afternoon of 24 October 2009, a rain band with embedded convective storms developed ahead of a strong cold front over the Blue Ridge Mountains. Similar to the 15 November 2008 event, temperatures were above normal ahead of the cold front, especially over the Delmarva Peninsula. A convective storm on the eastern flank of the rain band produced a strong downburst at Tolchester Beach, Maryland. The GOES imager microburst product, derived from from both the GOES-12 sounder and GOES-11 imager (full disk mode), indicated favorable conditions for downbursts three hours prior to the event.
Figure 1 compares the GOES-12 sounder and GOES-11 imager products. At 1746 UTC, the sounder derived product image displayed a small break in the large frontal cloud band over the upper Chesapeake Bay and northeastern Maryland. Brightness temperature difference (BTD) between 30 and 35K indicated wind gust potential of 30 to 35 knots. By 1800 UTC, the GOES-11 image product showed a small break in the frontal cloud band over the Chesapeake Bay near Tolchester. Again, output BTD in this region ranged from 30 to 35K, indicating wind gust potential of 30 to 35 knots. Overlying radar reflectivity imagery from Dover Air Force Base NEXRAD at 2059 UTC displayed the downburst-producing convective storm as a spearhead echo over Tolchester Beach, with reflectivities greater than 50 dBZ. At 2100 UTC, the Tolchester Beach PORTS station recorded a wind gust of 39 knots, well-marked in the wind histogram in Figure 2. In a similar manner to the 15 November 2008 event, steep low to mid-level temperature lapse rates as inferred by elevated BTDs and precipitation loading as inferred by high radar reflectivity in the parent storm favored strong downdraft instability.The RUC sounding profile displayed in Figure 2, over Tolchester Beach at 1800 UTC, suggests that downward momentum transport was also a factor in downburst generation with winds near 40 knots near the top of the mixed layer at 955 meters above the surface.
Figure 1. GOES-12 sounder microburst product at 1746 UTC 24 October 2009 (top) and GOES-11 imager microburst product at 1800 UTC (bottom). Radar reflectivity from Dover AFB NEXRAD at 2059 UTC is overlying both images.
Figure 2. Wind histogram for Tolchester Beach PORTS station (top) and RUC sounding profile over Tochester at 1800 UTC 24 October 2009 (bottom).
Comparing product images in Figure 1 reveals that the GOES-11 imager product in full disk mode provides a higher spatial resolution and a more precise display of output BTD than that produced by the GOES-12 sounder. Thus, the GOES-11 imager product may provide forecasters with more useful guidance pertaining to downburst risk over the Chesapeake Bay region. More information pertaining to downburst activity over the Chesapeake Bay region is available in this paper.
During the afternoon of 15 November 2008, a squall line developed ahead of a strong cold front over the western Piedmont region of Maryland and Virginia. Temperatures and dewpoints were much above normal for mid-November ahead of the cold front, especially over the Delmarva Peninsula where solar heating of the boundary layer during the afternoon resulted in strong static instability. The the squall line produced strong convective winds as it tracked from central Maryland through the Delmarva Peninsula during afternoon and evening. The GOES imager microburst product, derived from brightness temperature difference (BTD) between sounder band 11 (mid-level water vapor, 7μm), band 8 (longwave infrared window, 11μm), and split window band 7 (12μm), indicated favorable conditions for downbursts two to three hours prior to each event.
The first strong convective winds were observed in eastern Frederick County, Maryland between 1845 and 1900 UTC 15 November 2009. Downburst wind gusts were estimated near 35 knots based on radar velocity measurements by Sterling, Virginia NEXRAD (KLWX). Figure 1 shows the convective storm line moving through Frederick and western Montgomery Counties at 1847 UTC. Apparent is a spearhead echo embedded in the storm line east of Frederick. Immediately downstream of the line are NEXRAD radial velocities near 35 knots, as indicated by the red shading. Corresponding storm relative (outflow) wind (not shown) was measured near 30 knots. Two hours prior, the GOES imager microburst product indicated wind gust potential near 30 knots over nearby Carroll County.
Figure 1. Geostationary Operational Environmental Satellite (GOES) imager microburst product at 1647 UTC 15 November 2008, with overlying radar velocity data from Sterling, Virginia NEXRAD at 1847 UTC.
