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AUGUST 31, 1993

NATIONAL CLIMATIC DATA CENTER

RESEARCH CUSTOMER SERVICE GROUP

TECHNICAL REPORT 93-03

WATER EQUIVALENT VS RAIN GAUGE MEASUREMENTS

FROM THE MARCH 1993 BLIZZARD

NEAL LOTT

PHYSICAL SCIENTIST
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INTRODUCTION

 On March 12-14, 1993, the eastern seaboard of the U. S.

was struck by what is now referred to as: 1) The Storm of

the Century, 2) The Blizzard of 93, or 3) The Big One! One

of the interesting and more overlooked aspects of the storm

was the discrepancy between liquid water measurements by the

rain gauge and water equivalent 'core samples' of the

snow/ice on the ground. In examining these data, I found

that many stations appeared to either have a problem with

'undercatch' of snowfall by the rain gauges, or a systematic

problem with water equivalent measurements. This report

will attempt to show that: 1) The most likely scenario is a

problem with 'undercatch' of snowfall, and 2) the careful

measurement of water equivalent is an important element for

hydrological interests and the climatic records.

 Water equivalent measurements are taken by extracting

core samples, or slices, of the snow/ice on the ground, and

then melting the sample to calculate the water equivalent of

the snow/ice. The measurement can be done carefully with a

rain gauge (e.g., an 8-inch sample of 16 inches on the

ground melts to 1 inch of water, indicating an 8-1 ratio and

2 inches of water equivalent). It is very important that

the sample be representative of the full profile of the

snow/ice depth. Care must be taken not to compress or

compact the snow for the sample, since the density of snow

in the sample should be the same as that on the ground.
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 The problem with 'undercatch' of snowfall by rain gauges

has been documented in previous studies (Larson and Peck,

1974; Peck, 1972). These studies have established empirical

relationships between gauge measurements, water equivalent

measurements of snow on the ground, and wind speeds. From

these relationships and field testing of the ensuing

equations, unshielded gauges were shown to 'undercatch'

precipitation by 70% or more (40% or more for shielded

gauges) during snowfall events with wind speeds of 20 MPH or

higher. Suffice it to say that this storm presented an

excellent opportunity to observe this problem.

DISCUSSION

 The accompanying table presents statistics on the

measurements made during the storm at 40 stations. By

carefully studying the table, several aspects become readily

apparent. These aspects are discussed below.

 The snowfall storm totals (INCR1) were quite

substantial, with totals of 1 to 2 feet being common

throughout much of the eastern U.S. These totals are based

on snow depth reports before, during, and after the storm,

in order to better correlate them with water equivalent

'core samples' taken at about the same times. Generally,

the greatest depths were observed on the 14th.
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 The water equivalent storm totals (INCR2) were also

unusually high for a snow event in this part of the country,

with amounts of 2 to 4 inches being rather common. If these

totals are accurate, then the 'true' amounts of liquid

deposited by the storm should be equal to or greater than

those shown in the INCR2 column. In other words, the 'true'

liquid total from a storm is always equal to or greater than

the final water equivalent measurement. The observer's

measurement of the greatest water equivalent amount (WTEQ2)

does not account for melting of snow/ice near a ground

surface with above freezing temperatures at the beginning of

the storm. Since this storm was a late season event, most

of the affected areas did have above freezing ground

temperatures at storm onset, and, in some cases, several

hours of snow fell before actual accumulation began. Then,

as heavy snow fell and accumulated, some melting from

underneath occurred throughout the storm at many locations.

 This affect can be seen in looking at the synoptic-hour

observations (every three hours) of snow depth, which show a

decrease in snow depth for some stations during the hours

immediately after the storm--with temperatures still well

below freezing--an indication of melting from underneath.

Also, the true amount of liquid would tend to be higher than

'INCR2' since most stations reported some additional

snowfall after their report of the maximum water equivalent

amount (WTEQ2).
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 'RATIO1' is a measure of the apparent undercatch by the

gauges (rain gauge vs water equivalent), although the

factors mentioned above have to be considered. For the 28

stations reporting 'SNOW' as the predominant precipitation

type, 4 are within a range of .01 to .40, 15 are within .41

to .80, 8 are within .81 to 1.20, and 1 is over 1.20. In

light of previous studies mentioned above ('undercatch'

precipitation by 70% or more...), these figures are not

surprising since wind speeds generally averaged 10-30 MPH at

most stations during the heavy snowfall. Also, they show

more than half (15) of the values to be within the second of

these four ranges.

