Broadband Model Simulation of the Top-of-Atmosphere Longwave Emission

Example of a tropical cloud pass (day 263, 19/09/2006)

Below is a tropical radiative transfer simulation of the longwave emission associated with tropical cumulonimbus, for the satellite pass example shown by the Pacific ocean transection found at this subdomain. Heating rates are shown by thin black contours in K/day, the coloured contours show the cloud ice mixing ratio in mg/m3 with respect to logarithmic right hand height scale. The simulated longwave emission using EOS MLS data, is shown by the red curve with the precision in cloud ice represented by vertical error bars, according to the left hand longwave irradiance scale.

The example appears to show a stratospheric intrusion over a tropical convective cluseter in the tropical Pacific Ocean, where cold dry air is drawn into the troposphere from the stratosphere adjacent to the strong convection (d). This is evidence of a process of tropospheric-stratospheric exchange between the tropical tropopause. The tropopause is around 125hPa on this figure (-2.1 on RH scale), marked by the presence of weak longwave heating rate of 2K/day in the presence of thin anvil cirrus (a). Stratospheric temperature and longwave heating anomalies (c), appear to indicate dynamics in the stratosphere which are directly linked to the troposphere, likely to be Potential Vorticity anomalies where a vortex draws moist tropospheric air into its vortex from below, by a process of mass-conservation / displacement. Dry air is drawn into the edge of the tropical anvil from the stratosphere above resulting in atmospheric temperature anomalies and a locally reduced longwave heating rate. The mirrored-effect of the stratospheric dynamics is due to a vertical temperature inversion, where potential temperature increases with height. Instabilities in the lower stratosphere, appear to be linked intrinsically to the lower troposphere. The longwave heating rates indicate some thin cirrus, which is usually undetectable by the EOS MLS cloud algorithm. Heating rates are a useful measure of the rate of energy divergence or convergence between a hypothetical volume of air over time and its surroundings, given that the atmosphere is non-convective and all other cloud dynamics are "suspended" in animation over a geographical location.]
**These figures were produced using a simplified broadband cloudy-sky radiative transfer, which functions over tropics only, using water vapour, ozone, ice water content and temperature version 2 data products from the EOS MLS instrument. Parameterisation schemes were undertaken using the spherical ice crystal approximation. The IDL code is 4'000 lines in length, and calculations take about 3 minutes per 3495 (1 day of) MLS profiles on a modern desktop computer.

• The localised intense longwave heating rates (greenhouse warming) at high altitude is due to the lower density of the stratosphere. A unit volume of stratospheric air has a smaller heat capacity than lower and mid-tropospheric air, so that even small mixing ratios of greenhouse absorbers and cloudiness can have large effects on the stratospheric radiative fluxes and dynamics. Most of the heating above tropical convection is due to stratospheric-tropospheric exchange of water vapour, an abundant greenhouse gas in the Earth's atmosphere.

• It is likely that the majority of the observed heating rate anomalies within deep convective (cumulonimbus) type clouds are a direct result of vertical motion, which is almost equal and opposite in magnitude to the derived counteracting winds within deep convective plumes. Within the thin cirrus outflow however, heating rates are a "real" phenomenon.

Derived Meteorological Products of u(zonal) and v(meridional) wind for MLS were calculated and provided by Gloria Manney and William Daffer, with support from the MLS team at NASA's Jet Propulsion Laboratory, California Institute of Technology. All other parameters were derived and calculated by myself, or by my own model simulations with the exception of the raw hdf5 data products of ice water content and water vapour (v2.2 EOS MLS data fields).

*Counteracting vertical winds are defined as ... the theoretical winds required to reduce the modelled longwave heating rates to zero, and are scaled to the maximum vertical velocity of w~5m/s. Most of these counteracting vertical winds are a direct result of longwave heating caused by the ice water content. If these vertical wind vectors are added to the actual observed wind vectors, then the derived vertical component should be zero in a state of thermodynamic equilibrium.

Results from the computer code ...   Acknowledgments to NASA JPL for the MLS data, and to the NERC Phd funding body [WV(coloured contours)&Heating Rates(thin black contours)]; [IWC(coloured contours)&Heating Rates(thin black contours)], [IWC(coloured contours)&CERES and Model OLR Estimates]

KEY to abbreviations: [IWC=Ice Water Content from the NASA EOS MLS instrument, WV=Water Vapour from the NASA EOS MLS instrument, PV = Potential Vorticity from MetOffice horizontal wind vectors derived from numerical analysis and interpolated to MLS profile profiles]
The wind vectors shown are along track winds interpolated to a set of 43 EOS MLS vertical pressure levels, and to the tangent points of each individual MLS profile. The inclination of the A-Train satellites in their 750km altitude polar orbit is 82.883 degrees (from the W-E equatorial plane). The A-Train satellite passes are approximately parellel to the N-S meridional plane, and so wind vectors shown are close to v(j) vector components. Derived meteorological products, interpolated to A-Train EOS MLS profiles, can be obtained from the Jet Propulsion Laboratory.