The seasonal evolution of snow and ice on Lake Orajärvi, northern Finland, was investigated for three consecutive winter seasons. Material consisting of numerical weather prediction model (HIRLAM) output, weather station observations, manual snow and ice observations, high spatial resolution snow and ice temperatures from ice mass balance buoys (SIMB), and Moderate Resolution Imaging Spectroradiometer (MODIS) lake ice surface temperature observations was gathered. A snow/ice model (HIGHTSI) was applied to simulate the evolution of the snow and ice surface energy balance, temperature profiles and thickness. The weather conditions in early winter were found critical in determining the seasonal evolution of the thickness of lake ice and snow. During the winter season (Nov.–Apr.), precipitation, longwave radiative flux and air temperature showed large inter-annual variations. The uncertainty in snow/ice model simulations originating from precipitation was investigated. The contribution of snow to ice transfor...

Snow and ice on Bear Lake (Alaska) – sensitivity experiments with two lake ice models

Impact of partly ice-free Lake Ladoga on temperature and cloudiness in an anticyclonic winter situation - a case study using a limited area model

The effect of horizontal transports of momentum and heat by transient eddies (TE) on the time-mean flow is studied by examining the relevant terms in a local budget of quasi-geostrophic potential vorticity. Two long-term observational data sets are used, and results for the northern Hemisphere winter are presented. The results indicate that eddy beat fluxes in the free atmosphere exert a dissipative influence on both the zonally averaged flow and the stationary waves. On the oilier hand, eddy momentum transports tend to force cyclonic circulations over the semi-permanent Icelandic and Aleutian surface lows, and anticyclonic circulations over the oceanic high pressure cells in the subtropics. The forcing of the time-mean flow arising from horizontal TE heat transports is generally stronger than the forcing associated with eddy momentum transports. The net eddy of eddy transports of heat and momentum is to dissipate the potential enstrophy of the stationary waves. The characteristic time scale asso... Abstract The effect of horizontal transports of momentum and heat by transient eddies (TE) on the time-mean flow is studied by examining the relevant terms in a local budget of quasi-geostrophic potential vorticity. Two long-term observational data sets are used, and results for the northern Hemisphere winter are presented. The results indicate that eddy beat fluxes in the free atmosphere exert a dissipative influence on both the zonally averaged flow and the stationary waves. On the oilier hand, eddy momentum transports tend to force cyclonic circulations over the semi-permanent Icelandic and Aleutian surface lows, and anticyclonic circulations over the oceanic high pressure cells in the subtropics. The forcing of the time-mean flow arising from horizontal TE heat transports is generally stronger than the forcing associated with eddy momentum transports. The net eddy of eddy transports of heat and momentum is to dissipate the potential enstrophy of the stationary waves. The characteristic time scale asso...

A study on parametrization of orography-related momentum fluxes in a synoptic-scale NWP model

When short-wave (SW) radiation fluxes modelled with a numerical weather prediction (NWP) model or climate model do not match observed SW fluxes it can be challenging to find the cause of the differences. Several elements in the model affect SW fluxes. This necessitates individual testing of each of the physical processes in the NWP model. Here we present a focused study of the SW radiation schemes in the HIRLAM (HIgh Resolution Limited Area Model) Aladin Regional Mesoscale Operational NWP In Europe (HARMONIE) model, which is the primary NWP model used and developed by several National Weather Services in Europe. Detailed calculations have been made with the DISORT model run in the libRadtran framework, which is a collection of state-of-the-art radiative transfer software and data sets. These are used to test the NWP radiation calculations. Both models are given the same atmospheric properties as input. We also perform a separate test of cloud liquid optical property parameterisations with Mie calculations. This leads us to introduce a new parameterisation for calculating these properties. In addition, we show that the results of a simpler radiation scheme, introduced into HARMONIE, compare well with those of the comprehensive default parameterisations. The methodology applied here may be used for testing radiation schemes in other NWP or climate models.