General Information - GOES Atmospheric Soundings Display


The intention of this document is to give the user a detailed look at data being provided on the Skew-T diagrams being generated from GOES Sounder data at the Forecast Products Development Team (FPDT), NOAA/NESDIS.  A vast array of information is presented on these charts and the products can be used for any number of purposes. One caveat, however, is that this is a purely experimental product,and while this certainly does not imply low quality, it does mean that for brief periods data quality could degrade while science tests are ongoing. Also, data outages are possible since the Sounding System running at FPDT does not have around-the-clock support.

 OVERVIEW

GOES Soundings are produced once per hour. Both GOES-8 and GOES-9 each have two Sounder sector scans per hour. Atmospheric Soundings are produced from all of these four scans. Gradient winds (essentially geostrophic winds, since they are generated from the height field gradients) are then produced at ten levels for each of the sectors. The gradient winds are then co-located with each sounding, which is followed by a parameter generation program which accesses this data and produces a vast array of information pertaining to both the GOES soundings and the AVN first guess sounding.

GENERAL CHART DESCRIPTION

The GOES atmospheric temperature and moisture soundings are plotted on a 'SkewT-LogP' Chart. The chart's name reflects the parameters associated with the vertical and horizontal axes. Atmospheric pressure (in millibars) is plotted along the vertical axis using a logarithmic scale,

which represents the variation of atmospheric pressure with height observed in the earth's atmosphere. Atmospheric temperature is increasing from left to right along the horizontal axis. The lines of constant temperature are 'skewed' from the lower left to the upper right of the chart. The GOES soundings use an initial estimate of the vertical temperature and moisture profile obtained for the AVN numerical forecast model. This model is maintained and run by the National Centers for Environmental Prediction (NCEP), Environmental Modelling Center (EMC). The infrared radiation measured by the GOES sounder, for 18 different frequencies, is used to adjust the initial 'first guess' AVN profiles. Information of the magnitude and direction of these adjustments is useful for weather analysis and forecasting activities. For this reason, both the AVN 'first guess' sounding and the GOES satellite'adjusted' sounding. The darker red, solid line is the GOES atmospheric temperature as it varies with height, and the brighter red, solid line the AVN forecast temperature profile. The darker blue, dashed line is the GOES dewpoint temperature, and the lighter blue, dashed line the AVN forecast dewpoint temperature profile. The green line represents the temperature profile of a lifted parcel of low level air, based on the GOES data. The shaded in purple region denotes the area where that lifted parcel would be positively buoyant. In contrast, the area shaded in light blue signifies the area where the lifted parcel would be negatively buoyant. The thin black lines and numerical values along the right side of the chart represent 'gradient' wind estimates derived from the GOES sounding fields. Several stability, moisture and meteorological parameters derived from the GOES and AVN soundings are listed on the right-hand portion of the chart (described in detail below)  Further to the right, the actual numerical data from the GOES derived temperature and moisture profiles is shown.  Finally, since the GOES Soundings are not necessarily centered immediately at a given station location, directly underneath the position of the GOES Sounding, with respect to the station, is listed.
 
 

DETAILED INFORMATION

Pressure

Atmospheric pressure, in millibars (MB), is represented by the vertical axis of the chart. Values range from 1050MB to 100MB. Average surface atmospheric pressure is 1013MB. Decreasing pressure along the vertical axis represents increasing height above the earth's surface. Some reference values of pressure and altitude are given below, using average atmospheric conditions.
        850MB       1.4 Kilometers       4,400 Feet

        700MB       3.0 Kilometers       9,500 Feet

        500MB       5.5 Kilometers     17,500 Feet       

        250MB     10.5 Kilometers     33,300 Feet

        100MB     16.0 Kilometers     50,700 Feet

Temperature and Dewpoint (Moisture) Profiles

The dark red line represents the GOES satellite temperature profile as a function of height (decreasing pressure). In the example above, the atmospheric temperature is 22 Degrees Celsius (C) at the surface (1019mb), and decreases to -68C at 100mb (~50,000 feet).