As the squall line moved east of the Chesapeake Bay, it encountered a more unstable boundary layer due to a greater amount of surface heating through the afternoon. This was reflected in figure 2, the GOES microburst product at 1846 UTC with overlying radar reflectivity imagery from Sterling NEXRAD at 2203 UTC. The microburst product indicated elevated risk (yellow to orange shading) with output BTD of 32K, corresponding to wind gust potential near 32 knots. At 2203 UTC, the storm line produced a downburst wind gust of 32 knots, very well marked in the wind speed trace at Cambridge National Ocean Service (NOS) observing station (location "X"), also shown in Figure 2. High reflectivities (>45 dBZ) were indicated over Cambridge at the time of downburst occurrence.
Figure 2. GOES imager microburst product at 1846 UTC 15 November 2008, with overlying radar reflectivity data from Sterling, Virginia NEXRAD at 2203 UTC (top); wind histogram from Cambridge NOS observing station (bottom).
About two hours later, near 0000 UTC 16 November, a stronger downburst was observed farther south on the Chesapeake Bay Bridge. The 2200 UTC (15 November) imager microburst product, shown in Figure 3, indicated slightly higher risk values, with corresponding wind gust potential of 32 to 35 knots, in proximity to the Bay Bridge. A downburst wind gust of 40 knots was recorded at the Chesapeake Bay Bridge NOS observing station (location "X") at 0006 UTC. Similar to the Cambridge downburst, the downburst-producing segment of the line exhibited high radar reflectivity near 50 dBZ. As shown in Figure 3, this downburst event was also well-marked with a sharp peak in wind speed near 0000 UTC.
Figure 3. GOES imager microburst product at 2146 UTC 15 November 2008, with overlying radar reflectivity data from Wakefield, Virginia NEXRAD (KAKQ) at 0006 UTC 16 November(top); wind histogram from Chesapeake Bay Bridge NOS observing station (bottom).
This event demonstrated the usefulness of NOS meteorological observations in the microburst product validation process over the Chesapeake Bay region. The temporal resolution of NOS data, six minutes, is well-suited for downburst observation. In addition, the environment over the Bay region was favorable for a late Fall convective high wind event as inferred by the GOES imager microburst product, characterized by steep low to mid-level temperature lapse rates. Steep lapse rates in conjunction with heavy precipitation produced by the squall line fostered strong convective downdraft development.
Yes- downbursts do occur in Alaska- especially in the Central Interior region during the summer. During the afternoon of 23 May 2009, strong convective storms developed over the Central Interior region of Alaska when temperatures were much above normal for the season. Two strong downbursts were recorded in the Fairbanks area during the late afternoon and early evening. The first downburst wind gust of 40 knots was recorded at the Alaska Climate Research Center (ACRC) weather station at 0015 UTC 24 May (1615 AKDT). A weaker downburst (35 knots) was recorded at Eielson Air Force Base (AFB) near Fairbanks about three and a half hours later at 0350 UTC. The pre-convective environment during the afternoon was more typical for the interior northwestern United States. Strong solar heating resulted in the development of a deep convective mixed layer (ML) that was favorable for downbursts. This favorable downburst environment was effectively indicated by the Geostationary Operational Environmental Satellite (GOES)-11 imager microburst product, in which output brightness temperature difference is strongly correlated with microburst risk. The 2300 UTC GOES imager product did indicate elevated microburst risk with wind gust potential of 40 to 45 knots 75 minutes prior to the downburst at ACRC. Further explanation of the GOES-11 imager microburst product is available in Pryor (2009).
Figure 1. Geostationary Operational Environmental Satellite (GOES) imager microburst product at 2300 UTC 23 May 2009 with overlying radar reflectivity from Pedro Dome, Alaska NEXRAD (PAPD) at 0015 UTC 24 May.
Figure 2. Geostationary Operational Environmental Satellite (GOES) imager microburst product at 2300 UTC 23 May 2009 with overlying radar reflectivity from Pedro Dome, Alaska NEXRAD (PAPD) at 0350 UTC 24 May.
