Hannu Savijärvi

Comparison of surface radiative flux parameterizations Part II. Shortwave radiation

Abstract This paper presents a comparison of several longwave (LW) downwelling radiative flux parameterizations with hourly averaged pointwise surface-radiation observations made at Sodankyla, Finland, in 1997 and 1999. Both clear and cloudy nighttime conditions are considered. The comparisons covered a 2-m temperature range from +11°C all the way to −49°C. The clear-sky comparisons included eight simple LW parameterizations, which mainly use screen-level input data, and four radiation schemes from numerical weather prediction (NWP) models: the former European Centre for Medium-Range Weather Forecast (ECMWF) scheme, the Deutscher Wetterdienst (DWD) scheme, the High Resolution Limited Area Model (HIRLAM) scheme, and the new ECMWF LW scheme (Rapid Radiative Transfer Model, RRTM). Atmospheric-sounding profiles were used as input for the NWP schemes. For the cases with clouds, three simple cloud-correction methods were tested. Almost all LW schemes usually underestimated the downwelling clear-sky flux, particularly, in cold (surface inversion) conditions. Overall, the RRTM scheme performed best. The simple cloud-correction methods turned out to be useful in the LW region. Finally, some new simple parameterization formulas were developed using the present data.

Fast Radiation Parameterization Schemes for Mesoscale and Short-Range Forecast Models

Abstract Three fast and simple longwave broadband radiation parameterization schemes were tested against a reference narrow-band model in clear sky conditions. The three schemes gave rather similar results. The emissivity dependence on water vapor path length was tuned to give best fit to the reference. The smaller other gas, aerosol and continuum circus were added in a simple fashion both to the radiative cooling and to the longwave downward flux at the surface. Clouds are handled as blackbodies. In the shortwave scheme the attenuation of solar radiation due to variable water vapor and gray clouds, but average ozone and other gas absorption, aerosols and Rayleigh scattering are taken into account and tuned by surface observations of solar flux. The water vapor absorptivity for solar heating was fitted by line-by-line model results.

The Performance of a Medium-Range Forecast Model in Winter–Impact of Physical Parameterizations

Abstract We present the results of a series of forecasts on seven weather situations from February 1976 using two models which differ only in their physical parameterizations. One set of parameterizations was developed at the Geophysical Fluid Dynamics Laboratory (GFDL) some years ago, the other more recently at the European Centre for Medium Range Forcasts (ECMWF). The resolution of the model (N48, 15 levels) was that which ECMWF has used in the first phase of operations, which began in August 1979. The particular aim of the experiments was to study the importance of the differences in the parameterization schemes for the model; in addition, we obtained a general view of the forecast results that might be available in the first phase of operations. Both sets of parameterizations gave similar results in terms of forecast quality. When measured by the standard objective methods, the range of predictability was 5–6 days. A study of the systematic errors in the forecasts showed that these were mainly associa...

The Martian Slope Winds and the Nocturnal PBL Jet

Abstract The summertime Martian PBL diurnal wind variation, slope winds, and the nocturnal low-level jets were studied using Prandtls theory, a mesoscale numerical model, and Viking lander observations. During moderate prevailing large-scale flow, nocturnal jets were simulated that were rather similar to those on Earth. They were mainly caused by inertial oscillation after sunset with some contribution from the slope wind effects over sloping regions (which are very common in Mars). During weak large-scale flow, shallow nocturnal drainage flows with strong vertical shear developed over the cold Martian slopes. At middle and high latitudes, these katabatic winds tended to turn to flow along the slope by dawn (due to the Coriolis force). For sufficiently steep slopes, near-surface drainage winds could reach considerable speeds. In contrast, the typical afternoon upslope winds were vertically homogeneous up to 2–3 km and weak (only 1–3 m s−1 in magnitude), even over relatively steep large-scale slopes.

The Martian atmospheric boundary layer

The planetary boundary layer (PBL) represents the part of the atmosphere that is strongly influenced by the presence of the underlying surface and mediates the key interactions between the atmosphere and the surface. On Mars, this represents the lowest 10 km of the atmosphere during the daytime. This portion of the atmosphere is extremely important, both scientifically and operationally, because it is the region within which surface lander spacecraft must operate and also determines exchanges of heat, momentum, dust, water, and other tracers between surface and subsurface reservoirs and the free atmosphere. To date, this region of the atmosphere has been studied directly, by instrumented lander spacecraft, and from orbital remote sensing, though not to the extent that is necessary to fully constrain its character and behavior. Current data strongly suggest that as for the Earths PBL, classical Monin-Obukhov similarity theory applies reasonably well to the Martian PBL under most conditions, though with some intriguing differences relating to the lower atmospheric density at the Martian surface and the likely greater role of direct radiative heating of the atmosphere within the PBL itself. Most of the modeling techniques used for the PBL on Earth are also being applied to the Martian PBL, including novel uses of very high resolution large eddy simulation methods. We conclude with those aspects of the PBL that require new measurements in order to constrain models and discuss the extent to which anticipated missions to Mars in the near future will fulfill these requirements.

Error Growth in a Large Numerical Forecast System

Abstract Internal and external z500 global total rms errors followed quadratic growth laws quite well in NMC Medium-Range Forecast (MRF) Model 0–10-day forecasts for 1988–93. Growth parameters and model and analysis errors for many winter were estimated using the quadratic rms error growth assumption. Both the MRF model error and analysis error have nearly halved during 1988–93. But at the same time the growth parameters have nearly doubled: smaller errors grow faster. Thus while the limit of deterministic predictability (rms error 71% of saturation) has been going up, the limit of dynamic predictability (rms error 97.5% of saturation) seems to be set at around 20 days in large horizontal scales, dropping to 6–7 days in small scales.

