By measuring the profile of rain as a function of altitude, TRMM is providing the first reliable global latent heating estimates ever made.

The burning of fossil fuels, industrial emissions and deforestation have markedly increased the concentrations of some greenhouse gases in the atmosphere. Scientists have determined that higher greenhouse gas concentrations are "enhancing" the greenhouse effect and thus leading to global climate change: because the greenhouse gases trap heat, the enhanced effect has been called global "warming", but the true nature of the change is multi-faceted and quite complex.

Indeed, our understanding of the mechanisms behind long-term climate change is still woefully lacking. As far as rain is concerned, we do know that the ultimate condensation of water vapor into falling rain 1) decreases the main greenhouse gas (water vapor) in the atmosphere; 2) alters the warming budget of the atmosphere through cloud formation; and 3) through the latent heat released, drives the global circulation in the atmosphere and therefore plays a major role in shaping global weather patterns. However, we are only beginning to understand how these mechanisms interact with one another. The TRMM instruments are allowing us to track and study, for the first time, the small-scale atmospheric changes due to rain, thereby helping improve our models of global circulation, and hence of the evolution of our weather and of the planet's climate.

These are the forces which govern the atmospheric circulation. The two most important factors which generate wind flow in the first place are 1) the substantial variation in the angle of the sun's rays as one moves from the equator to the poles, which makes the effect of solar radiation strongly dependent on latitude and on the earth's attitude with respect to the sun (i.e. on the season), and 2) the significant differences between the temperature of the oceans and that of continents -- land reacts to the seasonal change in solar radiation much faster than oceans do. The variation in the solar radiation between the equator and the poles and the subsequent "meridional" (north-south) temperature gradient produce "zonally symmetric" circulation (i.e. independent of longitude), which, as one moves towards the pole, gets progressively more strongly affected by the coriolis force and develops a zonal (east-west) component, until, at higher latitudes, the circulation becomes mostly zonal. The thermal imbalance between oceans and continents, along with the topography, are responsible for zonal asymmetries in the circulation.

By measuring the profile of rain as a function of altitude, TRMM is providing the first reliable latent heating estimates ever made throughout the tropics.

• the normal higher-pressure zone over the Eastern Pacific and the usual lower-pressure zone over the Western Pacific get reversed;
• the prevailing surface winds, the easterly trade winds, weaken;
• the mass of warmer water usually accumulated in the tropical Western Pacific ocean moves to the Eastern Pacific.

The TRMM estimates of the ocean surface rainfall are being used in ocean circulation models in order to improve the ability of the models to predict changes in ocean circulation and weather patterns such as El Niño events.

• P = Tahiti MSLP - Darwin MSLP,
• P_m = long-term mean of P for the month in question,
• and s(P) = standard deviation of P for the month in question.

Negative values of the SOI are characteristic of El Niño episodes. The SOI was positive during 1996 and early 1997. It first dropped below 0 (to -8.5) in march of 1997, and reached a minimum of -28.5 in march 1998. By comparison, the previous record lows were -39.3 in may 1896, -36.7 in april 1905, -34.2 in february 1983, and -30.7 in december 1940.

Positive values of the SOI are associated with stronger trade winds and warmer sea temperatures in the western Pacific, sometimes referred to as "La Niña" episodes. Waters in the central and eastern tropical Pacific Ocean become cooler during this time. The most recent strong La Niña was in 1988-89, with an SOI of 21 in november 1988; a rather weak La Niña occurred in 1995-96, with a maximum SOI of 13.9 in june 1996. The record maxima were 32.3 in april 1917, 30.4 in november 1973, and 21.3 in september 1975.

The pattern and amount of rain associated with the wet monsoon varies greatly from year to year and, in addition to affecting the general atmospheric circulation, it affects the economy of certain parts of the world very significantly (the Indian sub-continent for example). This explains the strong interest in improving the predictions of the monsoonal circulation. TRMM is, for the first time, providing accurate estimates of the rainfall over the strong-monsoon areas. This information is being fed into forecasting models to improve their accuracy.

In the tropics, convective systems such as tropical cyclones and squall lines are low-altitude lows topped by upper-level highs resulting from the warming of the lower-level air. Otherwise the pressure in the tropical atmosphere generically does not vary with altitude as strongly as it does away from the tropics: that is why the well-understood theory for mid-latitude weather dynamics does not apply to the tropics.

By supplying us with systematic latent-heating estimates over the tropics, TRMM is providing quantitative information about the driving force behind tropical weather. This will allow us to improve our global circulation models, and therefore improve our short- and longer-term climate predictions.

The TRMM instruments can detect convection quite accurately, especially over the oceans, where no other reliable means of localizing convection are available.