The brighter red line represents the AVN forecast temperature profile as a function of height. In general, the GOES soundings tend to modify the dewpoint (moisture) profile more than the temperature profile. In the example above, there are no changes in the forecast temperature profile and, therefore, the AVN temperature profile cannot be seen beneath the GOES profile.

The dark blue dashed line represents the GOES dewpoint profile as a function of height. The dewpoint is the temperature at which condensation would occur as the air is cooled. Warmer dewpoint temperatures correspond to moister atmospheric conditions, and cooler dewpoints to dryer atmospheric conditions.

The brighter blue dashed line is the AVN forecast dewpoint profile. In the example above, the GOES satellite data indicated wetter conditions (as compared to the AVN forecast) for the entire profile below 450mb.

The solid green line represents the temperature a parcel of low level air would have if it were lifted through the atmosphere.  The temperature of this parcel drops dry adiabatically until it is saturated.   Thereafter, as the parcel is lifted upward its temperature falls moist adiabatically.  In the example above notice that through most of the atmosphere this lifted parcel remains warmer than the actual temperature profile.  The parcel represented by this line is the parcel derived from the GOES data.
 

Gradient Winds

The black lines at the right edge of the chart represent the "gradient" winds, in knots. The wind direction is depicted by the angle of the line. A line pointing to the left indicates a wind from the west, a line pointing up a wind from the north, a line pointing to the right a wind from the east, and a line pointing down a wind from the south. These winds are calculated using horizontal fields of geopotential heights derived from GOES soundings. By measuring gradients in temperature and moisture at a particular pressure level, atmospheric wind estimates can be derived.

It should be noted that since these winds are computed from fields of GOES soundings, they may not always be available.  In situations where extensive cloudiness surrounds the GOES sounding, there may not be sufficient data to derive a valid field.  In these cases, the gradient wind information will not be displayed.

In the example above, the gradient wind estimates are as follows:

      1000MB          NW, 9 Knots

        850MB          WSW, 9 Knots

        700MB          WSW, 17 Knots

        500MB          SW, 42 Knots

        400MB          SW, 50 Knots

        300MB          SW, 60 Knots

        250MB          SW, 67 Knots

        200MB          SW, 73 Knots

Chart

Diagonal orange lines extending from the lower left to the upper right represent air temperature, in Degrees Centigrade. The values range from -90C to +40C.

Diagonal brown lines extending from the lower right to the upper left depict the change in temperature with height as a parcel of dry air rises, known as the 'dry adiabatic lapse rate'. This is approximately 10 Degrees Centigrade per Kilometer. For example, a parcel of dry air starting at 1000MB with a temperature of 10C and rising to 700MB, will have cooled to approximately -18C.

Dashed gray, curved lines extending from the bottom of the chart and curving to the upper left are 'moist adiabats'. These represent the change in temperature with height of an air parcel which is saturated (relative humidity = 100%, usually a cloud). Note that the decrease in temperature with height is less than that for dry air. This is due to heat being released from the condensation process as the cloud forms, which slows the cooling rate of the parcel as it rises.

Dashed gray lines extending from the lower left to the upper right are lines of contant mixing ratio, which is measured as grams of water vapor per kilogram of dry air. The mixing ratio is an absolute measure of the amount of water vapor in the atmosphere.

The information above the chart includes the location (Southwest Pass, LA), and to the right of that, the 4 character identifier code (BURL), the day, month and year (9 DEC 99) and the time (20GMT, or Greenwich Mean Time). In this example, the day is the 9th day of December, 1999 and the time, 20GMT, is 3PM Eastern Standard Time. GMT, or Greenwich Mean Time, is one of the time standards used in meteorology, and is 6 hours ahead of CST (4 hours ahead of EDT) - 00GMT is equal to 8 pm EDT (7pm EST), 12GMT is 8am EDT (7am EST). You might also see GMT listed as Z, Zulu time, or UTC, Universal Coordinated Time.

