CHAPTER 3.2 GROUND-BASED OBSERVATIONS Franz-Josef L¨ubken1 and Keri A. Nicoll2 1 Introduction Surface and airborne observations of meteorological parameters have been made for many centuries, producing large datasets which span significant periods of time. Although satellites provide more comprehensive global coverage than ground-based observations, their short period of operation means that detecting the effect of long term changes in solar variability on climate is difficult, not least due to the additional complication of increased anthropogenic activity during the period of satellite measurements, which is likely to mask solar signals. Ground-based observations therefore provide a vital existing data source of the investigation of the effects of long term solar changes on climate, however, as with any long-term data source, caveats exist. For example, ground-based instruments are normally upgraded from time to time to take advantage of contemporary technological devel- opments. This might influence the statistics, e.g., when more sensitive instruments are applied. Generally speaking, trends are not monotonous in time because of non-uniform forcing and/or non-linear interactions. Physical and chemical param- eters in the upper atmosphere are typically impacted by the 11-year variation of solar radiation. This variation is small for the total solar radiation( 0.1%) but is large (up to a factor of 100) when UV and EUV radiation is considered∼ (see Chap- ter 4.1). Therefore, ground-based techniques should be in place for several decades in order to distinguish long term trends from variations due to the solar cycle. Ground-based measurements provide information at one location only. More information may be needed from several stations or from satellites to get a complete picture. Furthermore, trends of parameters measured from the ground may be influenced by several processes, sometimes with opposite effects. In this chapter, we concentrate on two altitude regions which can be measured using ground-based techniques: the troposphere{stratosphere region (0{50 km), and the mesosphere{lower thermosphere region (MLT, 50{150 km). We include sensors 1 Leibniz-Institute of Atmospheric Physics, 18225 K¨ulungsborn, Germany 2 Department of Meteorology, University of Reading, UK c EDP Sciences 2015 DOI: 10.1051/978-2-7598-1733-7.c117 140 Earth's climate response to a changing Sun measuring atmospheric parameters directly (in-situ) at the ground or from bal- loons, and techniques which are based on remote sensing from the ground. In-situ measurements are also available in the MLT from sounding rockets, but they only occur infrequently and are not considered further here. A summary of techniques is presented in Tables1 and2. Table 1. Summary of meterorological parameters and the most common measurement techniques used to observe them routinely in the troposphere and stratosphere. Only direct methods (not proxy methods) of observation are included. Parameter Sensor/Technique Remarks Temperature - liquid-in-glass thermometer Mounted at a height between 1.2- - thermograph 2 m above ground level, and shel- - electrical resistance thermome- tered from direct exposure to sun. ter Temperature above the surface - thermocouple measured by electrical thermome- ters on meteorological radioson- des. Sea Surface - sample of sea surface water with No standard device for measur- Temperature bucket ing sea surface temperature due to - reading temperature of the con- huge diversity in ship size/speed, denser intake water cost, ease of operation and main- - exposing electrical thermometer tenance. to sea water temperature - infrared radiometer mounted on ship to look down on sea surface Pressure - electronic barometer Should be placed in an environ- (sea level) - mercury barometer ment where external effects will - aneroid barometer not lead to measurement errors. Pressure above the surface mea- sured on meteorological radioson- des. Humidity - pyschrometer (Assman, whirling Screen pyschrometer and hair or screen) and electrical hygrometers should - hair hygrometer be mounted inside thermometer - chilled mirror dew point cell screen at height of 1.2-2 m above - electrical resistive and capacitive the surface. Relative humidity hygrometer above the surface measured on meteorological radiosondes. Wind Standard exposure of wind instru- - speed - cup/propeller anemometer ments is over level terrain, 10 m - direction - wind vane above the surface. Pitot-tubes, hot wire and sonic anemometers, as well as radar and lidar can all be used to measure wind proper- ties, but are not yet that com- mon in routine meteorological net- works. Wind above the surface is also measured by GPS from mete- orological radiosondes. F.