Accuracy Matters in Radiosonde Measurements / WHITE PAPER WEA-MET-G-Accuracy-Matters-in-Radiosonde-Measurements-White-Paper-B211548EN-A.indd 1 11.3.2016 10.37 Table of Contents Introduction ................................................................................................................................................. 3 Quality of Radiosonde Measurements is Critical ............................................................................................ 5 Convective Weather and Thunderstorms ...................................................................................................... 6 Radiosondes in Forecasting Convection ..................................................................................................... 6 Case Study 1: Convective Weather ............................................................................................................. 8 Case Study 2: Weak Convection ................................................................................................................10 Winter Weather ...........................................................................................................................................12 Radiosondes in Forecasting Winter Precipitation ......................................................................................13 Case Study 3: Freezing Rain and Ice Storm ................................................................................................15 Numerical Weather Prediction ..................................................................................................................... 17 Radiosondes in Validating NWP Models ....................................................................................................18 Case Study 4: Fog .....................................................................................................................................18 Meteorological Indices ...............................................................................................................................20 Impact of Radiosonde Data Quality ...........................................................................................................21 Radiosonde Pressure Measurement ............................................................................................................ 23 GPS-based Pressure ................................................................................................................................ 24 Sensor-based Pressure ............................................................................................................................ 25 Case Study 5: Tropical Cyclone ................................................................................................................ 26 Radiosonde Data in Climatology ................................................................................................................. 27 Radiosonde Accuracy Matters .................................................................................................................... 28 Further reading ...................................................................................................................................... 29 2 WEA-MET-G-Accuracy-Matters-in-Radiosonde-Measurements-White-Paper-B211548EN-A.indd 2 11.3.2016 10.37 Introduction Radiosondes measure critical atmospheric variables with accuracy and precision that cannot be obtained with other meteorological observations. Radiosondes are unique instruments as they provide continuous, detailed profiles from the ground to altitudes of 30 km and above. The quality and reliability of the measurements are essential. Even small inaccuracies in the profiles can prevent the forecaster from observing critical details and making correct conclusions. The chapters in this document provide information on the importance of radiosonde measurement accuracy, and how accuracy helps to successfully forecast high impact weather. Graphs, tables, and case study summaries illustrate how correct or incorrect observations of temperature inversion layers, ice forming layers, humidity, and other atmospheric properties can change a forecast in various weather situations. a) Deep convection d) Freezing rain Sounding Modified sounding: profile wet-bulb error Ice formation Shallow layer T < -10 °C ➔ Probable ice formation Elevated warm Tmax = 1.9 °C ➔ Partial melting of layer ice ➔ Solid and liquid can occur Surface Tsurface < 0 °C ➔ Rain will freeze on the ground ➔ Ice accumulation / sleet Forecast Ice pellets (more probable) or freezing rain or mix 3 WEA-MET-G-Accuracy-Matters-in-Radiosonde-Measurements-White-Paper-B211548EN-A.indd 3 11.3.2016 10.37 4 WEA-MET-G-Accuracy-Matters-in-Radiosonde-Measurements-White-Paper-B211548EN-A.indd 4 11.3.2016 10.37 Quality of Radiosonde Measurements is Critical The radiosonde is unique among important that radiosonde sensors or even entirely mask the feature other observation systems in work reliably in changing conditions in the profile. In summertime that it provides a complete throughout the harsh environment this may lead the meteorologist vertical profiling of the initial of the upper atmosphere. Erroneous to conclude that, for example, state of the atmospheric analysis measurements could entirely convection will start earlier in that drives numerical weather change the forecast conclusions. For the day with less energy released prediction models. Furthermore, example, the following challenging into convection, or to predict that meteorologists are interested conditions could occur in cloudy rain will start earlier and cool the in several phenomena that are weather situations: land surface, with thunderstorms visible in the radiosonde data, unlikely to form. including cloud layers, dry layers, • As the radiosonde emerges from temperature inversions, jet streams a cloud, the temperature sensor • If the humidity sensor freezes and wind shear. Radiosondes also is in danger of experiencing a while passing through clouds, have an important role in providing ‘wet bulb’ error, generated as the radiosonde may incorrectly long-term high-quality time series water or ice evaporates from the continue measuring high relative of climatology trends of various surface of the sensor. This may humidity. The sensitivity to detect parameters. lead to incorrectly measuring cloud layers is reduced. For the strength of the temperature example, erroneously predicted These applications place a high inversion. Such erroneous upper cloud layers would block demand on the accuracy and observations can reduce the the solar radiation from reaching precision of the measurement. It is magnitude of the detected layer the ground and thus inhibit convection. 5 WEA-MET-G-Accuracy-Matters-in-Radiosonde-Measurements-White-Paper-B211548EN-A.indd 5 11.3.2016 10.37 Convective Weather and Thunderstorms Figure 1. Model radiosonde profiles of temperature, dew-point and wind, showing characteristic features that predict various types of convection. See further explanations in Table 1. During convection, warm air near Radiosondes in Forecasting Convection the ground starts rising until it cools and becomes balanced with In a convective situation, the Radiosonde measurements its surroundings. The following analysis of the latest radiosonde summarize the state of the factors are needed for atmospheric profiles in the region is an essential atmosphere and give the basis for convection to occur: enough step in forecasting. Convection understanding how the weather will moisture at the height from which is still poorly represented in the evolve in the next hours. Figure 1 convection initiates, instability, numerical weather prediction shows examples of three basic types and a lifting mechanism. Strong models due to inadequate spatial of convective weather and how they convection in humid atmosphere and temporal resolutions and the can be forecasted from radiosonde can lead to thunderstorms. difficulty to quantify humidity. profiles. a) Deep convection The air near the surface is warm, moist and well-mixed. There is a strong temperature inversion and enough convective inhibition (CIN) to prevent convection from beginning too early and thus enables convective available potential energy (CAPE) to build up. A relatively rapid decrease in temperature with height in the middle troposphere results in small stability. Cloud layers affect the amount of solar warming on the surface. If air near the surface 6 WEA-MET-G-Accuracy-Matters-in-Radiosonde-Measurements-White-Paper-B211548EN-A.indd 6 11.3.2016 10.37 Factor Explanation Interpretation in radiosonde profile Temperature inversion Layer of warmer air above cool A temperature inversion prevents air. ascending air from penetrating it and helps build up convective energy. A strong inversion may prevent convection entirely. Dew-point temperature The temperature where Humid and dry layers in the profile condensation begins. Moisture indicate the amount of energy available content of air is high when in the atmosphere. the dew-point is close to air A deep dry layer can eat moisture from temperature. lower layers and thus prevent deep convection. CAPE and CIN Meteorological indices for Calculated from radiosonde profile. convective available potential Indicate the stability of the atmosphere. energy and convective inhibition. Vertical wind shear Change in wind speed and Vertical wind shear is needed for direction over height. thunderstorms to become severe and long-lasting. Lifted