Event Summary
     National Weather Service, Raleigh NC


February 12, 2001 Winter Weather Event and
the "Seeder - Feeder Mechanism"


Mike Moneypenny
Douglas Schneider

Background information on the February 12, 2001 Winter Weather Event.




Introduction



On February 12, an ice storm, consisting primarily of sleet, was expected over the RAH CWFA. The heaviest sleet was expected during the morning between 12Z and 18Z. In retrospect, the precipitation was considerably lighter and more of a mixed variety than expected due to poor model performance prior to the event. Easily lost in the larger scale, however, was a mesoscale snow outbreak, which occurred overnight prior to the main precipitation event. Upon examination, it was determined that the snow resulted from a very pronounced seeder-feeder mechanism in which low level supercooled clouds were seeded with ice crystals from above. This seeding modified the mid and low level temperature profiles, glaciated the low level clouds, and produced light snow for several hours in the foothills and northwest piedmont. Snowfall amounts were not large in RAH's CWFA, ranging from to 1 inch in the northwest piedmont, and were overshadowed by the larger scale icing event which evolved shortly thereafter. It is clear, however, that the seeder-feeder mechanism alone could easily produce an "advisory-scale" event, consisting of either frozen or freezing precipitation. The purpose of this paper is to explain how the seeder-feeder mechanism might be foreseen by providing a real-life and close-to-home example for future reference.

To review very briefly, the 'seeder-feeder' mechanism is the introduction of ice from above into a lower level liquid or supercooled liquid cloud. This introduction of ice provides condensation nuclei, thus initiating precipitation from the low level cloud layer. The low cloud layer may consist of 1) liquid droplets, 2) supercooled liquid droplets, or 3) if the temperature is cold enough (maximum low layer temperature of < 1C - figure 1) the cloud may glaciate, and produce snow. The resulting precipitation type is, of course, dependent upon the thermal profile from the cloud to the surface (as well as temperatures of exposed surfaces in the case of freezing rain).

The icing event is not the focus of this paper, thus we will not provide an analysis of synoptic scale features leading up to the event. The seeder-feeder mechanism, being mesoscale in size, would likely not be readily discernable beyond 24 hours. We will thus narrow our analysis to a time frame beginning 18 hours prior to the onset of precipitation at 12Z on the 11th, and concentrate mainly on forecast soundings and observed data.



Initial Mesoscale Conditions at 00Z - 2/12/01



From the attached zone forecast excerpt (figure 2), we were expecting the precipitation (sleet) to begin towards morning as late as the evening update on Sunday night, February 11. At sunset, western NC was blanketed by a uniform deck of stratocumulus based from 5 to 6 thousand feet, while an altocumulus deck around 10 thousand feet was approaching from the southwest. (figure 3). The 00Z Greensboro sounding (figure 4) was quite dry, and a significant portion of the sounding was warmer than 0C below 10 thousand feet. Of particular note is a warm (> 0) 2 to 3 thousand foot thick layer at the mid-levels (790-710 mb, ~7 to 10 thousand feet). The 00Z radar composite (figure 5) showed a narrow band of light rain over western SC, where Anderson was the only METAR site reporting rain. The rain was evidently in response to a small vorticity max, depicted on the 00Z Mesoeta over the Greenville-Spartanburg area, which was forecast to move quickly northeast across western NC. Temperatures at 04Z across the area were in the 40 to 45 degree range (figure 6) and, given significant precipitation amounts, would fall rapidly due to diabatic cooling as dewpoints were in the teens. This would produce wet bulb temperatures in the 30 to 35 degree range.



What Occurred Overnight



The light rain moved into the southern piedmont near Charlotte shortly before midnight. This initial surge of precipitation was not expected to be significant, as evidenced by the lack of its inclusion in the evening update. The precipitation's early arrival would not appear to pose a problem for the forecast of sleet, as diabatic cooling would quickly lower surface temperatures to freezing or below as the rain moved north of Charlotte into the foothills and northwest piedmont. Meanwhile, warm air advection, as evidenced by the veering wind profile at GSO (figure 4) and temperature profiles from downstream sites at Nashville and Atlanta (figure 7), would reinforce and even strengthen the warm air already in place at the mid-levels. We could then expect a changeover to primarily sleet as forecast. Catching us somewhat by surprise, the precipitation began to fall as snow (figure 8). Observations at Hickory and Statesville showed the precipitation beginning at 05Z as rain, then changing to snow by 06Z and remaining snow until around 09Z. At Greensboro, the precipitation began as snow at 08Z and continued until around 1030Z when it changed over to freezing rain, which lasted through the afternoon. The snow accumulated up to an inch in the northwest piedmont and foothills.