 Some of the snow/water ratios (RATIO2) seem quite low

(i.e., high water content). However, given that this was a

very deep 'spring-type' storm which drew in large amounts of

Atlantic and Gulf of Mexico moisture, it is likely that the

snow had a higher liquid content than would be expected in a

'normal' winter storm. Unfortunately, I am not aware of any

studies which have attempted to correlate the severity and

type of storm system with the resulting liquid water content

of snowfall. Such a study is not within the scope of this

paper.

 Although there were a wide range of temperatures among

stations during the heavy snowfall, the general range was

25-34 degrees Fahrenheit at storm onset, falling to 10-25

degrees Fahrenheit when heavy snowfall ended. Dew point
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depressions averaged about 2 degrees Fahrenheit throughout

the storm. The dew points are probably a better indicator

of the liquid content of snowfall in a storm system with

large amounts of moisture entrained, since they give us a

'reading' of atmospheric moisture content. Historically,

dry bulb temperatures at the surface have been used as a

'rough' indicator of liquid content, with readings of around

30 degrees Fahrenheit indicative of 'wet snow.' However, a

30-degree dry bulb with a 25-degree dew point would tend to

indicate less liquid content than a 30-degree dry bulb with

a 28-degree dew point.

 In looking at this aspect further, I chose 9 stations

along the Appalachian chain where geography and the synoptic

situation would be similar for this event. The table below

presents each station's mean dew point temperature during

the time of moderate to heavy snow (i.e., when most of the

snow fell) as compared to the snow/water ratio (RATIO2).

 STATION MEAN DEW PT RATIO2

 Asheville NC 30 4.2

 Hickory NC 29 6.0

 Charleston WV 24 6.8

 Roanoke VA 27 7.6

 Huntington WV 23 7.9

 Beckley WV 20 8.1

 Elkins WV 18 9.5

 Pittsburgh PA 20 11.9
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 Binghamton NY 14 14.0

 One would expect that as the dew points fell, the

snow/water ratios would increase. The table shows this to

generally be true. To investigate this further, the

correlation coefficient can be calculated which will

estimate the affect of the mean dew point on the snow/water

ratio. A 'perfect' correlation here would be -1.00 since a

decrease in the mean dew point yields an increase in the

snow/water ratio. The actual correlation coefficient for

these values is -.90, and 80.3% of the variation in

snow/water ratio is accounted for by a linear relationship

with the mean dew point. Also, by restricting the list to

Asheville, NC plus the four West Virginia stations (five

stations with very similar terrain influences), the

coefficient 'improves' to -.98, and 96.1% of the variation

in snow/water ratio is then accounted for by the

relationship with mean dew point. Although this cannot be

said to validate the water equivalent measurements for these

stations, it certainly adds more credibility to the values.

 Other factors that may come into play in a storm such as

this would be the proximity of the station to large bodies

of water, and the location of the station to the

leeward/windward side of mountain ranges. However, in my

evaluation of the data (for all 40 stations), I did not find

any direct correlations with these factors. As for the

distribution of 'RATIO2' values for the 28 'SNOW' stations,
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3 are within the .1 to 4.0 range, 13 are within 4.1 to 8.0,

10 are within 8.1 to 12.0, and 2 are over 12.0. Therefore,

nearly half (13) have ratios of from 4.1 to 8.0. This is

certainly a significant departure from the 10.0 ratio that

is commonly used when an actual measurement is not taken.

 The inconsistency of reports between stations is

especially apparent for 'RATIO1' and 'RATIO2.' As the

distributions shown above indicate, the data do show some

clustering of values within certain ranges. However, there

is considerable variability in the data and spatial (i.e.,

geographic) clustering cannot be shown. A few cases of

large differences over short distances (e.g., Rochester vs

Syracuse, NY) may indicate a problem in the validity of a

few of the values. Some of these variations can probably be

explained by geographic/terrain differences; snow melt

during the storm at some locations (melting underneath

mentioned above); and possibly some subtle differences in

observing practices between stations.

