NUCAPS Atmospheric Soundings Display Overview

General Information:

The intention of this overview is to give the user a detailed look at data being provided on the Skew-T Log-P diagrams being generated from the NOAA Unique Combined Atmospheric Processing System (NUCAPS) using (CrIS/ATMS, IASI/MetOp-A, and IASI/MetOp-B) Sounder data at the Office of Satellite and Product Operations (OSPO) of 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.

Overview

NUCAPS Soundings page is refreshed approximately every 30 minutes with new retrievals from whatever swath was added. NUCAPS algorithm produces soundings using only Microwave Physical (MW Phys) retrievals and Microwave plus Infrared Physical (MW+IR Phys) retrievals. Several meteorological parameters are generated using the sounding data and produces a detailed information pertaining to both the “MW+IR Phys” and “MW Phys”.

General Chart Description

The NUCAPS vertical profiles of atmospheric temperature and moisture are plotted on a 'Skew-T-Log-P' Chart. The chart's name reflects the parameters associated with the vertical and horizontal axes. Atmospheric pressure (in hPa) 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 NUCAPS uses a regression retrieval of vertical temperature and moisture as initial estimate. The regression retrieval coefficients are calculated by relating 4 global days of ECMWF temperature and moisture profiles to CrIS measurements. The only information that NUCAPS gets from GFS model is the surface pressure. The GFS model is maintained and run by the National Centers for Environmental Prediction (NCEP), Environmental Modelling Center (EMC).

The infrared radiation measured by the CrIS and IASI sounders. The Cross-track Infrared Sounder (CrIS), provides soundings of the atmosphere with 2207 spectral channels, over 3 wavelength ranges: LWIR (9.14 - 15.38um); MWIR (5.71 - 8.26um); and SWIR (3.92 - 4.64 um). CrIS scans a 2200km swath width (± 50°), with 30 Earth-scene views. Each field consists of 9 fields of view, arrayed as 3 × 3 array of 14-km diameter spots (nadir spatial resolution). Each scan (with an 8-second repeat interval) includes views of the internal calibration target (warm calibration point), and a deep space view (cold calibration point). The overall instrument data rate is < 1.5 Mbps. ATMS is a cross-track scanning 22-channel passive microwave radiometer. The channels are bands from 23 GHz through 183 GHz making its measurement capabilities similar to that of the Advanced Microwave Sound Unit (AMSU) and the Microwave Humidity Sounder (MHS). ATMS makes three scans (a scan set) every eight seconds. Each scan contains a single row of 96 FOVs. The FOV coverage sizes vary for each ATMS channel. ATMS scan sets are synchronized with those of the CrIS instrument. With each scan, the ATMS FOV coverage extends over each end of the associated CrIS scans. This is done to allow for footprint resampling of the smaller ATMS FOVs into larger AMSU-A like footprints (~40 km at nadir). The resampled ATMS radiances can be used as input into existing retrieval algorithms like that in NUCAPS.

Detailed Information

Pressure
Atmospheric pressure, in hPa, is represented by the vertical axis of the chart. Values range from 1018 hPa to 100 hPa. Average surface atmospheric pressure is 1013 hPa. 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.

Temperature and Dew Point (Moisture) Profiles
The red solid line represents the Infrared (CrIS/IASI) plus microwave (ATMS/AMSU) physical derived atmospheric temperature profile and red dashed line represents the dew point temperature profile. The blue solid line represents the microwave (ATMS/AMSU) only physical derived atmospheric temperature profile and blue dashed line represents the dew point temperature profile. The green solid line represents the microwave regression derived temperature profile and dashed green line represents the dew point temperature profile. The short red horizontal bars on the right side of the chart represent the cloud top pressure layer 1 and lower Cloud Top Pressure layer 2. The height of a position of a bar is determined by the cloud top pressure, and the percentage indicates the cloud top fraction.

Several stability, moisture and meteorological parameters derived from “MW+IR Phys” and “MW Phys” soundings are listed on the right-hand portion of the chart (described in detail below.). In addition to the sounding, several meteorological parameters shown on the chart are calculated from the sounding data.

In the example above, the atmospheric temperature is 25 °C at the surface (860 hPa), and decreases to -70 °C at 100 hPa (~50,000 feet).

The dew point is the temperature at which condensation would occur as the air is cooled. Higher dew point temperatures correspond to moister atmospheric conditions, and lower point to dryer atmospheric conditions.

Chart

Diagonal thin red lines extending from the lower left to the upper right represent constant air temperature (isotherms), in Degrees Centigrade. The values range from -130 °C to +40 °C.

Dashed gray lines extending from the lower left to the upper right are lines of constant 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.

