N. P. M. van Lipzig

Factors controlling the near-surface wind field in Antarctica

Abstract Using data from the regional atmospheric climate model RACMO/ANT1 the momentum budget of the Antarctic atmospheric surface layer (SL; taken as the lowest model layer located at 6–7 m above the surface) is presented. In July (winter), the katabatic pressure gradient force (PGF) dominates the SL momentum budget over the steep coastal slopes, which results in strong (10–15 m s−1) and directionally constant katabatic winds. Farther inland, where surface slopes are more gentle, the large-scale PGF may become equally important. With the circumpolar pressure trough north of the Antarctic coastline, the large-scale PGF acts along with the katabatic PGF in the downslope direction. In Wilkes Land, Dronning Maud Land, and on the western Ross Ice Shelf, the large-scale PGF causes equivalent geostrophic winds in excess of 10 m s−1. Thermal wind effects that oppose the downslope acceleration are especially strong in areas where large-scale forcing is weak, which allows cold air to pile up over the flat ice she...

Clouds enhance Greenland ice sheet meltwater runoff

The Greenland ice sheet has become one of the main contributors to global sea level rise, predominantly through increased meltwater runoff. The main drivers of Greenland ice sheet runoff, however, remain poorly understood. Here we show that clouds enhance meltwater runoff by about one-third relative to clear skies, using a unique combination of active satellite observations, climate model data and snow model simulations. This impact results from a cloud radiative effect of 29.5 (±5.2) W m−2. Contrary to conventional wisdom, however, the Greenland ice sheet responds to this energy through a new pathway by which clouds reduce meltwater refreezing as opposed to increasing surface melt directly, thereby accelerating bare-ice exposure and enhancing meltwater runoff. The high sensitivity of the Greenland ice sheet to both ice-only and liquid-bearing clouds highlights the need for accurate cloud representations in climate models, to better predict future contributions of the Greenland ice sheet to global sea level rise.

Momentum budget of the East Antarctic atmospheric boundary layer: Results of a regional climate model

Output of a regional atmospheric climate model is used to quantify the average January and July momentum budget of the atmospheric boundary layer (ABL) over the East Antarctic ice sheet and the surrounding oceans. Results are binned in nine elevation intervals over the ice sheet and six distance intervals over the ocean. In January, when surface cooling is weak, the large-scale pressure gradient force dominates the ABL momentum budget. In July, under conditions of strong surface cooling, a shallow katabatic jet develops over the gentle slopes of the interior ice sheet and a strong, deep jet over the steep coastal slopes. In the coastal regions the ABL thickens considerably, caused by the piling up of cold air over the adjacent sea ice and ice shelves. This represents the main opposing force for the katabatic winds. Horizontal and vertical advection are generally small. In the cross-slope direction the momentum budget represents a simple balance between surface drag and Coriolis turning. Intraseasonal variability of the large-scale wind field in the ABL can be explained in terms of the strength of the polar vortex, the background baroclinicity, and the topography of the ice sheet. Subsidence is found over the interior ice sheet and rising motion in the coastal zone, reflecting the acceleration and deceleration of the katabatic circulation. However, vertical velocities are generally small, because the downslope mass flux in the ABL is confined to a shallow layer below the wind speed maximum.

Validation and comparison of two soil-vegetation-atmosphere transfer models for tropical Africa

This study aims to compare and validate two soil-vegetation-atmosphere-transfer (SVAT) schemes: TERRA-ML and the Community Land Model (CLM). Both SVAT schemes are run in standalone mode (decoupled from an atmospheric model) and forced with meteorological in-situ measurements obtained at several tropical African sites. Model performance is quantified by comparing simulated sensible and latent heat fluxes with eddy-covariance measurements. Our analysis indicates that the Community Land Model corresponds more closely to the micrometeorological observations, reflecting the advantages of the higher model complexity and physical realism. Deficiencies in TERRA-ML are addressed and its performance is improved: (1) adjusting input data (root depth) to region-specific values (tropical evergreen forest) resolves dry-season underestimation of evapotranspiration; (2) adjusting the leaf area index and albedo (depending on hard-coded model constants) resolves overestimations of both latent and sensible heat fluxes; and (3) an unrealistic flux partitioning caused by overestimated superficial water contents is reduced by adjusting the hydraulic conductivity parameterization. CLM is by default more versatile in its global application on different vegetation types and climates. On the other hand, with its lower degree of complexity, TERRA-ML is much less computationally demanding, which leads to faster calculation times in a coupled climate simulation.

The Influence of Soil and Vegetation Parameters on Atmospheric Variables Relevant for Convection in the Sahel

Abstract A key issue in modeling the Sahelian climate is to correctly predict the energy fluxes between the land surface and the atmosphere. A problem faced by land surface models in the Sahel is the horizontal heterogeneity of soil and vegetation properties in the region, where measured data are scarce. Experiments have been designed to evaluate a land surface model both in offline mode and coupled to the Advanced Regional Prediction System (ARPS), a mesoscale atmospheric model. For the evaluation in offline mode, an observational dataset of 58 days from the Hydrological and Atmospheric Pilot Experiment in the Sahel (HAPEX-Sahel) is gathered to interpret the results. For the evaluation in the coupled mode, boundary layer development is simulated for 4 individual days. The model is able to reproduce the observations close to measurement errors. Sensitivity experiments are conducted to identify the most important parameters that affect the simulation of the convective available potential energy (CAPE) and ...