 Other important considerations are the wind speeds

associated with the storm and the existence (or not) of

windshields for each of the gauges. Some of the National

Weather Service (NWS) stations in the Northeast are equipped

with windshields, while most in the Southeast are not. Wind

speeds generally averaged 10-30 MPH for most stations during

the moderate to heavy snowfall, but were quite gusty.

Estimating an average wind speed for the storm for each
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station is of questionable value due to this gustiness, and

since the wind's affect on the gauges would depend on the

gauge exposure. However, data for mean wind speeds and the

existence of windshields are included in the data table (see

WND and SHLD columns) to provide some indication of how

these data correlate with the 'undercatch' of the gauges

(RATIO1). Following are correlation coefficients calculated

from these data by correlating 'RATIO1' with 'WND':

 19 'SNOW' stations without windshields = -.39

 9 'SNOW' stations with windshields = -.33

 Here, a 'perfect' correlation would be -1.00 since

'RATIO1' should decrease as 'WND' increases. These poor

correlations may be due not only to the factors mentioned

above, but also to variations in snow density (i.e., snow

weight, which is directly proportional to water content and

inversely proportional to 'RATIO2'). Heavier, wetter snow

would tend to be less affected by the wind than drier snow.

However, I am not aware of any previous research of this

effect. Correlations can be calculated using groups of

stations with similar values for 'RATIO2' and/or similar

geographical influences, but I found most combinations to

only yield coefficients of between -.30 and -.60. It is my

opinion that these low correlations are the result of the

uncertainties cited above as well as possible

inconsistencies in reporting practices among the various

stations.
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CONCLUSION

 Of the 28 stations which reported 'SNOW' as the

predominant precipitation type, 23 (82%) show gauge 'catch'

lower than the water equivalent storm total (see RATIO1

column). However, of the 12 which reported 'SNOW, IP' or

'MIXED' as the main type, only 4 (33%) show gauge 'catch'

lower than the water equivalent storm total. Therefore,

those stations which received less snowfall and more ice

pellets/rain showed a much lower tendency for 'undercatch.'

Also, 15 of the 28 'SNOW' stations (54%) have a RATIO1 value

of from .41 to .80, with a mean for all 28 stations of .68.

These statistics indicate significant 'undercatch' of

snowfall by the gauges.

 The mean for 'RATIO2' for 'SNOW' stations is 8.2. This

seems to indicate a rather wet snow as compared to the

typical 10-1 ratio that we're accustomed to using. (Perhaps

it's time to reevaluate this 'typical' ratio, since snowfall

often has a somewhat lower or higher ratio than 10-1.)

These statistics also point to a need for further study of

the methods used in measuring water equivalent of snow/ice

on the ground, as the variability of the data indicate

possible problems with a few of the values.

 However, this should not diminish the importance of

water equivalent measurements/data for climatic records and
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for hydrological interests (river forecasting, etc.). In

fact, considerable flooding occurred in parts of the eastern

U.S. shortly after the storm mainly due to snow melt. In

'extreme events' of this nature, it would be wise for

hydrologists and climatologists to take note of how the

water equivalent reports compare with the rain gauge

reports. This is especially true for a month such as March

1993 when this event contributed greatly to the month's

precipitation total (based on rain gauge measurements), but

where the 'official' totals for the month probably fell

significantly short (20% or more in some cases) of the

actual liquid amounts received. In summary--the water

equivalent reports are very important for the climatic

records--not only to get a true picture of the liquid

amounts received, but also to provide a baseline for

studying the problem of 'undercatch' by gauges.

 As to what can be gained from all of this, I suggest the

following:

 a. Although water equivalent has not been one of the

more used/studied meteorological elements, this storm is a

prime example of the usefulness of these data when measured

correctly. I would encourage future observing practice

standards (i.e., the FMH) to emphasize the measurement of

'core samples' (even in the age of automation). The

measurements are especially important to hydrological

interests and climatological records, as discussed above.
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 b. Recent studies have indicated that the Canadian

'Nipher' shield may be the best available due to its

structure allowing for better 'catch' of snowfall. The use

of this shield could be implemented on a 'test' basis at

several stations with frequent snowfall. Optimally, a

three-way test could be conducted with the Nipher shielded

gauge vs the standard NWS shield vs an unshielded gauge. Of

course, the gauge type would have to be the same in each

instance. These data could be compiled with follow-on

recommendations for precipitation measurements.

c. Some equations have been developed (Larson and Peck,

1974) for estimating the 'true' liquid amount during

snowfall events. These equations use (as input) the rain

gauge measurement and estimated average wind speeds. One

equation is used for shielded gauges and another equation

for unshielded gauges. This storm would be an excellent

case study for the application of these equations. Such a

follow-up study could add to the table shown in this report

by calculating:

1) Estimated 'true' liquid amounts using the

equations mentioned above.

