New observation systems for better weather forecasting

The first weath­er fore­cast­ing mod­els were developed in the early 20th cen­tury¹ with the idea that if the ini­tial state of the atmo­sphere – that is, the con­di­tions of wind, humid­ity, tem­per­at­ure and pres­sure at a giv­en time – are known pre­cisely, then its future state can be pre­dicted thanks to the equa­tions of phys­ics gov­ern­ing the tem­por­al evol­u­tion of the atmo­spher­ic vari­ables. For a weath­er fore­cast to be use­ful, it needs to be of a cer­tain qual­ity and provide pre­dic­tions for the short term: it can there­fore be thought of as a kind of race against time. Moreover, as the weath­er fore­cast at a giv­en point depends on the weath­er con­di­tions observed else­where, weath­er fore­casters set up col­lab­or­a­tions early on in the his­tory of the field to exchange their obser­va­tion­al data in real time and com­pare the per­form­ance of their models.

Ground and upper air observations

The first obser­va­tion­al sys­tems con­sisted of net­works of instru­ments that meas­ured atmo­spher­ic con­di­tions in situ (at the exact loc­a­tion of the sensor) at ground level, but met­eor­o­lo­gists soon real­ised that they also needed obser­va­tions at alti­tude. They thus developed radio­sondes – bal­loons car­ry­ing instru­ments to meas­ure tem­per­at­ure, pres­sure, humid­ity, and wind, launched sev­er­al times a day and reach­ing up to 20 km in alti­tude. Some bal­loons can also be launched from ships.

Obser­va­tion tech­no­lo­gies have evolved con­sid­er­ably and we are now wit­ness­ing the emer­gence of obser­va­tion pro­jects based on drones, which could be of oper­a­tion­al interest for sampling the atmo­sphere at alti­tude, par­tic­u­larly over the sea (where deploy­ment con­di­tions are less con­strained), fol­low­ing sampling strategies that are determ­ined by met­eor­o­lo­gic­al conditions.

The advent of remote sensing 

From the 1960–70s, remote sens­ing (the remote meas­ure­ment of atmo­spher­ic para­met­ers) began to be intro­duced into obser­va­tion net­works. Weath­er radar is an example of such remote sens­ing instru­ments. These devices emit elec­tro­mag­net­ic waves that propag­ate and inter­act with rain, snow and hail, and can thus be used to map pre­cip­it­a­tion over areas of sev­er­al hun­dred kilo­metres or more when the instru­ments are net­worked. Today, about 200 weath­er radars are in oper­a­tion in Europe. The num­ber is sim­il­ar in the United States and Japan. For many years, weath­er radar data have been assim­il­ated into numer­ic­al weath­er pre­dic­tion mod­els, sig­ni­fic­antly improv­ing the qual­ity of pre­cip­it­a­tion fore­casts².

Anoth­er example of a remote sens­ing instru­ment is lid­ar, which is sim­il­ar to radar except that the waves emit­ted are light waves. These waves are sens­it­ive to aer­o­sols – small dust particles sus­pen­ded in the atmo­sphere – or cloud droplets. Lid­ars can there­fore meas­ure the prop­er­ties of these particles from a dis­tance and are now part of the oper­a­tion­al net­works used by met­eor­o­lo­gic­al ser­vices³.

Satel­lite observations 

Anoth­er major advance has of course been the emer­gence of satel­lite obser­va­tions, which make it pos­sible to observe large areas with the same instru­ment. Satel­lites can be divided into two main fam­il­ies. The first are the geo­sta­tion­ary satel­lites, which remain per­man­ently above a single point on the equat­or (at an alti­tude of about 36 000 km). As they can only observe half the globe, inter­na­tion­al col­lab­or­a­tion is required to cov­er the whole planet.