-J. L¨ubken and K.A. Nicoll: Ground-based observations 141 Solar radiation Radiation sensors must be sited in - direct - pyrheliometer locations with freedom from ob- - global - pyranometer structions to the solar beam at all - diffuse - pyranometer times and seasons of the year, and - net global - net pyranometer correct alignment of radiation sen- - longwave - pyrgeometer sors, as well as regular calibration - total - pyradiometer is essential. Cloud - synoptic (cloud amount, type) Synoptic observations are visual - cloud base recorer/laser ceilome- observations made by an observer ter (cloud amount, height) at the surface. Cloud radars can also be used to measure cloud properties above the surface but these are primarily research in- struments and not yet used in widespread global operational me- teorology. Rainfall - manual rain gauge Gauges are typically mounted be- - electronic rain gauge (tipping tween ground and 1.5 m above the bucket, weighing or float) surface. - X, C or S band weather radar (GHz) Ozone - Dobson ozone spectrophotomer Dobson and M-124 instruments - total ozone - Brewer spectrophotometer stored indoors and transported - M-124 filter ozonemeter outside to take a measurement. Brewer instrument permanently mounted outdoors. - vertical profile - ozonesonde (from balloon) Lidar technique only operates at - lidar night and when no cloud cover is - microwave radiometer present. Electric field - radioactive probe Typically mounted between 1{3 m - electric field mill above the ground. Measurements must be corrected for distortion of the electric field by the mounting pole and other surrounding metal objects. Lightning - thunder days (local lightning Ground-based lightning receivers presence) are typically deployed in networks - VLF direction finder (lightning consisting of many sensors at a location and strike rate) variety of locations to ensure the - VLF time of arrival receivers best coverage. (lightning location and strike rate) 142 Earth's climate response to a changing Sun Table 2. Summary of ground-based remote sensing techniques for the mesosphere/lower thermosphere. Technique Parameters Approx. height coverage Neutral density, Ground to upper Rayleigh temperatures mesosphere Lidars aerosols, winds Resonance 80{120 km Na, K, Fe density, temperatures aerosols, winds MF (0.3{3 MHz) Winds, electron 60{100 km densities Radars Meteor Winds, meteor heights 80{110 km VHF (30{300 MHz) Aerosols, winds 80{90 km Incoherent Ionospheric 80 to several (UHF, >300 MHz) (electrons, ions) hundred km Ionosondes HF (1{40 MHz) Electron densities 100{400 km 2 Ground-based observations in the troposphere and stratosphere Here, we describe observations of the troposphere and stratosphere. We focus on meteorological parameters in which solar influences have already been detected: temperature, sea surface temperature, rainfall, ozone, cloud and atmospheric elec- tricity. Table1 details commonly measured meteorological parameters in the tro- posphere as well as their measurement techniques. More information can be found, e.g., from WMO(1988). 2.1 Temperature Air temperature is perhaps the most commonly measured meteorological param- eter, typically measured at a height of 1.5 m using a thermometer, housed inside a ventilated screen to protect the thermometer from overheating from the effects of direct solar radiation. Traditionally thermometers used mercury or alcohol in glass, but nowadays electronic thermometers using resistive methods are com- mon. The longest instrumental time series of daily temperature measurements is the Central England Temperature series, dating back to 1772 (see Figure1). Although such time series are invaluable for detecting long term trends in data Bremer and Berger(2002), it is important to consider that changes in thermometer instrumentation, meteorological site characteristics, and the state of the protective screen can all contribute to unwanted biases in temperature data. This is parti- cularly true for upper air measurements of temperature made from meteorological radiosonde balloons, which have seen significant changes in the development and accuracy of the temperature sensors since their inception in the 1940s. This has resulted in inhomogeneities within the long-term time series, which must be F.-J. L¨ubken and K.A. Nicoll: Ground-based observations 143 Fig. 1. Annual temperature anomalies from the longest time series of temperature mea- surements in the world { the Central England Temperature series, which is representative of a roughly triangular area of the United Kingdom enclosed by Lancashire, London and Bristol. Daily values date back to 1772, and monthly values to 1659. Anomalies shown are relative to the 1961{1990 average and the red line is a 21-point binomial filter, which is roughly equivalent to a 10-year running mean (Source: