Tracking the Tropics With TRMM

EDIT: On 9 April, 2015, the TRMM satellite was turned off due to the exhaustion of its fuel supply after nearly 18 years of service. It burned up upon re-entry on 16 June, 2015.

Recently I wrote a post about NASA’s Terra and Aqua satellites, with a focus on the MODIS instrument they both carry. In this post I’ll focus on my favorite satellite: TRMM. The Tropical Rainfall Measurement Mission, or TRMM (pronounced like “trim”), is a joint mission between the US and Japan that revolves around a very unique satellite, also referred to a TRMM, launched in November of 1997.

The TRMM satellite and its instruments

The aim of the mission is to closely study tropical rainfall patterns, something that had never been done before. In order to constantly observe the tropics, and not waste time tracking over the mid-latitudes or the polar regions, TRMM has an unusual orbit path that is at a 35 degree angle to Earth’s equatorial plane as opposed to polar orbiting satellites which are at an angle close to 90 degrees, thus everything TRMM observes are in the Tropics or Subtropics. On board are five instruments: the Clouds and the Earth's Radiant Energy System (CERES), the Lightning Imaging Sensor (LIS), the Visible and Infrared Scanner (VIRS), the TRMM Microwave Imager (TMI), and the Precipitation Radar (PR).





CERES

LIS

The first two, CERES and LIS, are not directly part of the mission, they are instruments for other Earth observing purposes that were able to fit on the spacecraft. The other three instruments, VIRS, TMI, and PR, are designed to work together to paint a complete picture of rainfall.

VIRS

VIRS:
The VIRS package is a relatively standard system for observing weather in much the same way other meteorological satellites do. However, since it observes the same swath at the same time as the other instruments, VIRS allows for direct comparison of traditional IR and visible imagery with data obtained by the TMI and PR. For example, it can be used to see a particular cloud cluster, while the other instruments gather data on the rain (or ice crystals) that are being produced by those clouds.

TMI

TMI:
This microwave imager allows TRMM to look through the clouds and observe the structure at many different levels. The TMI gathers the same types of data that many other microwave observing meteorological satellites, but at a very high resolution. However, in order to make such detailed observations, the TMI’s swath is much narrower than that of the systems on other orbiters. Of the several frequencies of microwaves it can detect, perhaps the most notable are 85GHz and 37GHz, which provide a view of the details near the top of tall rain clouds and near the base of the clouds, respectively.

PR

PR:
The Precipitation Radar is TRMM’s primary instrument and the first space borne radar. With PR’s radar data the 3-D structure of clouds can be visualized and the intensity of precipitation anywhere within the clouds can be determined. These observations, along with data from the TMI, enable rainfall and the mechanisms that produce it to be understood in a more complete manner than ever before. One important characteristic of the PR is that unlike the other instruments on TRMM, or most satellites for that matter, is that it is an active system, as opposed to a passive one. This difference can be better understood with the aid of an analogy using more familiar tools. Suppose you are at the park and wish to take a photo of a tree. All that must be done is to press a button, the camera’s shutter will open for a split second, light will enter the camera, and the image of the tree will be recorded on the film. This example would be analogous to the TMI recording the microwaves that it collects. Now suppose that at the park is a gathering of all your family and extended family and you wish to take a group photo. First you would have to go around the park and gather all your family members together and arrange them for the picture, then you would set the timer on the camera and race over to the group so you would also be in the photograph, finally, after the camera takes the picture and the film is developed you will get to see how the picture turned out. This situation is sort of like the PR which has to first emit a beam that is scatted by precipitation , then it must receive any of the beam that is scattered back toward TRMM and use it determine the location and intensity of precipitation.






TRMM PR data of Hurricane Isaac

One significant use of TRMM observations is in the study of tropical cyclones. Using PR data the full 3-D structure of the storms’ features, such as the eyewall and rainbands, can be analyzed based on the precipitation within them. Databases now exist that provide this data for every time the PR’s swath passed over a tropical cyclone.

Worldview 3

In this shot of tropical storm 13S Felleng the highly asymmetric structure demonstrates the effect of shear on a tropical cyclone.

This rapidly developing mid-latitude cyclone (MLC) would eventually produce hurricane force winds. At this time it had just begun to be occluded and was in the process of ingesting a tropical disturbance, the remnants of which are at the end of the main front near the Philippines.

 

I've posted several images of von Karman waves before, but although this shot is of clouds formed in the lee of an island, this is clearly a very different type of wave.

 

This wide shot captures two tropical cyclones impacting opposite ends of Australia. On the left is TS Peta, which was very short lived, and TD Oswald, which lasted for a surprisingly long time over land before dissipating.

 

A major source of large scale weather features in the tropics are pressure waves that ripple around the Earth, each at different speeds, direction, and intensity. Parts of these waves favor cloud formation near the surface, such as convective cloud clusters and tropical cyclones. This very wide shot appears to show signs of one of these waves in the form of the rather evenly spaced systems that happen to be centered in each of the swaths, just north the equator. From the left the systems are: an invest area in the North Indian basin, TS Sonamu, an invest area just east of the Philippines, and some other unorganized cloud cluster.