The most obvious difference between tornadoes and hurricanes is that they have drastically different scales. They form under different circumstances and have different impacts on the environment. Tornadoes are "small-scale circulations", the largest observed horizontal dimensions in the most severe cases being on the order of 1 to 1.5 miles. They most often form in association with severe thunderstorms which develop in the high wind-shear environment of the Central Plains during spring and early summer, when the large-scale wind flow provides favorable conditions for the sometimes violent clash between the moist warm air from the Gulf of Mexico with the cold dry continental air coming from the northwest. However, tornadoes can form in many different circumstances and places around the globe. Hurricane landfalls are often accompanied by multiple tornadoes. While tornadoes can cause much havoc on the ground (tornadic wind speeds have been estimated at 100 to more than 300 mph), they have very short lifetimes (on the order of minutes), and travel short distances. They have very little impact on the evolution of the surrounding storm, and basically do not affect the large-scale environment at all.

On the other hand, tropical cyclones (the term includes hurricanes and typhoons) are large-scale circulations with horizontal dimensions from 60 to well over 1000 miles in diameter. They form at low latitudes, generally between 5 and 20 degrees, but never right at the equator. They always form over the warm waters of the tropical oceans (sea-surface temperatures must be above 26.5° C, or about 76° F) where they draw their energy. They travel thousands of miles, persist over several days, and, during their lifetime, transport significant amounts of heat from the surface to the high altitudes of the tropical atmosphere. While their sporadic occurence prevents them from systematically influencing the large-scale circulation, they still affect it in ways which must be accounted for and need to be better understood.

The TRMM satellite is providing unique information about the structure and evolution of hurricanes. Indeed, some of the first data obtained from the satellite revealed the fine three-dimensional structure of the tropical cyclone Pam.

The orbit of the TRMM satellite brings it over Los Angeles relatively frequently, but it misses all points North of Santa Barbara. The follow-on mission will cover most of the United States.

However, away from the tropics, there is indeed more rain and snow in winter. That is due mainly to "fronts". Frontal systems are the most common weather systems at mid-latitudes, and winter systems are far stronger than summer ones, mainly because the temperature difference between the equator and the poles is greatest in winter (during summer any colder air mass quickly gets warmed up by the continents).

The TRMM instruments are helping us quantify the total rainfall over the tropics. By comparing with the measurements of identical instruments on other satellites, TRMM is also helping us improve our estimates of rain over the entire globe.

TRMM is providing important information about the size and frequency of occurence of rain storms in the tropics. This is filling a significant gap in our observations and, thus, increasing our knowledge about the water cycle and the atmospheric circulation over the globe: while small-scale rainstorms occur much more frequently in the tropics, only the large ones have a significant impact on the global circulation.

Since moister air is more unstable and has a much greater potential for producing clouds than drier air, precise knowledge of the dew point temperature distribution is of crucial importance for accurate prediction of storm development, propagation and evolution. By telling us where rain is falling, and how much, the TRMM data will improve our knowledge of the available (remaining) moisture over the oceans.

How do these particles form? First, tiny cloud droplets are born when the water vapor in the air is cooled and starts to condense around tiny "condensation nuclei" (particles so small they are invisible to the naked eye). The presence of these aerosols is crucial: without them, in absolutely clean air, condensation would not start until the relative humidity has reached several hundred percent (this suggests that the "saturation" level of 100% humidity is poorly defined; in fact, the atmosphere always contains more than enough nuclei of all sorts for condensation to start as soon as the dew point temperature is reached). The more particles there are in the atmosphere, the easier cloud droplets will be formed and the smaller they will be (since more particles will be competing for the same amount of water, so each one of them will attract less). This is why clouds over land have more droplets of smaller sizes than clouds over oceans where the air is generally much cleaner.

The process of ice formation similarly requires the presence of nuclei. However, there are much fewer particles which make suitable ice nuclei. This is why freezing often does not start until the temperature of the air reaches -15° C (if there are no ice nuclei at all, freezing will not occur before the temperature drops to -40° C). Hence, clouds with temperatures below 0° C can still consist of water droplets called "supercooled" water. These drops freeze immediately upon contact with any surface. When they fall to the ground as freezing rain, they can form a thin layer of sleet on roadways, an almost invisible and very dangerous hazard for drivers.

While the heat released during cloud formation plays a very important role in the development of the cloud and affects the atmosphere around it, there are no easy ways to measure it. The rain quantities and drop distributions measured by the TRMM satellite will be used in conjunction with cloud models to estimate for the first time the "latent" heat released during cloud growth and decay.

