In the Baltic Sea region, a significant increase in solar radiation has been detected during the past half-century. Changes in shortwave irradiance ar…In the Baltic Sea region, a significant increase in solar radiation has been detected during the past half-century. Changes in shortwave irradiance are associated with atmospheric transparency and cloudiness parameters like cloud fraction and albedo. One of the most important reasons for day-to-day changes in cloudiness is the synoptic-scale atmospheric circulation; thus, we look for reasons for solar radiation trends due to changes in atmospheric circulation. We analysed regional time series and trends from satellite-based cloud climate data record CLARA-A2 for the Baltic Sea region in 1982–2018. As the rise in solar radiation depends on the seasonally averaged values of total fractional cloud cover (CFC), surface incoming shortwave radiation (SIS) and occurrences of circulation types were analysed. We show that the shift in seasonality connected to the earlier accumulated sums of SIS is at least partly explained by the changes in synoptic-scale atmospheric circulation.more
In recent years, the amount of water used for agricultural purposes has been rising due to an increase in food demand. However, anthropogenic water us…In recent years, the amount of water used for agricultural purposes has been rising due to an increase in food demand. However, anthropogenic water usage, such as for irrigation, is still not or poorly parameterized in regional- and larger-scale land surface models (LSMs). By contrast, satellite observations are directly affected by, and hence potentially able to detect, irrigation as they sense the entire integrated soil-vegetation system. By integrating satellite observations and fine-scale modelling it could thus be possible to improve estimation of irrigation amounts at the desired spatial-temporal scale. In this study we tested the potential information offered by Sentinel-1 backscatter observations to improve irrigation estimates, in the framework of a data assimilation (DA) system composed of the Noah-MP LSM, equipped with a sprinkler irrigation scheme, and a backscatter operator represented by a water cloud model (WCM), as part of the NASA Land Information System (LIS). The calibrated WCM was used as an observation operator in the DA system to map model surface soil moisture and leaf area index (LAI) into backscatter predictions and, conversely, map observation-minus-forecast backscatter residuals back to updates in soil moisture and LAI through an ensemble Kalman filter (EnKF). The benefits of Sentinel-1 backscatter observations in two different polarizations (VV and VH) were tested in two separate DA experiments, performed over two irrigated sites, the first one located in the Po Valley (Italy) and the second one located in northern Germany. The results confirm that VV backscatter has a stronger link with soil moisture than VH backscatter, whereas VH backscatter observations introduce larger updates in the vegetation state variables. The backscatter DA introduced both improvements and degradations in soil moisture, evapotranspiration and irrigation estimates. The spatial and temporal scale had a large impact on the analysis, with more contradicting results obtained for the evaluation at the fine agriculture scale (i.e. field scale). Above all, this study sheds light on the limitations resulting from a poorly parameterized sprinkler irrigation scheme, which prevents improvements in the irrigation simulation due to DA and points to future developments needed to improve the system.more
The satellite-based cloud climate data record CLARA-A2 has been used to analyse regional average time-series and regional maps of trends in the Baltic…The satellite-based cloud climate data record CLARA-A2 has been used to analyse regional average time-series and regional maps of trends in the Baltic Sea region, 1982–2015. The investigated cloud parameters were total fractional cloud cover and cloud top height. Cloud observations from the Tartu-Tõravere meteorological station were used as reference data for the same period. Fractional cloud cover from CLARA-A2 was in a good agreement with in situ data regarding the maxima and minima years and a downward trend in March over the 1982–2015 period. In June the fractional cloud cover interannual variability was very high and no clear trend was seen. For cloud top heights summer and spring regional averages showed opposite signs of the trend: for June positive and for March negative. Winter and autumn seasons have been left out of analysis due to too large uncertainties in cloud products over latitudes higher than 60∘.more
The near-global and all-sky coverage of satellite observations from microwave humidity sounders operating in the 183 GHz band complement radiosonde an…The near-global and all-sky coverage of satellite observations from microwave humidity sounders operating in the 183 GHz band complement radiosonde and aircraft observations and satellite infrared clear-sky observations. The Special Sensor Microwave Water Vapor Profiler (SSM/T-2) of the Defense Meteorological Satellite Program began operations late 1991. It has been followed by several other microwave humidity sounders, continuing today. However, expertise and accrued knowledge regarding the SSM/T-2 data record is limited because it has remained underused for climate applications and reanalyses. In this study, SSM/T-2 radiances are characterised using several global atmospheric reanalyses. The European Centre for Medium-Range Weather Forecasts (ECMWF) Interim Reanalysis (ERA-Interim), the first ECMWF reanalysis of the 20th-century (ERA-20C), and the Japanese 55-year Reanalysis (JRA-55) are projected into SSM/T-2 radiance space using a fast radiative transfer model. The present study confirms earlier indications that the polarisation state of SSM/T-2 antenna is horizontal (not vertical) in the limit of nadir viewing. The study also formulates several recommendations to improve use of the SSM/T-2 measurement data in future fundamental climate data records or reanalyses. Recommendations are (1) to correct geolocation errors, especially for DMSP 14; (2) to blacklist poor quality data identified in the paper; (3) to correct for inter-satellite biases, estimated here on the order of 1 K, by applying an inter-satellite recalibration or, for reanalysis, an automated (e.g., variational) bias correction; and (4) to improve precipitating cloud filtering or, for reanalysis, consider an all-sky assimilation scheme where radiative transfer simulations account for the scattering effect of hydrometeors.more
The estimation of tropical cyclone (TC) intensity using Ku-band scatterometer data is challenging due to rain perturbation and signal saturation in th…The estimation of tropical cyclone (TC) intensity using Ku-band scatterometer data is challenging due to rain perturbation and signal saturation in the radar backscatter measurements. In this paper, an alternative approach to directly taking the maximum scatterometer-derived wind speed is proposed to assess the TC intensity. First, the TC center location is identified based on the unique characteristics of wind stress divergence/curl near the TC core. Then the radial extent of 17-m/s winds (i.e., R17) is calculated using the wind field data from the Haiyang-2B (HY-2B) scatterometer (HSCAT). The feasibility of HSCAT wind radii in determining TC intensity is evaluated using the maximum sustained wind speed (MSW) in the China Meteorological Administration best-track database. It shows that the HSCAT R17 value generally better correlates with the best-track MSW than the HSCAT maximum wind speed, therefore indicating the potential of using the HSCAT data to improve the TC nowcasting capabilities.more
Buoys provide key observations of wind speed over the ocean and are routinely used as a source of validation data for satellite wind products. However…Buoys provide key observations of wind speed over the ocean and are routinely used as a source of validation data for satellite wind products. However, the movement of buoys in high seas and the airflow over waves might cause inaccurate readings, raising concern when buoys are used as a source of wind speed comparison data. The relative accuracy of buoy winds is quantified through a triple collocation (TC) exercise comparing buoy winds to winds from ASCAT and ERA5. Differences between calibrated buoy winds and ASCAT are analyzed through separating the residuals by anemometer height and testing under high wind-wave and swell conditions. First, we converted buoy winds measured near 3, 4, and 5 m to stress-equivalent winds at 10 m (U10S). Buoy U10S from anemometers near 3 m compared notably lower than buoy U10S from anemometers near 4 and 5 m, illustrating the importance of buoy choice in comparisons with remote sensing data. Using TC calibration of buoy U10S to ASCAT in pure wind-wave conditions, we found that there was a small, but statistically significant difference between height adjusted buoy winds from buoys with 4 and 5 m anemometers compared to the same ASCAT wind speed ranges in high seas. However, this result does not follow conventional arguments for wave sheltering of buoy winds, whereby the lower anemometer height winds are distorted more than the higher anemometer height winds in high winds and high seas. We concluded that wave sheltering is not significantly affecting the winds from buoys between 4 and 5 m with high confidence for winds under 18 ms(-1). Further differences between buoy U10S and ASCAT winds are observed in high swell conditions, motivating the need to consider the possible effects of sea state on ASCAT winds.more
Abstract. This paper analyzes the differences between ERA-Interim
and ERA5 surface winds fields relative to Advanced Scatterometer (ASCAT) ocean vecto…Abstract. This paper analyzes the differences between ERA-Interim
and ERA5 surface winds fields relative to Advanced Scatterometer (ASCAT) ocean vector wind
observations, after adjustment for the effects of atmospheric stability and
density, using stress-equivalent winds (U10S) and air–sea relative motion
using ocean current velocities. In terms of instantaneous root mean square (rms) wind speed
agreement, ERA5 winds show a 20 % improvement relative to ERA-Interim
and a performance similar to that of currently operational ECMWF forecasts.
ERA5 also performs better than ERA-Interim in terms of mean and transient wind
errors, wind divergence and wind stress curl biases. Yet, both ERA products
show systematic errors in the partition of the wind kinetic energy into
zonal and meridional, mean and transient components. ERA winds are
characterized by excessive mean zonal winds (westerlies) with too-weak mean
poleward flows in the midlatitudes and too-weak mean meridional winds (trades)
in the tropics. ERA stress curl is too cyclonic in midlatitudes and high latitudes,
with implications for Ekman upwelling estimates, and lacks detail in the
representation of sea surface temperature (SST) gradient effects (along the equatorial cold tongues
and Western Boundary Current (WBC) jets) and mesoscale convective airflows (along the Intertropical Convergence Zone and the warm
flanks for the WBC jets). It is conjectured that large-scale mean wind
biases in ERA are related to their lack of high-frequency (transient wind)
variability, which should be promoting residual meridional circulations in
the Ferrel and Hadley cells.more