Parameters

Several parameters are listed along the right hand side of the chart. The first line; 'PARAM GOES AVN' identifies the three columns of information, i.e. parameter, value derived from the GOES sounding, and the value derived from the AVN forecast profile, which is used as a first approximation.

 The various parameters available for each sounding are:

 TIME - The exact time of the sounding, (when the GOES sounder was scanning the particular location). In this example, the time is 2009Z (20 hours, 9 minutes), or 3:09am EST, and is the same for the GOES sounding and the AVN forecast. NOTE: The AVN forecast is available at 6 hour intervals. Therefore, two forecast times (in this case, 18Z and 00Z) are linearly interpolated to the time of the GOES scan.

 ELEV - The surface elevation above sea level, in meters (not necessarily the same as the surface elevation of the station). The actual location of the sounding can be up to 30 nautical miles from the station; this can be critical in regions of greatly varying terrain. This information is given below the sounding plot.

 PARP - Pressure of a "parcel of air" used to determine some stability parameters. An initial parcel is determined by examining the lowest three levels of the atmosphere. The pressure level with the most potential buoyancy (i.e., the highest equivalent potential temperature) is determined to be the best parcel for determining stability. Using this parcel, rather than a simple surface parcel is critical, especially for morning soundings when a surface inversion can reflect unrealistically stable conditions. Note that in the example above, while the surface pressure is 1019mb, the parcel chosen is from the 950mb level.

 PART - The temperature (C) of the parcel being lifted for stability calculations.  This is simply the actual profile temperature at the PARP pressure level.

 PARD - The dewpoint temperature (C) of the parcel being lifted for stability calculations. It is important to note that the moisture is mixed from the surface to the parcel level. Therefore, while the temperature of the parcel (PART) is exactly the same as the profile temperature at the parcel pressure, the dewpoint of the parcel (PARD) may very well not be equal to the profile dewpoint at the parcel pressure level.

 TSKIN - The surface skin temperature(C).  This is the estimated temperature of the ground surface as derived from the GOES satellite (note that no value is available for the AVN).

 PW - The total precipitable water (millimeters) derived from the soundings. The TPW is a measure of the liquid content of a vertical column through the atmosphere.In this example, the AVN forecast TPW is 30mm, and the GOES sounding TPW is 36mm. This is consistent with the increased dewpoint temperatures of the GOES sounding in the profile. Precipitable water estimates are one of the most important products generated from GOES sounder data. While the changes to the AVN forecast temperature profile using GOES sounder data are usually small, large changes to the AVN forecast dewpoint profile and resulting TPW occur frequently. Overall statistics as well as case studies have consistently showed the GOES TPW values to be more accurate than the first guess TPW. As such, examining the TPW values from the GOES and AVN is useful in adjusting forecasts of severe weather, cloud cover and precipitation.

 L.I. - The Lifted Index is calculated by lifting (frontal, orographic, upper air dynamics, etc.) a parcel of air dry adiabatically while conserving moisture until it reaches saturation. At that point the parcel is lifted moist adiabtically up to 500mb. The Lifted Index is the ambient air temperature minus the lifted parcel temperature at 500mb. If the parcel is warmer than the environment (negative L.I.), it has positive buoyancy, and will tend to continue to rise, favoring convection. L.I. values less than -5C indicate very unstable conditions. A positive L.I. value indicates negative parcel buoyancy, and the parcel will tend to sink. This is representative of stable conditions where convection is unlikely. Increasingly negative numbers correspond to increasing instability and likelihood of severe weather. At times, very high (stable) lifted index values in cold air are indicative of frozen or freezing precipitation versus rain during warm advection events. The extreme stability does not allow air to lift out, resulting in cold air "damming", which restricts the advance of warm air at the surface.

