Parametric model to estimate clear‐sky longwave irradiance at the surface on the basis of vertical distribution of humidity and temperature

Abstract

The surface downwelling longwave irradiance in clear‐sky situations is an important component of the global radiation balance. It can be measured directly using ground‐based pyrgeometers or computed using a radiative transfer code given precise information on atmospheric composition (water vapor, ozone, and aerosols) and temperature. Discrepancies between instantaneous observed and simulated values of the clear‐sky longwave irradiance are typically at the 3–10 W m−2 level (root‐mean‐square error). The discrepancies depend both on pyrgeometer and atmospheric composition uncertainties. Over the past century, many authors have worked on deriving control parameters to simulate the clear‐sky longwave irradiance using simple parameterizations. The most common control parameters found in the literature are screen‐level temperature, screen‐level water vapor density, and column integrated precipitable water. We show that reference parameterizations are able to simulate the clear‐sky longwave irradiance with an uncertainty of about 10 W m−2. Uncertainties are greater during nighttime than daytime periods. We propose a new parameterization that uses the standard input parameters and their diurnal variations to account for important effects of their vertical distribution on the simulation of clear‐sky longwave irradiance. The new parameterization allows us to reduce uncertainties in clear‐sky surface downwelling longwave irradiance simulations to better than 5 W m−2 for both daytime and nighttime situations.