In addition to the isobars (thin black), isotherms (thin red), and the lines of constant moisture (grey) saturated adiabats and the lines of constant potential temperature are drawn. Saturated adiabats are represented by the green dashed lines, and potential temperatures — with solid blue lines.

The information to the right of the chart includes the location: latitude in degrees North and longitude of degrees East. The time, shown as UTC (Universal Coordinated Time) indicates the time the satellite instruments have completed the measurement for the sounding.

Underneath the satellite name the status of the retrieval computations, which have produced the sounding, is written. The possible status expressions are "Accepted", "Rejected by Physical (MW + IR)", "Rejected by MW File", "Rejected by Regression File", "Rejected by MW", "Rejected by Physical and MW", "Rejected by Regression", "Rejected by Physical and Reg.", "Rejected by MW and Regression", and "Rejected by Phys., MW, and Reg."

Parameters

The various parameters available for each sounding are:

VIEW: The view angle, in degrees, for the sounding location.

SOLZ: The solar zenith angle, in degrees, for the sounding location.

ELEV: The surface elevation above sea level, in meters, for the sounding location.

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 1019 hPa, the parcel chosen is from the 950-hPa 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 dew point 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 dew point of the parcel (PARD) may very well not be equal to the profile dew point at the parcel pressure level.

TSKIN: The surface skin temperature (°C). This is the estimated temperature of the ground surface as derived from the NUCAPS satellite (note that no value is available for the FG).

PW: The total precipitable water (millimeters) derived from the soundings. The TPW is a measure of the liquid water content of a vertical column through the atmosphere. In this example, the FG forecast TPW is -19 mm, and the NUCAPS sounding TPW is - 23 mm. This is consistent with the increased dew point temperatures of the NUCAPS sounding in the profile. Precipitable water estimates are one of the most important products generated from NUCAPS sounder data. While the changes to the FG forecast temperature profile using NUCAPS sounder data are usually small, large changes to the FG forecast dew point profile and resulting TPW occur frequently. Overall statistics as well as case studies have consistently showed the NUCAPS TPW values to be more accurate than the first guess TPW. As such, examining the TPW values from the NUCAPS and FG 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 adiabatically up to 500 hPa. The Lifted Index is the ambient air temperature minus the lifted parcel temperature at 500 hPa. 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 1000 J/kg represent moderate amounts of atmospheric potential energy. Values exceeding 3000 J/kg are indicative of very large amounts of potential energy, and are often associated with strong/severe weather. 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/s2). 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, 10-15 cm) 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 J/kg. 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 is 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 J/kg also require a decent amount of forcing. CINH values under 10 J/kg 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 850 hPa
T500 = Temperature at 500 hPa
TD850 = Dew point temperature at 850 hPa
T700 = Temperature at 700 hPa
TD700 = Dew point temperature at 700 hPa

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 850 hPa. That is, the 850 hPa parcel is lifted to saturation, then moist adiabatically to 500 hPa. The difference between the parcel and environment at 500 hPa 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 -5 °C is highly unstable).

SWEAT: The SWEAT index is currently not calculated

LR8-5: The 850 to 500 hPa 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 (K/km).

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) (hPa).

LFC: Level of Free Convection, the lower boundary of the most significant region of CAPE in the troposphere. It is the point at which a lifted parcel of air will become equal in temperature to that of the environmental temperature (hPa).

EL: Equilibrium Level is the level above the level of free convection (LFC) at which the temperature of a rising air parcel again equals the temperature of the environment (hPa). In meteorology, the equilibrium level (EL), or level of neutral buoyancy (LNB), or limit of convection (LOC), is the height at which a rising parcel of air is at the same temperature as its environment.

ELT: Equilibrium Level Temperature, a state of a system in which all parts are at the same temperature (K).

CCL: Convective Condensation Level represents the height (or pressure) where an air parcel becomes saturated when heated from below and lifted adiabatically due to buoyancy (hPa).

MCL: Mixing Condensation Level is the lowest height at which saturation may occur if the near surface layer is or will be mixed completely by wind action. You may relate this to a situation in which you have a radiation inversion keeping the boundary layer wind away from the surface (hPa).

-20C: Minus-20-°C Level. Pressure level where the temperature is -20 °C (m). Minus-20-°C Level. Pressure level where the temperature is -20 °C (m).

15TH: The distance or thickness between the 1000-hPa and 500-hPa pressure levels (m).

87TH: The distance or thickness between the 850-hPa and 700-hPa pressure levels (m).

FRZL: Freezing Level, a pressure level where the temperature is Zero Degree Centigrade (m).

RCPT: Precipitation type: “R”, “SG”, or “MIX”.