The second type of satel­lite is the polar-orbit­ing satel­lite, which, as its name sug­gests, con­tinu­ally orbits the Earth (at an alti­tude of between 300 and 800 km), typ­ic­ally circ­ling the globe in about 100 minutes. These satel­lites can provide obser­va­tions at all lat­it­udes, includ­ing the poles, with the same spa­tial res­ol­u­tion, unlike geo­sta­tion­ary satel­lites. And because they are about ten times closer to the Earth, they offer bet­ter res­ol­u­tion of clouds and surfaces.

A wide vari­ety of instru­ments oper­at­ing in dif­fer­ent wavelengths are car­ried on these satel­lites and allow the prop­er­ties of the atmo­sphere to be meas­ured: clouds, pre­cip­it­a­tion, aer­o­sols, wind, tem­per­at­ure and humidity.

A new gen­er­a­tion of European geo­sta­tion­ary satel­lites has been launched.

The num­ber of met­eor­o­lo­gic­al satel­lites has increased dra­mat­ic­ally in recent years and ambi­tious pro­grammes, par­tic­u­larly in Europe, have been launched. Last Decem­ber, for example, a new gen­er­a­tion of European geo­sta­tion­ary satel­lites was launched⁴, as part of the European Agency’s pro­gramme for oper­a­tion­al weath­er satellites.

Timely obser­va­tions 

Finally, there are the so-called timely obser­va­tions – that is, those that come from an infra­struc­ture that was not ori­gin­ally designed to carry out atmo­spher­ic meas­ure­ments. An import­ant example: mobile phone net­works. These net­works use anten­nas (there are sev­er­al thou­sand of them in France) that com­mu­nic­ate with each oth­er at microwave wavelengths. Oper­at­ors quickly real­ised that the level of recep­tion between anten­nas was reduced when there were areas (or “cells”) of rain­fall between them. This is because microwave wavelengths are affected by precipitation. 

« Oper­at­ors reacted to this prob­lem by equip­ping the sys­tems with a capa­city to increase recep­tion levels in the event of observed atten­u­ation, but for met­eor­o­lo­gists, the meas­ure­ment of this observed atten­u­ation was very inter­est­ing because it provided inform­a­tion on the intens­ity of pre­cip­it­a­tion occur­ring in a giv­en area, » explains Pierre Tabary, Deputy Dir­ect­or of Oper­a­tions at Météo France. This indir­ect inform­a­tion, prop­erly pro­cessed, can help to improve pre­cip­it­a­tion maps⁵. « Nobody had ima­gined at the out­set that mobile phone net­works could be used in this way.

Pos­i­tion­ing satel­lites – the GPS sys­tem in the US and Galileo in Europe – are anoth­er example of a timely-based meas­ure­ment. « These ded­ic­ated pos­i­tion­ing satel­lites are con­stantly emit­ting sig­nals. So, the ingeni­ous idea was to put oth­er, much smal­ler satel­lites, that oppor­tun­ist­ic­ally pick up these sig­nals, into orbit. The sig­nals that are detec­ted are slightly refrac­ted on their way from the trans­mit­ting satel­lite to the receiv­ing satel­lite as they pass through the atmo­sphere, so that they are some­what ‘bent’. The level of this bend­ing can be quan­ti­fied by the receiv­ing satel­lite and indir­ectly provides valu­able inform­a­tion about ther­mo­dy­nam­ic con­di­tions, such as humid­ity, in the stra­to­sphere and upper troposphere.

Research­ers have veri­fied the rel­ev­ance of this meas­ure­ment prin­ciple (called radio occulta­tion) and there are now sev­er­al dozen recep­tion satel­lites of this type in ser­vice, the data from which are ana­lysed by oper­a­tion­al weath­er fore­cast­ing mod­els⁶. « Here again, we are able to carry out meas­ure­ments at lower cost: we do not emit waves into the atmo­sphere ourselves, but we exploit the waves already emit­ted by others.”

Today, about 90% of the data input into glob­al weath­er fore­cast­ing mod­els comes from satel­lites and this trend will con­tin­ue. « That said, ground-based meas­ure­ments will always be import­ant – espe­cially for cal­ib­rat­ing satel­lite data, » con­cludes Pierre Tabary.

 Isabelle Dumé