Romote Sensing of Weather

It was the stunning camera image of storms taken from a rocket launched in 1954 that ultimately propelled the concept of satellite meteorology, which would become the most obvious application of orbital viewing. Weather satellites have since become more and more complex with greater sensing capability. Whereas the Television InfraRed Observation Satellite (TIROS) in 1960 only sent back pictures, today weather imaging vehicles in both equatorial and polar orbits measure many different things. By 1975 geosynchronous orbiting satellites were being launched and quickly became the mainstay of remote imaging in meteorology. The Geosynchronous Orbiting Environmental Satellite (GOES) program places a series of imaging platforms around the globe equidistantly at an altitude of 22,240 statute miles over the equator. This "parks" a satellite in an orbit, precisely matching rotation speed of the earth. The first such vehicle launched was GOES-A, later designated GOES-1 once it successfully achieved orbit. In the 38 years since this program began, we have progressed to GOES 12, 13, 14 and 15, representing the current generation of satellites on station. This modern cadre of weather monitors see more than clouds. They also measure other quantities, such as carbon dioxide and ozone levels, outgoing low wave radiation and other exotic fields. GOES satellites also look outward to monitor space weather (solar x-rays and high energy particles). They keep tabs on the earth's magnetic field, and even carry Emergency Locator Transmitter (ELT) receivers assisting in search and rescue operations. GOES R and S are more advanced still, and will be launched in 2015 and 2017, respectively.

Polar orbiting satellites provide a closer, more detailed look at the ground from a much nearer vantage (450-530 miles altitude). In solar synchronous orbit, high resolution images of the same swath of earth are generally available twice a day. This is particularly advantageous at high latitudes which are farther away from equatorial satellites. Together, all the orbiting weather resources play a tremendous role for meteorologists helping them know what is happening at virtually every point above the planet. For computer model initialization, estimated wind vectors can be derived from these sensors. Vertical temperature, humidity and wind profiles can now be displayed for any location thanks to the current generation of weather satellites.

The other well known and even more immediate type of remote sensing involves weather radar at 200 locations across the United States and its possessions. The concept of radar, with meteorological application, comes from World War II when rainfall was treated as a nuisance for military radar operators. Reflectivity of transmitted electromagnetic energy was displayed at that time on a Cathode Ray Tube with a map overlay. Through the 1960s, intensive research was performed at several locations in the US bringing improvement to basic radars. Soon, Doppler radar was developed, measuring particle movement within storm systems so that incipient circulations could be detected. At about the same time, research also began on dual polarization technology which transmits both horizontally and vertically polarized signals. Returns from atmospheric targets using this method reveal specific shape and size of hydrometeors. Initially,the Doppler radar application was built into a national network in the late 1980s and 1990s. Today, a Dual Polarization update to existing Doppler radars has been completely on most of the 155 WSR-88D and 45 specialized aerodrome Doppler radars. Short range weather forecasting has been greatly improved thanks to real time radar tracking of inclement weather. Each Doppler radar around the country is a highly advanced apparatus, displaying what is tantamount to an MRI of the atmosphere every couple of minutes. Precipitation type, intensity, circulation, vertically integrated liquid, echo top, wind profile and many other routine first order products are generated about every 6 minutes. Second order calculated products are also produced in near
real time. This capability has place modern weather forecasters quantum leaps beyond where they were only 25 years ago. We are all beneficiaries of this resource.

The next technological bound is Phased Array Radar, something first researched in the glorious 1960s along with other breakthroughs of that era. This is a new form of radar, disposing of the parabolic dish rotating around the horizon as most people imagine a radar to be. Instead, a wall of transmitters on a grid electronically aim radar energy at oblique angles- accomplishing the same
task as a parabolic dish without any moving parts, and doing it almost instantaneously. Our current radar network is slated to be updated to a Phased Array network during the 2020s.

Twice a day at 71 locations across the United States, weather balloons are released to sample a column of atmosphere from the surface up through about 100,000 feet. This type of remote sensing has been critical in assessing the vertical specifics of the atmosphere since a balloon was first launched in 1896 for this purpose in France. Radiosondes transmit temperature, humidity and pressure data back to the release site. Wind vectors are calculated by a balloons movement. Upper air soundings became very important in World War II for military aircraft operations. Today, the instruments are used as ground truth for satellite sensors also measuring atmospheric conditions. Nearly 800 locations worldwide send up weather balloons at midnight and noon Greenwich Mean Time. These data are crucial input to numerical computer models worldwide.

Surface weather observations used to be a completely human endeavor with manual observation of the sky and reading of thermometers and wind equipment. Today, routine ceiling and visibility measurements are obtained through ceilometers and transmissometers. They utilize lasers to automatically determine cloud heights and horizontal visibility. In fact, nearly everything that can be seen in the atmosphere can also be identified and quantified from great distances thanks to modern technology.

Weather buoys are deployed in every ocean to aid in the meteorological surveillance of vast expanses of water, covering 70% of the earth's surface. These lonely outposts, where only sea creatures may venture near, provide valuable information on sea temperature and other bathymetric sub-surface data which further our understanding of potential tropical storm conditions as well as larger entities, such as El Nino.

The remarkable American weather technology legacy includes advances in remote sensing, which have paid great dividends through the decades. Although it wasn't the case 100 years ago, we couldn't even imagine weather forecasting today without satellites, radar, weather balloons and the host of instrumentation at weather stations everywhere. This is the infrastructure that has literally made modern meteorology possible.