Fire emission heights in the climate system – Part 1: Global plume height patterns simulated by ECHAM6-HAM2

Abstract

Abstract. We use the global circulation model ECHAM6 extended by the aerosol module HAM2 to simulate global patterns in wildfire emission heights. Prescribed plume heights in ECHAM6 are replaced by an implementation of a simple, semi-empirical plume height parametrization. In a first step, the global performance of the plume height parametrization is evaluated for plumes reported in the Multiangle Imaging Spectroradiometer (MISR) Plume Height Project (MPHP) data set. Our results show that the parametrization simulates a largely reasonable global distribution of plume heights. While the modeled global mean plume height (1411±646 m) is in good agreement with the observed mean (1382±702 m), the upper and lower tails of the plume height distribution tend to be slightly underrepresented. Furthermore, we compare plume heights simulated by the simple parametrization to a more complex, analytical plume model. Major differences in global plume height distributions are found for the lowest 1.5 km, but reasonable agreement is observed for higher plumes. In a second step, fire radiative power (FRP) as reported in the global fire assimilation system (GFAS) is used to simulate plume heights for observed fires globally for the period 2005-2011. The global fraction of simulated daytime plumes injecting emissions into the free troposphere (FT) ranges from 3.7 ± 0.7 to 5.2 ± 1.0 %. This range is comparable to results from observational studies, but it is much lower than results for prescribed plume heights in the ECHAM6-HAM2 standard setup. Nevertheless, occasionally deep emission injections exceeding 5-7 km in height are simulated for intense fires and favorable meteorological conditions. The application of a prescribed diurnal cycle in FRP turns out to be of minor importance. For a hypothetical doubling in FRP, moderate changes in plume heights of 100-400 m are simulated. These small changes indicate that a potential future increase in fire intensity will only slightly impact the emission heights on a global scale.

Fire emission heights in the climate system – Part 2: Impact on transport, black carbon concentrations and radiation

“…Kaiser et al (2012) found that GFAS emissions implemented in the global circulation model ECMWF are only able to reproduce AOT observations in a reasonable way, if global GFAS wildfire emissions are multiplied by a global factor of 3.4. This zero-order approximation also provided reasonable global modeling results in studies by Huijnen et al (2012) andvon Hardenberg et al (2012) well as ECHAM5-HAM1. Basically, the underestimation of AOT in GFAS and other bottom-up inventories could have various reasons including an underestimation of emission fluxes (e.g., due to underestimation of wildfire emission factors or burned area as well as FRP) as well as shortcomings in the representation of aerosol micro-physics in the model (impacting aging and removal rates).…”

“…In the first part of this two-paper series (Veira et al, 2015), we presented globally simulated plume height patterns. Through a comparison of simulated plume heights to observations from the Multiangle Imaging Spectroradiometer (MISR) Plume Height Project (MPHP) data set, we evaluated the performance of different plume height implementations.…”

“…Overall we carry out five simulations with different implementations of the SP: the original and most simple one-step model as described in Sofiev et al (2012) called "SOFIEV-ORIGINAL", one simulation with additional application of a diurnal cycle in fire emissions and FRP called "SOFIEV-DCYCLE" and one simulation which applies a diurnal cycle as well as a tuning of high plumes "SOFIEV-MODIFIED". Simulation SOFIEV-MODIFIED represents the plume height parametrization which provides the best agreement to global plume height observations (see Veira et al, 2015). A hypothetical future scenario with a doubling in FRP and emissions, "SOFIEV-2X-EMISSIONS-FRP", enables a comparison of the impact of changes in emission fluxes and emission heights.…”

“…For more details on the SP, a description of the normalizing constants N 0 and P f0 as well as the tuning parameters Veira et al (2015) and Sofiev et al (2012). Overall we carry out five simulations with different implementations of the SP: the original and most simple one-step model as described in Sofiev et al (2012) called "SOFIEV-ORIGINAL", one simulation with additional application of a diurnal cycle in fire emissions and FRP called "SOFIEV-DCYCLE" and one simulation which applies a diurnal cycle as well as a tuning of high plumes "SOFIEV-MODIFIED".…”

“…Only 5.2±1.0 % of all daytime plumes were injecting emissions into the FT. On average, plume heights simulated by the semi-empirical parametrization are 1.1-2.0 km lower than prescribed standard plume heights in ECHAM6-HAM2. Based on the simulations introduced in Veira et al (2015) -Does the diurnal cycle of fire intensity and emission release enhance, dampen or change the sign of the averaged climate response?…”