Mars Science Laboratory relative humidity observations: Initial results

The Mars Science Laboratory (MSL) made a successful landing at Gale crater early August 2012. MSL has an environmental instrument package called the Rover Environmental Monitoring Station (REMS) as a part of its scientific payload. REMS comprises instrumentation for the observation of atmospheric pressure, temperature of the air, ground temperature, wind speed and direction, relative humidity (REMS-H), and UV measurements. We concentrate on describing the REMS-H measurement performance and initial observations during the first 100 MSL sols as well as constraining the REMS-H results by comparing them with earlier observations and modeling results. The REMS-H device is based on polymeric capacitive humidity sensors developed by Vaisala Inc., and it makes use of transducer electronics section placed in the vicinity of the three humidity sensor heads. The humidity device is mounted on the REMS boom providing ventilation with the ambient atmosphere through a filter protecting the device from airborne dust. The final relative humidity results appear to be convincing and are aligned with earlier indirect observations of the total atmospheric precipitable water content. The water mixing ratio in the atmospheric surface layer appears to vary between 30 and 75 ppm. When assuming uniform mixing, the precipitable water content of the atmosphere is ranging from a few to six precipitable micrometers. Key Points Atmospheric water mixing ratio at Gale crater varies from 30 to 140 ppm MSL relative humidity observation provides good data Highest detected relative humidity reading during first MSL 100 sols is RH75%

Modelling of the combined late-winter ice cap edge and slope winds in Mars Hellas and Argyre regions

Abstract Towards the end of southern hemisphere winter (Ls ≈ 180°) the Martian southern polar cap extends equatorward to 40°S and covers at least, the southern slopes of the Hellas and Argyre impact basins. Subsequently, during retreat of the seasonal ice cap, varying configurations of ice coverage on these slopes occur. Since both sloping topography and ice-edge effects can independently drive mesoscale circulations, the superposition of these two processes may then generate interesting wind patterns. A set of numerical experiments has been performed with the University of Helsinki 2-D Mars Mesoscale Circulation Model (MMCM) in order to study the characteristics of circulations driven by these combined forcings. A model-centre latitude of 57°S and a slope angle of 0.6°, both representative of Hellas southern slope, are used. When compared with the winds arising in the ice-free slope case, ice coverage in the upper extent of the slope results in diminished upslope (daytime) winds, while the down-slope (nighttime) flow is enhanced. Ice coverage in the lower section of the slope in turn causes enhanced upslope (daytime) and attenuated downslope (nocturnal) flows. This arises due to the daytime off-ice near-surface flow induced by the thermal contrast at the ice cap edge. The surface winds are persistently downslope over a fully ice-covered slope. Inclusion of atmospheric dust (τ = 0.3) reduces the ice-edge forcing. In comparison with the dust-free situation, the resulting circulation is almost unchanged in the case of ice-covered upper part of the slope, in the opposite case the daytime flow is attenuated and the nocturnal downslope flow enhanced. When the entire slope is ice-covered, the flow is amplified due to the increased direct atmospheric heating. Inclusion of a large scale circulation component (7 m⧸s southerly wind) in conjunction with an ice-covered slope top results in the generation of a downslope windstorm (fohn, or bora-type of event) with near surface winds exceeding 30 m⧸s. Winds of this magnitude, not realised in any of the other experiments, approach speeds deemed capable of lifting dust from the surface.

Global Energy and Moisture Budgets from Rawinsonde Data

Abstract Vertically integrated budgets of moisture and energy for the global atmosphere are presented, based on the GFDL 10-year rawinsonde data analyses of Oort. The extratropical free atmospheric mean wind divergence is derived from vorticity balance, and mass balance is imposed. The results of data so modified are shown in the form of DJF and JJA maps of mean vertical velocity, evaporation minus precipitation, mean diabatic beating, total energy flux divergence and its potential. Also the annual poleward enemy transport are shown together with the implied oceanic transports. The results indicate that the procedure of getting divergent winds from vorticity balance works wed at mid-latitudes and produces budgets comparable to FGGE data estimates. The results for the tropics are free of any model assumptions such as convection schemes (which may influence data-assimilation scheme climates as is briefly discussed) but are quite sensitive to the lack of data over oceans.

The United States Great Plains Diurnal ABL Variation and the Nocturnal Low-Level Jet

Abstract Observations made from an Oklahoma City TV tower are first used to test the one- and two-dimensional versions of a numerical mesoscale model. Both model versions produce a fairly good reproduction of the observed typical diurnal variation in the wind, temperature, and turbulent mixing coefficient profiles. Next, the models are used to study slope winds and the nocturnal low-level jet (NLLJ) formation, starting with the Prandtl slope wind theory conditions and eventually adding more complex physics (e.g., relating to slope angle, mixing, radiation, background flow, and heat input conditions). The models are able to reproduce the Prandtl analytical solution in the appropriate simple conditions. The time scale to reach this solution from an initial rest state is, however, very long for gently sloping plains. Results with the full model indicate that the main mechanism for the Great Plains “average” NLLJ is the inertial oscillation due to frictional decoupling after sunset. Under normal conditions th...