2) Comparison of these estimates to the water

equivalent storm totals (INCR2).
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 This report has shown that the March 1993 "Storm of the

Century" presented an excellent opportunity to study the

problem of 'undercatch' of snowfall by rain gauges. Also,

it has shown that the accurate measurement of water

equivalent is important for both hydrological interests and

for the climatic records. In fact, these measurements

provide one of the bases for studying the afformentioned

'undercatch' problem. For further information about this

storm, you may contact the National Climatic Data Center

(phone 704-271-4800, fax 704-271-4876, internet

orders@ncdc.noaa.gov) in Asheville, NC. We have a complete

report about the storm, along with several digital datasets

of observations taken during the "Storm of the Century."

REFERENCES

L. Larson and Peck, E.L., 1974: Accuracy of Precipitation

Measurements for Hydrologic Modeling. "Water Resources

Research," 857-862.

Peck, E.L., 1972: Snow Measurement Predicament. "Water

Resources Research," 244-248.
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DATA TABLE

 MARCH 12-14, 1993 "STORM OF THE CENTURY"

 All stations which reported at least 1 inch of water equivalent

 of snow/ice on ground at some point during storm:

STATION DPTH1 DPTH2 INCR1 WTEQ1 WTEQ2 INCR2 DPTH3 GAUGE PRECIP RATIO1 RATIO2 WND SHLD

Albany NY 4 28 24 1.2 3.2 2.0 28 1.83 SNOW .91 12.0 20 YES

Allentown PA 0 18 18 0 1.7 1.7 16 2.01 SNOW 1.18 9.4 23 YES

Asheville NC 0 18 18 0 3.8 3.8 16 1.85 SNOW .49 4.2 15 NO

Beckley WV 0 30 30 0 3.7 3.7 30 2.00 SNOW .54 8.1 18 YES

Binghamton NY 13 35 22 3.2 4.7 1.5 34 1.00 SNOW .67 14.0 23 YES

Bristol TN 0 13 13 0 1.1 1.1 12 1.35 SNOW 1.23 10.9 15 NO

Buffalo NY 4 16 12 .3 2.0 1.7 15 .82 SNOW .48 6.5 26 NO

Burlington VT 13 31 18 2.6 3.7 1.1 25 .59 SNOW .54 10.9 18 NO

Caribou ME 20 36 16 6.5 11.8 5.3 36 1.05 SNOW .20 3.0 21 YES

Charleston WV 0 19 19 0 1.9 1.9 13 1.16 SNOW .61 6.8 13 NO

Chattanooga TN 0 20 20 0 1.8 1.8 18 1.44 SNOW .80 10.0 18 NO

Cleveland OH 3 11 8 .7 1.7 1.0 7 .55 SNOW .55 4.0 25 NO

Concord NH 6 23 17 2.2 4.5 2.3 17 .75 SNOW .33 4.8 17 NO

Elkins WV 1 19 18 0 1.9 1.9 19 1.22 SNOW .64 9.5 13 NO

Erie PA 4 17 13 .3 1.3 1.0 16 .70 SNOW .70 12.0 31 NO

Hickory NC 0 10 10 0 1.5 1.5 9 1.76 SNOW 1.17 6.0 17 NO

Huntington WV 0 22 22 0 1.9 1.9 15 1.08 SNOW .57 7.9 16 YES

Jackson KY 0 20 20 0 3.1 3.1 20 .47 SNOW .15 6.5 16 YES

Knoxville TN 0 15 15 0 1.6 1.6 12 1.49 SNOW .93 7.5 12 NO

Mansfield OH 2 9 7 .3 1.3 1.0 9 .51 SNOW .51 7.0 25 NO

Pittsburgh PA 0 25 25 0 2.1 2.1 25 1.12 SNOW .53 11.9 22 NO

Portland ME 17 34 17 4.4 6.1 1.7 34 1.58 SNOW .93 10.0 24 NO

Roanoke VA 0 16 16 0 1.7 1.7 13 1.97 SNOW 1.16 7.6 18 NO

Rochester NY 7 25 18 1.7 8.1 6.4 25 1.09 SNOW .17 2.8 32 NO

Syracuse NY 5 37 32 1.6 3.5 1.9 34 2.03 SNOW 1.07 15.3 20 YES

