ARPSView Guide

Description of Products

Parameter Description Usage
Bndy Lyr Conv Boundary layer convergence (m/s/m), computed from the mean 0-2 km AGL wind vector, multiplied by 1000 Positive values indicate areas of convergence which can act as a focus for convection.
BRN Bulk Richardson Number, defined as the ratio of CAPE to the square of the horizontal wind shear from 0 - 6 km The BRN is a useful predictor of storm type. Values of 5-50 are associated with supercells, while values of 35 - 400 are associated with for multicell storms.
BRN Shear Bulk Richardson Number shear (m s-1), defined as the magnitude of 0 - 6 km horizontal wind difference. This is the shear value used in computing the BRN.
Cap Strength Cap strength (C), defined as the difference between the ambient temperature and the parcel temperature at the Lifted Condensation Level (LCL). A value of zero indicates that there is no capping inversion. The higher the value, the stronger the cap.
CAPE Convective available potential energy (J/kg), computed by integrating the positive area on a Skew-T diagram from the Level of Free Convection (LFC) to the Equilibrium Level (EL) for a hypothetical undilute parcel rising pseudoadiabatically from the surface. The theoretical maxiumum updraft speed is twice the square root of CAPE. Severe thunderstorms are associated with values in excess of 1000 J/kg.
CIN Convective inhibition (J/kg), computed as the integral of the negative area on a Skew-T diagram between the surface and the LFC. CIN is the amount of work required to lift a parcel to its LFC. Values less than -100 J/kg indicate a significant cap.
Dew Point Dewpoint temperature (oC) The T-Td spread is useful for determining areas of visibility, cloud cover, and potential fog.
Divergence Divergence (m/s/m), defined as the dot product of the gradient operator and the wind vector, multiplied by 1000 Positive values indicate divergence. Divergence aloft coupled with low-level convergence delineates rising motion.
Geopotl Hgt Geopotential height (dam), defined as the integral of potential energy, divided by 9.8 m/s2 Falling heights at a given pressure level indicate temperature advection increasing with height and positive vorticity advection. A 5 m/h height fall is significant.
LI Lifted index (C), defined as the temperature difference between the ambient temperature at 500 mb and a surface parcel lifted to 500 mb Negative values indicate potential instability. Values of -6 C or less are associated with severe convection.
Moist Conv Moisture convergence (g/kg/s), computed from the horizontal wind and mixing ratio, multiplied by 1000 Positive values indicate increasing moisture in a given area. Convection is associated with areas of maximum convergence.
RH Relative humidity (decimal fraction) RH is useful for determining areas of visibility, cloud cover, and potential fog.
Richardson Nmbr Richardson Number, defined as the ratio of the buoyancy frequency to the square of the horizontal wind shear Values of less than 0.25 indicates significant turbulence.
Sea-Level Pres Sea-level pressure (mb) Wind speed is proportional to the pressure gradient.
Storm Motion, Est Estimated Storm Motion (m/s), computed as the mass-weighted 0-6 km AGL mean wind, empirically adjusted to account for right-moving storms. For mean wind of 20 m/s or greater, the storm speed is 0.89 of the mean speed and the direction is 18 degrees to the right of the mean wind. At lower speeds the speed and directional changes are greater. This is the approximate motion vector for severe storms.
Stm-rel Helicity Storm-relative Helicity (m2/s2), defined as the vertically integrated component of vorticity parallel to the storm inflow from 0 - 3 km AGL. Helicity is a measure of the potential for updraft rotation if convection occurs. Values below 100 m2/s2 are considered small while 300 m2/s2 is a lower threshold for tornadoes.
Temp Temperature (oC) At upper levels, temperature is useful for determining precip type and icing potential.
Theta-E Equivalent potential temperature (K) This form of potential temperature includes moisture in its definition. Higher values generally indicate greater instability.
Wind Barbs Horizontal wind speed (kts) and direction Wind barbs are useful in locating fronts and other boundaries.
Wind Speed Isotachs of horizontal wind speed (kts) Areas of higher wind speed are likely to be more turbulent.
Vort Vorticity (s-1), multiplied by 105 Positive values indicate cyclonic flow.
Vort, Abs Absolute vorticity (s-1), defined as the sum of the relative vorticity and earth's vorticity, multiplied by 105 At midlatitudes, a value greater than ~105 s-1 indicates cyclonic flow.
Stm-rel Wind Estimated storm-relative wind (m/s), defined as the difference between the estimated storm speed and the mean wind speed in the layer. The storm motion is computed from the mean wind, with an empirical adjustment for right-moving storms. This value is useful in determining storm type and intensity. The lower threshold for supercells is 10 m/s.
Reflectivity Reflectivity (dBZ) A lower threshold for hail is 55 dBZ, while anything above 70 dBZ is almost definitely hail.
Reflectivity, Composite Composite reflectivity (dBZ), defined as the maximum reflectivity at any altitude for a specific point The composite reflectivity can be used to quickly find the location of the strongest echoes regardless of altitude.
Clouds Vertically integrated condensate (kg/m2), including the five model water species of cloud water, cloud ice, rain, snow, and hail/graupel. Closely related to, but more general than, VIL (vertically intergrated liquid). Contours of condensate denote areas of cloud cover as well as the most intense parts of a storm.
Precip Type Vertically integrated mixing ratios of snow, rain, and hail (kg/m2) The contours will outline likely areas of snow, rain, and hail within a storm. This is particularly useful for determining icing potential. (Colors indicate coding scheme on the product image.)
Precip Amount Precipitation amount (mm), since the forecast began.
Vert Shear Vertical wind shear (m/s/m), multiplied by 1000 Areas of high vertical shear are associated with turbulence.
Vert Velocity Vertical velocity (m/s) Values of up to 0.1 m/s are typical of synoptic scale motion. Values greater than 5 m/s indicate significant storm-scale updrafts and downdrafts.

* Some of the parameters shown in green are currently only available for forecasts.

Maintained by Kevin W. Thomas. Your comments, inquiries, problem reports, and suggestions are welcome.