Remote sensing winds in complex terrain : a review

Ground-based remote sensing is now essential for wind energy purposes. Currently available remote sensing instruments construct a wind vector from wind components measured at several spatially separated volumes, leading to errors on complex terrain where the flow is inhomogeneous. Wind estimation errors are found to be fully described by only two parameters: the flow curvature and flow inclination above the instrument. However, neither parameter is measured directly, nor are they simple products of flow models, so the challenge is to adequately estimate them. Linearized flow models are attractive in being fast and requiring few inputs, but make several limiting assumptions that can lead to their failure to adequately predict corrections for remote sensing. It is found that sophisticated CFD models can also over-correct. The status of such corrections is reviewed here, from a number of diverse measurement campaigns, and it is found that generally remote sensed winds can be corrected to within 1.5% of nearby mast winds. Alternative methods, using multiple receivers sensing several wind components within one volume, are also reviewed. Such systems show promise but are under development and further improvements are likely

HOAPS precipitation validation with ship-borne rain gauge measurements over the Baltic Sea

Global ocean precipitation is an important part of the water cycle in the climate system. A number of efforts have been undertaken to acquire reliable estimates of precipitation over the oceans based on remote sensing and reanalysis modelling. However, validation of these data is still a challenging task, mainly due to a lack of suitable in situ measurements of precipitation over the oceans. In this study, validation of the satellite-based Hamburg Ocean Atmosphere Parameters and fluxes from Satellite data (HOAPS) climatology was conducted with in situ measurements by ship rain gauges over the Baltic Sea from 1995 to 1997. The ship rain gauge data are point-to-area collocated against the HOAPS data. By choosing suitable collocation parameters, a detection rate of up to about 70% is achieved. Investigation of the influence of the synoptic situation on the detectability shows that HOAPS performs better for stratiform than for convective precipitation. The number of collocated data is not sufficient to validate precipitation rates. Thus, precipitation rates were analysed by applying an interpolation scheme based on the Kriging method to both data sets. It was found that HOAPS underestimates precipitation by about 10%, taking into account that precipitation rates below 0.3 mm h−1 cannot be detected from satellite information

The Impact of North Atlantic-Arctic Multidecadal Variability on Northern Hemisphere Surface Air Temperature

The 20th century Northern Hemisphere surface climate exhibits a long-term warming trend, largely caused by anthropogenic forcing, and natural decadal climate variability superimposed on it. This study addresses the possible origin and strength of internal decadal climate variability in the Northern Hemisphere during the recent decades. We present results from a set of climate model simulations that suggest natural internal multidecadal climate variability in the North Atlantic-Arctic Sector could have considerably contributed to the Northern Hemisphere surface warming since 1980. Although covering only a few percent of the earth’s surface, the Arctic may have provided the largest share in this. It is hypothesized that a stronger Meridional Overturning Circulation in the Atlantic and the associated increase in northward heat transport enhanced the heat loss from the ocean to the atmosphere in the North Atlantic region, and especially in the North Atlantic portion of the Arctic due to anomalously strong sea ice melt. The model results stress the potential importance of natural internal multidecadal variability originating in the North Atlantic-Arctic Sector in generating inter-decadal climate changes not only on a regional, but possibly also on a hemispheric and even global scale

Precipitation Data Retrieval and Quality Assurance from Different Data Sources for the Namoi Catchment in Australia

Within the Horizon 2020 Project WaterSENSE a modular approach was developed to provide different stakeholders with the required precipitation information. An operational high-quality rainfall grid was set up for the Namoi catchment in Australia based on rain gauge adjusted radar data. Data availability and processing considerations make it necessary to explore alternative precipitation approaches. The gauge adjusted radar data will serve as a benchmark for the alternative precipitation data. The two well established satellite-based precipitation datasets IMERG and GSMaP will be analyzed with the temporal and spatial requirements of the applications envisioned in WaterSENSE in mind. While first results appear promising, these datasets will need further refinements to meet the criteria of WaterSENSE, especially with respect to the spatial resolution. Inferring information from soil moisture-derived from EO observations to increase the spatial detail of the existing satellite-based datasets is a promising approach that will be investigated along with other alternatives

Corrections to sodar Doppler winds due to wind drift

Refraction of the acoustic beam from a sodar, or translation of the beam due to the wind (known as wind drift), both affect the scattering angle and hence the Doppler shift of the return signal. Wind drift has been the subject of a number of previous studies, which have shown that errors increase with wind speed, giving about 6 % error in the estimated wind speed when the actual wind speed is 7 ms−1. Since previous studies have not treated the general case of finite angular beam width, a new analytic treatment is given here for monostatic sodars. Surprisingly, it is found that the Doppler error contributed by the transmitted beam is exactly compensated by the Doppler error contributed by the received beam, if the two beam widths are the same. If the transmitter beam width is not equal to the receiver beam width, then either over-estimation or under-estimation of wind speed results, depending on which beam is wider. This has implications for bi-static sodars, in which it is not possible to match beam widths. Contrary to this new theory, examples of field comparisons with mast instrumentation show a non-linear relationship between wind speeds measured by a sodar and wind speeds measured by mast-mounted instruments. It is proposed that this is due to the acoustic baffle clipping the received beam and changing its width, since the received beam must return at lower elevations from scattering downwind. This is shown to be feasible for a Metek sodar. The sign and magnitude of the observed non-linear dependence of estimated wind speed on actual wind speed are consistent with this proposed mechanism

Precipitation Data Retrieval and Quality Assurance from Different Data Sources for the Namoi Catchment in Australia

Within the Horizon 2020 Project WaterSENSE a modular approach was developed to provide different stakeholders with the required precipitation information. An operational high-quality rainfall grid was set up for the Namoi catchment in Australia based on rain gauge adjusted radar data. Data availability and processing considerations make it necessary to explore alternative precipitation approaches. The gauge adjusted radar data will serve as a benchmark for the alternative precipitation data. The two well established satellite-based precipitation datasets IMERG and GSMaP will be analyzed with the temporal and spatial requirements of the applications envisioned in WaterSENSE in mind. While first results appear promising, these datasets will need further refinements to meet the criteria of WaterSENSE, especially with respect to the spatial resolution. Inferring information from soil moisture-derived from EO observations to increase the spatial detail of the existing satellite-based datasets is a promising approach that will be investigated along with other alternatives