 CAPE - Convective Available Potential Energy, a measure of the cumulative buoyancy of a parcel as it rises, in units of Joules per kilogram. CAPE values larger than 1000J/kg represent moderate amounts of atmospheric potential energy. Values exceeding 3000J/kg are indicative of very large amounts of potential energy, and are often associated with strong/severe weather.  Graphically, the CAPE is the positively buoyant area (shaded purple) on the skew-t diagram.  It is important to note, however, that for the purposes of this CAPE calculation only the lowest positively buoyant region is included.  There may be times when a small negatively buoyant region may break up two positive areas.  This is critical, especially if the lower positive area is larger than the negative area.  In this case, disregarding other outside influences, the parcel would have enough buoyancy from passing through the lower positive region to successfully pass through the entire negative region and back into the high positive area.  In a case such as this, the listed CAPE value will be deceivingly low and visual examination of the areas of positive and negative buoyancy are very important.

 NCAP - Normalized CAPe is a measure of the structure of the positively buoyant parcel.  The NCAP is equal to the CAPE divided by the depth of the positive area.  This value is equal to the acceleration of the parcel (cm/s^2).  Therefore, a high CAPE value over a great depth will result in a slowly accelerating parcel, whereas, a high CAPE value over a shallow depth will result in a much greater parcel acceleration.  The NCAP value can be helpful in determining the potential for tornadic activity.  High NCAP values (>20) are typical for such severe cases; lower NCAP values (roughly, 10-15cm) in combination with high CAPE values typically indicate conditions more conducive for heavy rain and, possibly, hail.

 MXHAIL - MaXimum HAIL, is a rough estimate of the maximum hail size that can be expected (cm).  Given the acceleration (NCAP), disregarding outside vertical forcing, one can calculate the parcel speed at the top of the positively buoyant layer of the atmosphere.  The fall velocity of hail can be roughly estimated as a function of size.  As such, using a fall velocity equal to the parcel's maximum velocity can yield a prediction of hail size.  Of course, with various unknowns in this calculation (outside vertical forcing, formation level of the hail, etc.), it is a very rough approximation and is intended to be on the high side, representing the maximum possible hail size under the given conditions.  Note that under conditions of Convective Inhibition (see below) greater than 20 J/kg, forcing required for convection is great enough such that the MXHAIL parameter is not produced.

 CINH - Convective INHibition, a measure of negative buoyancy below the layer of positive buoyancy (if it exists), in Joules pre kilogram. Below the "positive area" which defines the CAPE, there can exist some negative area, where the parcel is colder than the environment. The atmosphere in these situations are sometimes referred to as "capped". In these cases, either lifting of a parcel through some forcing mechanism, or heating of the lower atmosphere to eliminate the negative buoyancy area is need for initiation of convection. Dynamically, once the parcel gets through this negative area it is free to rise through the positive area. Thus, occasionally a sounding may have more than one negative region, but only the lowest negative area is considered the Convective Inhibition. Since CINH is not reported unless some CAPE is present, the CINH values are typically fairly low. CINH values above 50 J/kg are typically enough to inhibit convection, unless dynamic forcing is extreme.  Values from 25-50 J/kg require significant forcing, but can be overcome with reasonable dynamics or heating.  Values from 10-25 also require a decent amount of forcing.  CINH values under 10 indicate a requirement for only minimal forcing.

 K.I. - The K-Index is a simple index using data from discreet pressure levels, instead of a lifted parcel. It is based on vertical temperature changes, moisture content of the lower atmosphere, and the vertical extent of the moist layer. The formula for K.I. is:

K.I.=(T850-T500)+(TD850-(T700-TD700)) where:

T850=Temperature at 850mb
T500=Temperature at 500mb
TD850=Dewpoint temperature at 850mb
T700=Temperature at 700mb
TD700=Dewpoint temperature at 700mb

It is more correlated to convective activity in general as opposed to severe weather. The higher the K-Index the more conducive the atmosphere is to convection. K.I. values below 20 imply little support for thunderstorm activity, while values exceeding 30 are quite supportive of thunderstorm activity. Values in the Central and Eastern U.S. typically need to be slightly higher than in the Western U.S. in order to indicate the same level of potential thunderstorm activity.

 TT - The Total Totals Index, like the K-Index, is computed using discreet pressure level information, but is more indicative of severe weather potential. It's formula is:

TT=(T850+TD850)-2(T500)

Generally, TT values below 40-45 are indicators of little or no thunderstorm activity, while values exceeding 55 in the East and Central or 65 in the West are indicators of considerable severe weather, including the potential for tornadic activity. Total Totals values tend to be somewhat higher over higher elevations, therefore higher TT values in the Western U.S. are required to indicate the same level of storm severity as lower TT values in the Central and Eastern U.S.

SHOW - The Showalter Index is a parcel-based index, calculated in the same manner as the Lifted Index, using a parcel at 850mb. That is, the 850mb parcel is lifted to saturation, then moist adiabatically to 500mb. The difference between the parcel and environment at 500mb is the Showalter Index. Again, the calculation is environment minus parcel, so negative numbers indicate instability. The SHOW values are similar to the LI values as far as references for severe weather (negative is unstable, below about -5C is highly unstable).

 SWEAT - The SWEAT index is calculated only for the GOES Sounding, using the gradient wind information. Current processing contraints preclude the use of AVN forecast winds. The SWEAT index is computed from five terms that contribute to severe weather potential:

Low-level moisture (850mb dewpoint temperature)
Instability (Total Totals Index)
Low-level winds (850mb wind speed)
Upper-level winds (500mb wind speed)
Warm advection (Veering of the wind between 850mb and 500mb)

 he formula used to calculate the SWEAT Index is:

SWEAT=12(TD850)+20(TT-49)+2(WS850)+WS500+125(S+0.2), where:

TD850 = Dewpoint temperature (Degrees C) at 850mb
TT = Total Totals Index (Degrees C)  (If TT < 49, the term 20(TT-49) is set to zero

WS850 = 850mb wind speed in knots
WS500 = 500mb wind speed in knots
S = SIN(WD500-WD850), where:

WD500=500mb wind direction
WD850=850mb wind direction

NOTE: The entire wind shear term [125(S+0.2) is set to zero when any of the following conditions are met:

WD850 is between 130 and 250 degrees
WD500 is between 210 and 310 degrees
(WD500-WD850) is positive
WS850>15 knots AND WS500>15 knots

NOTE: No term in the formula may be negative.

Higher SWEAT indices correspond to greater potential for severe weather, given some triggering mechanism. SWEAT values under 200 indicate little potential for severe weather, values > 300 are adequate for severe weather, and tornadic activity is possible with SWEAT values exceeding 400.  The greatest value of the SWEAT index is that it is the only one of the various stability parameters on the chart which includes a term to account for wind shear (which can enhance tornadic potential).

 LR8-5 - The 850 to 500mb lapse rate (C/km).

 CVT - Convective Temperature, the temperature at which convection will begin without any aid from lifting. That is, if the surface air temperature reaches the convective temperature, the initial parcel will become buoyant, regardless of whether or not any lifting mechanism is present.

 LCL - The Lifting Condensation Level is the pressure at which a parcel, when lifted, will reach saturation. This is determined by lifting a parcel dry adiabatically while conserving moisture (constant mixing ratio). The LCL is defined as the pressure at which the saturation mixing ratio of the parcel equals the parcel mixing ratio (i.e., the parcel is saturated).

 LFC - The Level of Free Convection is the pressure at which a parcel becomes buoyant. This is found by raising a parcel to the LCL and then continuing to lift it moist adiabatically. The pressure where the parcel temperature becomes greater than the environmental (profile) temperature is the level at which the parcel is buoyant. In a stable atmosphere a raised parcel may never become buoyant and, therefore, may not have a level of free convection.  In some complex profiles there may be more than one buoyant area, broken up by a negatively buoyant area.  In such cases the first (lowest) LFC is output.

 EL -The Equilibrium Level is defined as the pressure at which a buoyant parcel becomes cooler that the environment. If the environment is stable and there is no level of free convection, there is obviously no equilibrium level.  In some complex profiles there may be more than one buoyant area, broken up by a negatively buoyant area.  In such cases the first (lowest) EL is output.

 ELT - The Equilibrium Level Temperature is the actual environment (profile) temperature at the above defined Equilibrium Level (EL).

