Assistant professor in Glaciology

I am an assistant professor in the field of glaciology and polar meteorology at the Institute for Marine and Atmospheric research Utrecht (IMAU), part of Utrecht University, The Netherlands. I'm active on Mastodon.

The large-scale weather pattern over the North Atlantic is variable, and it impacts Greenland surface mass balance, and firn mass. We find that a more positive phase of NAO (i.e., more zonal flow) since 2012 has slowed down the loss of firn over most of the ice sheet.

Using a spectral unmixing technique, we map daily fractions of blue-ice area over Antarctica for the 22-year period since 2000, using the full available MODIS archive. This FABIAN product is published in Remote Sensing of Environment.

In the past few years, I have been developing a model that calculates radiative transfer of solar radiation through the atmosphere, clouds and snow. I started out with the monochromatic DAK model (Doubling-Adding KNMI) [1,2,3], and adapted it for broadband calculations using the correlated-k technique.

The doubling-adding technique can be subdivided into two parts. "Doubling" starts with a very small atmospheric layer, which is assumed to have single-scattering properties only, the properties of which can be derived analytically. An identical layer is then added, and the optical properties of the combined layer are calculated including internal scattering. This doubling procedure is repeated until the layer has reached the desired thickness. In this way, multiple scattering of light is taken into account.

The "adding" procedure is very similar to the doubling mechanism, but is designed to combine two layers with different optical properties instead. Two layers are combined to a single one, and this combined layer is added to a third layer, etc. This process is repeated until the radiative fluxes at the boundaries of all model layers are known.

In order to perform calculations for the entire shortwave spectrum (300-3,000 nm), one should do line-by-line calculations which include absorption by the main atmospheric gases. This is necessary because absorptive spectra of e.g. water vapour are highly irregular and strongly dependent on temperature and pressure. These line-by-line runs over the entire spectrum are very time-consuming however, and typically require 10,000+ calculations.

The correlated-k technique has been developed [4] to significantly reduce computation time for numerical broadband calculations. The idea is to put absorption lines within a certain wavelength interval in order of absorption strength rather than of wavelength. The absorption spectra then become smooth curves which can be evaluated using only typically 5 to 15 calculations for a wavelength interval. The entire shortwave spectrum is subdivided into 32 bands [5], requiring a total of 150 - 300 radiative transfer calculations.

In calculations on radiative transfer, clouds and snow are treated in a similar way. They are regarded as particulate media, the scattering properties of which are described by scattering phase functions (SPF). Clouds can consist of (im)perfect hexagonal ice crystals (ice clouds), or of spherical water droplets (liquid water clouds). An expansion of the SPFs in Legendre polynomials and in generalized spherical functions is calculated using a ray-tracing programme [6]. Optical properties of the cloud and snow layers are extracted from the expansion coefficients in DAK.

The broadband version of DAK has been subject to a thorough validation study by post-doc researcher Dr. Ping Wang at the Royal Netherlands Meteorological Institute (KNMI). The model was validated against the radiative transfer model SMARTS and solar radiation measurements from Cabauw, The Netherlands.

These are listed on my publications page.

1. De Haan, J.F., P.B. Bosma and J.W. Hovenier. 1987. The adding
method for multiple scattering calculations of polarized light.
*Astron. Astrophys.* **183**, 371-391.

2. Stammes, P., J.F. de Haan and J.W. Hovenier. 1989. The polarized
internal radiation field of a planetary atmosphere. *Astron. Astrophys.*
**225**, 239-259.

3. Van de Hulst, H.C. 1963. A new look at multiple scattering. *Tech.
Rep., Inst. Space Studies, NASA, New York.*

4. Lacis, A.A. and V. Oinas. 1991. A description of the correlated
k distribution method for modeling nongray gaseous absorption,
thermal emission, and multiple scattering in vertically inhomogeneous
atmospheres. *J. Geophys. Res.*, **96**(D5), 9,027-9,063.

5. Kato, S., T.P. Ackerman, J.H. Mather and E.E. Clothiaux. 1999. The
k-distribution method and correlated-k approximation for a
shortwave radiative transfer model. *J. Quant. Spectrosc. Radiat.
Transfer*, **62**, 109-121.

6. Hess, M., R.B.A. Koelemeijer and P. Stammes. 1998. Scattering
matrices of imperfect hexagonal ice crystals. *J. Quant. Spectrosc.
Radiat. Transfer*, **60**, 301-308.