Diagnosing the 'Seeder-Feeder' Mechanism Responsible for the Snow



00Z Soundings

The GSO sounding at 00Z shown in figure 4 points towards a sleet/freezing rain scenario, as it exhibits an 80mb deep warm layer aloft (790 - 710 mb) as well as the potential for significant diabatic cooling at the surface. If we go beyond a cursory glance, however, there are a couple of subtle details to note as well: 1) The warm layer, while deep, is only 1 to 2 degrees above freezing and very dry - thus we should expect diabatic cooling here as well; 2) we see the initial hint of mid-level moisture at 560 mb, just above the -10C isotherm (snow formation likely); 3) the maximum warm layer temperature (note - the warm layer is the lower cloud layer, not the entire boundary layer) is around -2C at 800 mb.

Upstream soundings at Nashville and Atlanta in figure 7 show that the mid- level moisture which will be arriving in the CWFA has a base around 750 mb (~ 8 kft) and is extremely deep. Notice also that the -10C isotherm is around 550 mb, so little thermal advection will occur at this level downstream at GSO.



06Z GSO Sounding

At the 06Z sounding (figure 9), the mid-level moisture has deepened and now has a base below 600 mb, with the vast majority of the layer colder than -10C. The mid-level melting layer has cooled to less than 0C as well, and only the surface layer remains above freezing.

At this point, the low level cloud has glaciated and snow began to fall in GSO shortly before 07Z. METAR observations of ceilings indicate that the mid-level altocumulus deck had progressed to GSO by 06Z. A few hours prior, rain had begun in the Charlotte area around 04Z, shortly after the arrival of the mid-level cloudiness. It is apparent that the mid-level cloudiness was responsible for seeding the clouds below, thus causing the unexpected surge of precipitation prior to the principal, more widespread precipitation. Approximately one hour separated the arrival of the mid-level cloud deck and the onset of snow at GSO.



Comparison of Observed Soundings and ETA Forecast Soundings



Forecast soundings from the ETA model's 12Z run on February 11 and 00Z run on February 12 were analyzed and compared to the actual soundings that were taken during the event. The presence of a warm nose, any above freezing layers, low level winds, and dry layers were noted.

The 12Z model sounding initialized well with a warm nose between 850 and 700mb and little moisture in the sounding (not shown). At 18Z (figure 10), the model sounding correctly showed increasing moisture, especially above 400mb, and continued to show a deep above-freezing layer. The model sounding did not show the stratocumulus deck at 810mb.

Figure 11 compares the ETA 12 hour forecast sounding valid at 00Z with the observed sounding at GSO. The observed sounding showed that the upper level moisture (above 400mb) continued to increase, with a thin moist layer evident at 560mb. This layer was the leading edge of the mid-level cloud deck that was approaching from the southwest. The model sounding underestimated the moisture above 400mb, and did not pick up on the approach of mid-level cloud deck from the southwest at 560mb. The observed sounding showed the presence of a stratocumulus deck at 800mb, while the model had a 15 degree dewpoint depression at this level. A dry 2C warm nose remained between 800 and 700mb in the forecast sounding. This compared well to the actual 00Z sounding, although the model greatly underestimated the amount and depth of the dry air from 800 to 600mb.

These model inaccuracies had large implications on its p-type forecast. The observed cloud layer that was approaching from the southwest near 560mb, which was not depicted in the model sounding, began to precipitate into the dry warm nose between 00Z and 06Z. This caused evaporative cooling in that layer, and reduced the temperature of the warm nose to near freezing isothermal. Because the model underestimated the amount of dry air that the precipitation was falling into (between 800 and 600mb), the model underestimated the potential for evaporative cooling in that layer. The precipitation from the mid-level clouds proceeded to seed the stratocumulus deck at 810mb with ice, which later produced snow. Since the model did not account for the stratocumulus deck, the mid-level cloud layer, or the very dry air between 800 and 600mb, it did not correctly portray the seeder-feeder process.

The 6 hour forecast sounding valid at 06Z is shown in figure 12. At this time, the model did pick up on the stratocumulus deck near 810mb and the precipitating mid-level cloud layer. At 06Z, the actual sounding (figure 9) showed an isothermal layer at 0C between 750 and 700mb with the rest of the sounding below freezing, except at the surface. However, the model showed that the layer was not near-freezing isothermal; it remained near 2C. Because the model forecast the layer to be too warm (underestimated the amount of evaporative cooling), the model p-type forecast of sleet and freezing rain at 06Z was incorrect. A warm nose between 1C to 3C warm nose will produce a snow/sleet mix if ice is introduced, and freezing rain if ice is not introduced (see figure 1, which was taken from the VISIT training session "P-Type Forecasting - The Top-Down Approach"). Since the ETA did not account for the mid-level cloud layer that introduced ice into the stratocumulus deck between 00Z and 06Z, the model did not account for ice seeding into the lower cloud layer early enough, and freezing rain was the predominant p-type that was forecast by the model.



Evaluation of partial thickness scheme and TRENDS



The 1000-850mb thicknesses and 850-700mb thicknesses were computed from the actual soundings and compared to the ETA model forecast thicknesses. The ETA forecast thicknesses were taken from the model run closest to that time. The partial thicknesses from the observed soundings and the forecast soundings were plotted on a partial thickness nomogram. The observed thickness nomogram correctly portrayed snow changing to mainly freezing rain and a little sleet. However, the model forecast thicknesses painted a different p-type scenario - one that was dominated by only freezing rain and sleet. Very slight differences in the observed and forecast soundings led to the inaccurate model forecasts.

The following chart compares the actual partial thicknesses and their corresponding p-type based on the nomogram with the model forecast thicknesses and p-type:





Observed thickness ETA forecast thickness Observed p-type from nomogram ETA forecast p-type from nomogram
18Z 2/11 1297/1556 1292/1561 Mostly FZRA, trace PL Mostly FZRA, trace PL
00Z 2/12 1305/1553 1305/1557 Mostly FZRA, trace PL Mostly FZRA, trace PL
06Z 2/12 1298/1549 1294/1557 Snow if isothermal near 0 Mostly FZRA, trace PL
12Z 2/12 1294/1551 1292/1552 Mostly FZRA, trace PL Measurable PL w/ FZRA



Figure 13 shows the low level thicknesses from the observed soundings plotted on a nomogram. The corresponding ETA model thicknesses were plotted on a separate nomogram, which is shown in figure14. Note that at 18Z, the ETA overestimated the thickness of the above-freezing layer and underestimated the low level thickness. Although the model correctly lowered the thickness in the above-freezing layer at the initialization time of 00Z, it was still too strong compared to reality. From 00Z to 06Z, the observed 850-700mb thickness decreased slightly, from 1553m to 1549m, placing the sounding in the "snowy nose" category of the nomogram. In this area of the nomogram, the p-type will be snow if a near-freezing isothermal layer is present; otherwise freezing rain and sleet will be the dominant p-type. In the observed sounding at 06Z, a near-freezing isothermal layer was present, and snow was able to reach the ground. At the same time, the model forecast 850-700mb thickness did not change, while the 1000-850mb thickness dropped by 11m. A near-freezing isothermal layer was not present in the 06Z forecast sounding. Instead, the ETA forecast sounding showed a 2warm nose between 800 and 700mb. This overestimation of the above-freezing layer led to an incorrect model p-type forecast of mostly freezing rain with a trace of sleet. The model overestimated the amount of low-level cold advection that was occurring and overestimated the temperature of the above-freezing layer, leading to a freezing rain and sleet forecast.

Between 06Z and 12Z, the model forecast 850-700mb thickness dropped 5m, while the observed drop in 850-700mb thickness was between 00Z and 06Z. The model may have been too slow to forecast evaporative cooling in this layer, which was due to the absence of the mid-level clouds that approached GSO at 00Z and precipitated into the above- freezing layer.

The partial thickness nomogram performed very well in predicting p-type, even though the thickness changes were very subtle. While the ETA forecast thicknesses were close to the observed thicknesses, they were off just enough to cause an incorrect p-type forecast. The model underestimated the effects of evaporative cooling in the warm nose layer because it did not account for the presence of the mid-level clouds, which precipitated into the layer and seeded the stratocumulus deck.



Conclusions



The feeder-seeder mechanism can be diagnosed by carefully examining the entire depth of the soundings. Very small changes in the vertical temperature profile can profoundly affect the p-type, and models often cannot resolve these details. Therefore, observed data must be analyzed intensively to improve the forecast. Comparing the differences between the observed sounding and the forecast sounding is critically important to determine when the model is deficient or accurate. The presence of any cloud layers or dry layers that the model is not accounting for must be carefully considered when making a p-type forecast.

This sort of event is not particularly difficult to diagnose beforehand - IF one is familiar with the mechanism responsible and thoroughly analyzes the observed and forecast soundings accordingly. This case study was drafted for such a purpose - to show that the seeder-feeder mechanism does exist and can cause potentially significant precipitation type problems, and that it can be forecast, at least in the short term.




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