 

Rain shadows are well known, but here the Olympic Mountains in Washington State are creating a noticeable "snow shadow".



In this image the location of the subtropical jet, a powerful upper level air flow, is betrayed by streaks of high cirrus over Southeast Asia.





This shot, which happens to be from just east of the image of Felleng above, captures a cloud cluster that happens to look a whole lot like the head of a dragon.

101: Conveyor Belt Model II

This is the image I ended with last time. It is an idealized model of a mid-latitude cyclone (MLC) as described by the Conveyor Belt Model (CBM), a concept to help resolve many of the issues that have arisen with the Norwegian Cyclone Model (NCM). Developed over the past few decades, the CBM considers the atmosphere in three dimensions, as opposed the NCM which mainly depicts the atmosphere in two dimensions. The key difference between the NCM and CBM is that the latter downplays the role of fronts as driving mechanisms of the system; instead it emphasizes the movement of the air masses involved in the system. The CBM also identifies several key air flows within the air masses referred to as conveyor belts, hence the name of the model. There are three belts (which are all depicted above in color): the Dry Tongue (green), the Cold Conveyor Belt (blue), and the Warm Conveyor Belt (red). It is these air flows that help define the fronts and also account for some of the features the NCM missed.

The Dry Tongue

This flow is responsible for much of the relative lack of cloudiness behind the cold front in the cold air mass. The dry tongue is a jet of air from high in the troposphere that dives down in the general direction of the low center, becoming very dry in the process. Right as nears the low center, the flow abruptly ascends back up and joins the upper level flow, which is generally towards the east or northeast. This dry air flow is often easy to locate on visible satellite images based on the effect it has on cloud cover. Being a dry jet, this conveyor belt inhibits cloud formation, thus it will often scour out higher level clouds in the large cloud shield associated with the MLC. This effect is often noticeable in imagery, since the clouds adjacent to the dry flow will often cast a noticeable shadow on the low clouds beneath the dry air, as seen in the visible satellite image below of a developing MLC in the Northwest Pacific on March 5, 2013.

For the other two conveyor belts I’ll be using data from the October, 2005 storm that was introduced in the first “101: Conveyor Belt Model” post. The most important product of this data are air parcel trajectories. This type of display, show here in animations, uses wind data to calculate the path an arbitrary parcel of air would take over some period of time. Following a few air parcels allows the general air flow to be inferred, and thus the orientation of the cold and warm conveyor belts can be identified. All of these animations feature a few parcels that begin at lower levels, although each animation begins at a different time and are of different durations in order to best capture air flow that best exemplifies the feature being discussed. On the “ground” of each animation is the corresponding infrared satellite image. The MODIS image (reproduced below) of the storm that was used previously along with the corresponding surface map shows the relative placement of the fronts, thus a basic sense of the locations of the fronts can be estimated throughout most of the animations. The colors in these animations gives a sense of the temperature of the air, so air near the surface will be warmer (red and orange) and the air high up will be much colder (green and blue).

The Cold Conveyor Belt

Its best to being this explanation by showing the trajectories of air in the cold air mass near the cold front, which, for the purpose of this discussion, is defined in the same way as it has been traditionally defined by the NCM. Notice how most of the parcels remain relatively low and all of them stay just behind the dense line of clouds, which happens to contain the cold front.

Starting from an earlier time shows air moving roughly towards the center of the low from the east. This air is quite chilly and like the air near the cold front, it stays pretty low. An important detail is that all of these parcels remain north of the large fan of clouds that contain the warm front. In fact, this conveyor belt generally slips in right under the warm front and just north (or nearly north) of the surface warm front.

The Warm Conveyor Belt

It is the depiction of the warm air mass that really gets expanded upon in the CBM. In the animation below, air parcels have been placed directly in front of the cold front, which is the dense line of clouds. The display itself is looking roughly to the south and the cold front is moving from west to east, with the cold air mass on the right (west). Notice how at first the parcels seem to essentially stay at the same location relative to the cold front, but ascend rather abruptly. Because this air is from the warm air mass, it is less dense than the cold air behind the cold front, so it is being forced upward. Eventually the parcels level off as they reach an altitude in which there is no longer a well-defined front and begin to track above the cold air mass. This part, at least, is pretty consistent with the NCM.

This final animation shows the trajectory of warm conveyor belt air parcels that begin the warm air mass and ascend up the warm front. It is this flow that might be the most significant change made by the CBM. In the classic NCM, the warm front is usually depicted as the boundary between cool air to the north and warm air from the south gliding over the dense cool air. Such a definition tends to paint the warm front as a relatively low energy boundary, after all, there is no cold air plowing through the warm air, it is simply sliding up a “ramp”. How things have changed. The CBM’s depiction of the warm air flow near the warm front is less of a glide, and more like a rocket. In this new concept, the air in the warm conveyor belt abruptly ascends as it reaches the warm front and levels back off at a very high altitude. From here, it follows the upper level flow (i.e. jet stream) along with the other two conveyor belts, although they are at a somewhat lower altitude.

To summarize all of this, I’ve included the image below. This is the same model that was used at the beginning of the post, but in a different view, and with some of the features labeled.