Figures 1 and 2 displayed scattered convective storms developing over the central interior region of Alaska. Apparent in both images are downburst-producing convective storms occurring in a region of elevated microburst risk as indicated by the orange shading near ACRC ("X" in Figure 1) and the yellow shading near Eielson AFB ("X" in Figure 2). In general, output brightness temperature difference (BTD) of 35 to 45K corresponded to wind gust potential of 35 to 45 knots. A downburst wind gust of 40 knots was recorded at the Alaska Climate Research Center (ACRC) weather station at 0015 UTC 24 May (1615 AKDT) followed by a weaker downburst (35 knots) recorded at Eielson Air Force Base (AFB) near Fairbanks at 0350 UTC.
Figure 3. Radiosonde observation (RAOB) at Fairbanks, Alaska at 0000 UTC 24 May 2009.
The pre-downburst environment in the Fairbanks area is most effectively illustrated by the sounding profile shown in Figure 3. Similar to favorable downburst environments over the interior northwestern U.S., the boundary layer was well-mixed with low relative humidity near the surface and a high cloud base near the 700-mb level. This environment fosters strong convective downdrafts through evaporational cooling and negative buoyancy generation as precipitation descends in the sub-cloud layer. Thus, the downbursts that occurred in the Fairbanks area were "dry" type associated with relatively light precipitation (radar reflectivity near 30 dBZ) and an "inverted-v" sounding profile with a deep mixed layer (ML) as shown in Figure 3. Noting the elevated output BTD associated with each downburst, it is evident that the GOES-11 imager microburst product effectively captured favorable conditions for strong convective winds in the Fairbanks area during the afternoon and evening of 23 May.
On the last full day of the Summer season, 21 September 2009, strong convective storms developed along a cold front as it was tracking eastward over Oklahoma. In general, the pre-convective environment downstream of the cold front over eastern Oklahoma was moist and very unstable with a shallow, well-mixed boundary layer. These conditions favored convective storms that produced strong outflow winds (downbursts) associated with heavy rain. Elevated Geostationary Operational Environmental Satellite (GOES) microburst index values, indicated in the 1800 UTC product images in the vicinity of downburst occurrence over eastern Oklahoma, served as evidence of the presence of this favorable environment. Strong downbursts that were recorded by Oklahoma Mesonet stations between 2100 and 0000 UTC 22 September resulted from a combination of heavy precipitation and sub-cloud evaporation of precipitation.
Figure 1. Geostationary Operational Environmental Satellite (GOES) imager microburst product at 1800 UTC 21 September 2009, with the location of Oklahoma mesonet observations of downburst wind gusts plotted on the image.
Figure 2. Geostationary Operational Environmental Satellite (GOES) Microburst Windspeed Potential Index (MWPI) product at 1800 UTC 21 September 2009, with the location of Oklahoma mesonet observations of downburst wind gusts plotted on the image.
Figures 1 and 2 display elevated microburst risk (yellow color) ahead of a cold front tracking eastward over Oklahoma. The cold front served as a trigger for convective storms over eastern Oklahoma during the afternoon and evening of 21 September 2009. Elevated Geostationary Operational Environmental Satellite (GOES) microburst index values displayed in the 1800 UTC product images in the vicinity of downburst occurrence over eastern Oklahoma, indicated wind gust potential of 35 to 50 knots. Downbursts wind gusts between 39 and 50 knots were recorded by the Oklahoma Mesonet stations plotted in Figures 1 and 2 between 2040 and 0000 UTC 22 September.
Validation results for the 2007 to 2009 convective seasons have been completed for the MWPI and imager microburst products. GOES sounder-derived MWPI values have been compared to mesonet observations of downburst winds over Oklahoma and Texas for 168 events between June 2007 and September 2009. The correlation between MWPI values and measured wind gusts was determined to be .62 and was found to be statistically significant above the 99% confidence level, indicating that the correlation represents a physical relationship between MWPI values and downburst magnitude and is not an artifact of the sampling process. Comparison of GOES-11 imager microburst risk values (output brightness temperature difference (BTD) in degrees K) to measured downburst wind gusts for 61 events in Oklahoma between June and September 2009 yielded a correlation of .40. This correlation was higher than the correlation computed between MWPI values and downburst wind gusts (.27) for the same time period. The correlation between output BTD and measured wind gusts was determined to be statistically significant at the 82% confidence level, indicating a high confidence that the correlation represented a physical relationship between output BTD values and downburst magnitude.
