Tropical Clouds

Tropical Cloud Research

Why should we study Tropical Clouds?

Clouds are critical regulators of climate and weather and understanding their radiative impact is a challenge on any scale and in any region. For example, although the tropical western Pacific is known to be a source of energy on a planetary scale, the specifics of this heat engine are not well understood. There also remain important and outstanding questions of how clouds effect ocean temperature and weather processes in the tropics. Although tropical temperatures provide a more amenable environment than the Arctic for running instruments, other challenges are encountered in characterizing the vast ocean regions such as high seas and tropical storms, ship-borne engineering difficulties, and high-humidity environments.

Which lidars have obtained cloud measurements in the Tropics?
What engineering modifications were made in order to take data from a moving ship?

R/V Brown during EPIC. Mini-mopa lidar is on the left of the upper deck.

Three lidar instruments (DABUL, mini-MOPA, and HRDL) have been deployed on board NOAA research vessels to obtain measurements over the Tropical Oceans. The mini-MOPA and the HRDL have been most re-engineered, modified, and optimized in order to handle the added challenges of:

• high-humidity marine environments (because laser energy is absorbed by water vapor) through the use of different (i.e. less absorbing) laser wavelengths, and
• the constant and sometimes wild ship's motion, through real-time scanner stabilization with high resolution GPS integration.

Examples of Lidar Tropical Cloud Observations

Tropical clouds

DABUL observation of bimodal tropical clouds.

Cloud boundaries derrived from DABUL data.

Cirrus Clouds

One example of unique lidar measurements in the tropics is detection of very high and sometimes sub-visual cirrus clouds that are often missed by radars. Additionally, lidars like the DABUL can discriminate between the cloud and the precipitation (see Figure). From lidar measurements, taken during a 3 week period in the vicinity of Nauru Island (near Papua New Guinea), clouds were detected 46% of the time and displayed a bimodal distribution in the vertical (predominantly occurring below 1500 m and above 8000 m).

• All of the cloud measurements from DABUL during the NAURU experiment can be viewed online.

Tropical Boundary Layer Wind Profiles

During the EPIC cruise in 2001, the mini-MOPA Doppler lidar system was successfully deployed aboard the NOAA Ship Ronald H. Brown obtaining over 400 hours of atmospheric boundary layer and cloud measurements from August 24th through October 22nd, 2001.

During our two month cruise from Seattle, Washington, to Arica, Chile, the lidar obtained high-resolution DIAL (water vapor) and Doppler (wind speed and direction) profiles and cloud measurements from the surface up to as high as 6 km. The lidar data set spanned a variety of atmospheric regions; from the northern U.S. coastal zone where we observed water vapor pressures of 13 mb, to the tropical warm pool where water vapor pressures were as high as 31 mb and finally into the equatorial cold tongue region where water vapor dramatically dropped to 18 mb. In addition to the water vapor data, the lidar simultaneously measured wind speed and direction. The lidar documented many different wind features throughout the cruise including low-level wind shear layers, jets, and occasional thunderstorm outflow boundaries. Wind speeds ranged from dead-calm to 13 m/s. Boundary layer depths (deduced from aerosol returns) and cloud heights are also documented as part of the lidar data set. Detailed stratus cloud measurements were obtained as well as turbulent kinetic energy information in a variety of cloud conditions and transitions.