Wilkes-Barre PA 1 21 20 0 1.4 1.4 12 1.24 SNOW .89 7.9 18 NO

Williamsport PA 1 15 14 0 1.8 1.8 13 1.10 SNOW .61 6.7 15 NO

Worcester MA 9 26 17 3.2 6.1 2.9 26 1.23 SNOW .42 5.9 24 YES

Baltimore MD 0 9 9 0 2.3 2.3 9 2.48 SNOW, IP 1.08 3.9 25 NO

Hartford CT 0 16 16 0 2.0 2.0 16 2.05 SNOW, IP 1.03 8.0 18 NO

Philadelphia PA 0 12 12 0 1.5 1.5 11 1.80 SNOW, IP 1.20 7.3 31 NO

Washington-Dulles 0 13 13 0 1.8 1.8 13 1.55 SNOW, IP .86 7.2 19 NO

Boston MA 1 12 11 0 5.0 5.0 9 1.95 MIXED .39 1.6 44 NO

Bridgeport CT 0 10 10 0 2.2 2.2 9 2.64 MIXED 1.20 4.1 34 NO

JFK Apt NY 0 9 9 0 2.1 2.1 8 2.39 MIXED 1.14 3.8 28 NO

LaGuardia Apt NY 0 9 9 0 3.7 3.7 8 2.49 MIXED .67 2.2 35 NO

Newark NJ 0 13 13 0 2.7 2.7 13 2.81 MIXED 1.04 4.8 23 YES

Providence RI 0 6 6 0 2.0 2.0 4 2.58 MIXED 1.29 2.0 23 NO

Washington-Natl. 0 6 6 0 1.3 1.3 5 2.31 MIXED 1.78 3.8 20 YES

Wilmington DE 0 10 10 0 2.4 2.4 9 2.33 MIXED .97 3.7 21 NO

KEY:

DPTH1 = Snow depth in inches before the storm (on March 11).

DPTH2 = Greatest snow depth in inches during storm.

INCR1 = Snowfall storm total in inches (DPTH2 - DPTH1) as calculated by

 subtracting the depth before the storm from the greatest depth

 reported. The actual snowfall totals may be slightly higher.

WTEQ1 = Water equivalent of snow/ice on ground before storm (on March 11)

 in inches and tenths.

WTEQ2 = Greatest water equivalent of snow/ice on ground during storm

 in inches and tenths.
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INCR2 = Water equivalent storm total (WTEQ2 - WTEQ1) in inches and tenths.

DPTH3 = Snow depth in inches at time of WTEQ2 report.

GAUGE = Liquid 'catch' by rain gauge (storm total) in inches and hundredths.

PRECIP = Predominant precipitation type during storm:

 IP = ice pellets

 MIXED = snow, ice pellets, and rain

RATIO1 = GAUGE / INCR2. This is the amount of precipitation caught

 by the gauge as a proportion of the WTEQ storm total. In

 theory, this value should always equal or exceed 1.00

 since melting from underneath the snow cover (due to above

 freezing ground temperature) and precipitation after the

 WTEQ2 report are not accounted for in the INCR2 column.

 In effect, this is an estimate of the gauge 'undercatch.'

RATIO2 = (DPTH3 - DPTH1) / INCR2. This is a measure of the water

 content of snow/ice from the storm by calculating the ratio

 of snow/ice accumulation to water equivalent. DPTH3 minus

 DPTH1 is the storm's snow/ice total at the time of the water

 equivalent measurement used for calculating INCR2.

 In effect, this is an indication of the average weight of the

 snow and/or ice from the storm. Over the years, stations not

 taking water equivalent measurements have often assumed a

 value of 10.0 for this calculation.

WND = The average wind speed in MPH during the storm for the period when

 moderate or heavy snow and/or ice pellets were reported.

SHLD = The existence (YES or NO) of a windshield for the rain gauge.