Figure 3.Scatterplot of Geostationary Operational Environmental Satellite (GOES) Microburst Windspeed Potential Index (MWPI) versus observed downburst wind gust speed as recorded by mesonet stations in Oklahoma and Texas between June 2007 and September 2009.
Figure 4.Scatterplot of Geostationary Operational Environmental Satellite (GOES) -11 imager output BTD values (K) versus observed downburst wind gust speed as recorded by mesonet stations in Oklahoma between June and September 2009.
Figures 3 and 4 are scatterplots of MWPI values and GOES-11 imager output BTD values (K) versus observed downburst wind gust speed as recorded by mesonet stations in Oklahoma and Texas. The MWPI scatterplot identifies two clusters of values: MWPI values less than 50 that correspond to observed wind gusts of 35 to 50 knots, and MWPI values greater than 50 that correspond to observed wind gusts of greater than 50 knots. Similarly, the GOES-11 imager microburst risk scatterplot identifies two clusters. The dominant cluster contains output brightness temperature difference (BTD) values less than 50K that correspond to observed wind gusts between 35 and 50 knots. The scatterplots illustrate that both microburst products demonstrate effectiveness in distinguishing between severe and non-severe convective wind gust potential.
During the afternoon of 9 September 2009, scattered convective storms developed along a weak cold front as it was tracking southeastward over Oklahoma. Although there was a weak temperature contrast across the front, the front acted as a convergence zone and a trigger for deep, moist convection. The pre-convective environment downstream of the cold front over western Oklahoma was dominated by vertical mixing that fostered the development and evolution of a convective boundary layer. Elevated Geostationary Operational Environmental Satellite (GOES) imager brightness temperature difference (BTD) values in the vicinity of downburst occurrence over western Oklahoma served as evidence of the presence of a well-developed mixed layer. Strong downbursts that were recorded by Oklahoma Mesonet stations in southwestern Oklahoma between 2145 and 2250 UTC resulted from a combination of precipitation loading and sub-cloud evaporation of precipitation. These downbursts occurred in proximity to high microburst risk values as indicated in the 2000 UTC imager microburst product. The images above are a Geostationary Operational Environmental Satellite (GOES) imager microburst product with overlying radar reflectivity from Frederick, Oklahoma NEXRAD (KFDR) at 2145 UTC (top) and 2249 UTC (bottom), times of downburst occurrence at Medicine Park and Apache Oklahoma Mesonet stations, respectively. The product image displayed scattered convective storms developing along the cold front over western Oklahoma. Downburst wind gusts of 54 and 41 knots were recorded at Medicine Park and Apache stations at 2145 and 2250 UTC, respectively. Apparent in both images is the spearhead echo associated with both downbursts, occurring in a region of high microburst risk as indicated by the orange shading. Output BTD near 50K corresponded to wind gust potential near 50 knots. Fujita and Byers (1977) related the spearhead echo signature in radar reflectivity imagery to the occurrence of downbursts as illustrated below. The Rapid Update Cycle model (RUC) analysis sounding above at 2000 UTC at Medicine Park shows an "inverted v" profile, and indicates the presence of a deep and dry mixed layer that favored the development of intense downdrafts due to the evaporation of precipitation in the sub-cloud layer. This profile corresponded well with high output BTD associated with downbursts and resultant spearhead echoes over southwestern Oklahoma.
References
Fujita, T. T., and H.R. Byers, 1977: Spearhead echo and downbursts in the crash of an airliner. Mon. Wea. Rev., 105, 1292-146.
Preliminary validation results for August 2009 have been completed for the Geostationary Operational Environmental Satellite (GOES) Microburst Windspeed Potential Index (MWPI) and imager microburst products. GOES sounder-derived MWPI values have been compared to mesonet observations of downburst winds over Oklahoma and Texas for 156 events between June 2007 and August 2009.The correlation between MWPI values and measured wind gusts was determined to be .64 and was found to be statistically significant above the 99% confidence level, indicating that the correlation did represent a physical relationship between MWPI values and downburst magnitude and was not an artifact of the sampling process. Comparison of GOES-11 imager microburst risk values (output brightness temperature difference (BTD) in degrees K) to measured downburst wind gusts for 49 events in Oklahoma between June and August 2009, yielded a correlation of .42. This correlation was higher than the correlation computed between MWPI values and downburst wind gusts (.32) for the same time period. The correlation between output BTD and measured wind gusts was determined to be statistically significant at the 85% confidence level. The results of this t-test also indicated a high likelihood (85%) that the correlation represented a physical relationship between output BTD values and downburst magnitude.