The radar on board the TRMM satellite can automatically distinguish between precipitating cumulus clouds and the harmless cirrus which TRMM's infrared sensor also detects.

In convective areas, the warming prevails at all levels. However, since melting and evaporation are strongest at lower levels, their effect leads to the decrease in low-level warming and, thus, explains the observed upper-level maximum in the warming.

In area-wide steady ("stratiform") rain which trails the convective storms, the low pressure created under the cloud by the heating draws air which, coming from outside the storm, is drier and therefore encourages the falling rain to evaporate, thus becoming cooler than the air before the storm.

So we have the contrast between the steady warming within a convective storm and the higher-altitude-warming/lower-altitude-cooling in stratiform rain. These two "latent heating" mechanisms are crucial to understanding the contribution of an individual tropical storm to the heat budget in the atmosphere. TRMM is providing the first reliable latent heating estimates ever made globally. While this latent heating occurs on the short time-scale of an individual storm's lifetime, it is a major driver of the global circulation, thus affecting the long-term atmospheric heating budget.

The TRMM radar is quite accurate in determining when a rainy column extends up past the freezing level, i.e. when the precipitation involves ice processes. The answer so far seems to be "less often than was once thought". TRMM is thus providing qualitative information about the frequency of occurence and intensity of ice processes in the tropics, and in this manner, could help evaluate their relative importance in rain formation. However, more data will be needed to obtain a more definite answer to the question. The radar on the follow-up mission will be equipped with additional capabilities to address this issue.

The passive microwave imager on the TRMM satellite can detect hail although it cannot determine at what altitude it is occuring. The follow-on mission will have a more sophisticated radar equipped with additional channels to estimate the amount of hail and melting ice particles below the freezing level.

where D is the diameter in cm. Hence a hailstone with a diameter on the order of 15 cm will fall at 75-80 m/s (170-180 miles/hour)!! This implies that updrafts of a comparable magnitude must exist in the cloud to support the hailstones long enough for them to grow. Because of this, hail is found only in very intense thunderstorms. Therefore, hail detection in storms is a clear indicator of their severity.

TRMM can detect the presence of hail in tropical storms but cannot determine at what height it is occuring. The radar on the follow-on mission will have specific channels which will detect frozen particles in general and hail in particular. This will provide crucial information about storm severity.

The TRMM instruments are not sensitive enough to identify fog or supercooled liquid clouds, but they are sensitive enough to detect minute amounts of rain, as little as 0.7 mm/hr or about one-tenth the rate at which a typical sprinkler system sprays water.

The TRMM mission is producing estimates of (not very accurate) three-hour, (moderately accurate) five-day and (unprecedentedly accurate) monthly totals of the accumulated precipitation over areas about the size of the Los Angeles basin. The follow-on mission will aim to produce very accurate three-hourly rain maps at a resolution that is sufficiently fine to predict flash-floods.

The reflectivity of a drop when illuminated by radar is roughly proportional to the square of its volume. It is this property which radar meteorologists exploit to estimate the total volume of rain from the reflectivity observed. This estimation process is rather difficult because the radar-rain relation is not linear, and the range of drop sizes within a single storm can vary greatly.

Because TRMM has passive instruments as well as the active radar, it is capable of estimating total rain volume and drop size distributions.

The TRMM radar does not have the capability to measure the fall speed of precipitating particles. Future spaceborne precipitation radars, however, will. This capability is important because it helps characterize the type of rain being measured.

In most storms, the TRMM radar is very good at detecting the freezing level. Unfortunately, it is not very good at discriminating between ice and liquid drops. The follow-on instrument will have separate channels to try to sort out the concentrations of frozen, melting, and liquid particles, thereby providing very detailed information about the evaporative cooling and/or condensational heating released into the atmosphere by rainstorms.

The TRMM estimates of the ocean surface rainfall are being used in ocean circulation models in order to improve the ability of the models to predict changes in ocean circulation and weather patterns such as El Niño events.

Unfortunately, the crucial ground observations are often simply not available over the tropics because these regions are typically inaccessible. In addition, the large-scale numerical models are still woefully inaccurate over the tropics, mainly because we do not yet have a solid understanding of the often severe dynamics which govern the circulation in the warm moist tropical atmosphere, and which have a determining effect on the weather far away from the tropics. The unprecedented precision of the TRMM measurements is replacing the missing ground measurements over the tropics, and it is helping fill the gap in our understanding of the processes which start around the equator but affect the weather all over the globe.

The TRMM radar is revealing the extent of the sensitivity of space-borne radar to the various types of precipitation (snow, ice, hail, rain) as well as to the presence of vertical up- and down-drafts. This information will help make system design choices for future airborne weather radars.