 CCL - The Convective Condensation Level is the pressure at which condensation will occur in a parcel providing the only mechanism for convection is heating. A parcel with the convective temperature, rather than the actual temperature, is lifted normally (physically, this lifting would be from heating alone) until saturation. The pressure level of saturation, found in the exact same manner as the LCL, defines the CCL.

 MCL - The Mixing Condensation Level is the pressure at which the CAPE is equal to the negative potential energy (Convective Inhibition), following complete vertical atmospheric mixing. As lifting occurs to produce convection, the negative buoyancy area acts as an inhibitor until it is "mixed" with the positive area. That is, if there are 100 J/kg of negative potential energy beneath the area of positive potential energy, the MCL is the pressure at which 100 J/kg of positive potential energy is reached.

 -20C - The -20C level is the height in the atmosphere at which the temperature reaches -20C.  Because water droplets can become supercooled before freezing, it is common to locate the level of either -15C or -20C temperatures to aid in hail prediction.  For example, if the CAPE value were very high, but over a very shallow layer (thus making the NCAP quite high and, likely, making
the MXHAIL parameter rather high) and that shallow layer was quite low in the atmosphere, the top of the positively buoyant parcel of air (the EL) may not extend up to the -20C level.  As a result, in this instance, hail formation would be unlikely, regardless of the MXHAIL indicator.

 15TH - The distance (thickness) between 1000 mb and 500mb pressure levels. As a general rule of thumb, 5400m (540dm) is used as a typical rain versus snow line, where thicknesses below 5400m indicate snow and thicknesses above indicate rain. Since the thickness is based on a mean temperature profile in the layer, relatively warm air in the upper levels of this layer can still be below freezing, but still result in thicknesses above 5400m, supporting snowfall. Likewise, a shallow, warm boundary layer can result in rain with thicknesses below 5400m. Generally, the GOES soundings show little change from the AVN first guess since the temperature profiles exhibit little modification. However, significant changes can occur with subtle temperature adjustments and major moisture adjustments, as thickness calculations are based on virtual temperature, which is slightly impacted by moisture. Thickness adjustments are typically on the order of one to ten meters, but changes as high as about fifty meters have been seen.

 87TH - 850mb - 700mb thickness. At very high elevations (low surface pressure), a thickness from 1000mb-500mb cannot be calculated. In these cases, an 850mb-700mb thickness is more appropriate (though it is still calculated for the lower elevation stations). The same assumptions apply for this thickness as for the 1000mb-500mb thickness; a general rule of thumb regarding snow/rain line is 1555m.

 FRZL - Freezing Level, defined as the lowest pressure at which the temperature drops below freezing. Because of the small temperature differences between the GOES and AVN first guess temperature profiles, the GOES and AVN values for the freezing level are often extremely close.

 WBFR - The Wet Bulb Freezing Level is the lowest pressure at which the wet bulb temperature drops below freezing. Frequently, the wet bulb temperature profile is more useful than the dry bulb (normal) temperature profile when attempting to determine precipitation type. Also, because of the differences in dewpoint profiles between the GOES and AVN first guess, the wet bulb temperature profiles are frequently noticeably different.

 TADV - The Thermal Advection is the mean advection in the 850-500mb layer, in degrees C/hour. The gradient winds derived from the soundings are used to calculate the mean temperature gradient for the 850-500mb layer.

 PCPT - The Precipitation Type as indicated from the sounding, and is derived primarily from the wet bulb temperature profile. Since GOES soundings are not available in cloudy areas, the precipitation type indicator is obviously present only in locations where precipitation is not falling. However, it can still be useful several hours prior to the onset of precipitation, providing overcast conditions have not developed. It is especially useful if the GOES Sounding is indicating a different precipitation type than the first guess.


Questions or comments? Please contact

Henry F. Drahos, NOAA/NESDIS, Team Leader - FPDT   hdrahos@nesdis.noaa.gov
or
Donald G. Gray, NOAA/NESDIS, GOES Product Manager   Donald.G.Gray@noaa.gov

 



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Last Updated 12/10/99