The images above are scatterplots of Geostationary Operational Environmental Satellite (GOES) Microburst Windspeed Potential Index (MWPI) values (top) and GOES-11 imager output BTD values (K) (bottom) versus observed downburst wind gust speed as recorded by mesonet stations in Oklahoma and Texas between June 2007 and August 2009 (top) and mesonet stations in Oklahoma between June and August 2009 (bottom).The MWPI scatterplot identifies two clusters of values:MWPI values less than 50 that correspond to observed wind gusts of 35 to 50 knots, and MWPI values greater than 50 that correspond to observed wind gusts of greater than 50 knots.Similarly, the GOES-11 imager microburst risk scatterplot identifies two clusters. The dominant cluster contains output BTDs less than 50K that correspond to observed wind gusts between 35 and 50 knots. The scatterplots illustrate that both microburst products demonstrate effectiveness in distinguishing between severe and non-severe convective wind gust potential.
During the afternoon of 26 August 2009, strong convective storms developed along a weak, slow-moving cold front as it was tracking eastward over Oklahoma. Although there was very little temperature contrast across the front, the front acted as a convergence zone and a trigger for deep, moist convection. The pre-convective environment downstream of the cold front over western Oklahoma was dominated by vertical mixing that fostered the development and evolution of a convective boundary layer. Elevated Geostationary Operational Environmental Satellite (GOES) imager brightness temperature difference (BTD) values and Microburst Windspeed Potential Index (MWPI) values in the vicinity of downburst occurrence over western Oklahoma served as evidence of the presence of a well-developed mixed layer. Strong downbursts that were recorded by Oklahoma Mesonet stations between 0000 and 0100 UTC 27 August resulted from a combination of precipitation loading and sub-cloud evaporation of precipitation. These downbursts occurred in proximity to moderate to high microburst risk values as indicated in the 2200 UTC GOES microburst products. The images above are a Geostationary Operational Environmental Satellite (GOES) imager microburst product with overlying radar reflectivity from Oklahoma City NEXRAD (KTLX) at 0033 UTC 27 August (top) and a corresponding GOES sounder Microburst Windspeed Potential Index (MWPI) product at 2200 UTC 26 August 2009 (bottom), with the location of Oklahoma mesonet observations (i.e BESS, WEAT, etc.) of downburst wind gusts plotted on the MWPI image. Both product images displayed convective storms developing along the cold front over western Oklahoma. Convective storm activity increased in coverage near the cold front during the following three hours. Downburst wind gusts between 41 and 56 knots were recorded by the Oklahoma Mesonet stations plotted in the MWPI image above between 0000 and 0100 UTC 27 August. The following are significant downbursts recorded by the Oklahoma Mesonet during this event: Station - Time (UTC) - Wind Gust (knots) Bessie - 0005- 50 Kingfisher - 0020 - 43 Weatherford - 0030 - 41 El Reno - 0040 - 50 Medford- 0055 - 56
OO33 UTC NEXRAD reflectivity overlying the imager microburst product displayed downburst-producing convective storms in progress west of Oklahoma City in a region of elevated microburst risk (orange shading). Also important to note the general increase in MWPI values from southwest (BESS) to northeast (MEDF) associated with a progression from hybrid to stronger wet type downbursts. Downbursts over western Oklahoma were predominantly "hybrid" type, while over north-central Oklahoma (MEDF, BREC), downbursts were "wet" type associated with heavy rainfall. The two RUC sounding profiles above at 2200 UTC at Weatherford (top) and Medford (bottom), respectively, show contrasting downburst environments over Oklahoma. The Weatherford sounding, an "inverted v" profile, indicates an overall deeper and drier mixed layer over western Oklahoma that favored the development of intense downdrafts due to the evaporation of precipitation in the sub-cloud layer. The Medford sounding, with a shallower, moister mixed layer and larger CAPE, indicated that precipitation loading was a significant factor in downdraft generation. Thus, this cold front downburst event demonstrates that favorable environments can vary over a relatively small geographic region.
