Melting-layer cloud observed over the tropical western Pacific

1.Introduction

Johnson et al. (1996) described prominent stable layers at heights of 2 km, 5 km, and 15-16 km during the TOGA COARE. Johnson et al. (1999) found that maxima in the vertical distributions of radar-echo (cloud) tops exist in the vicinity of these three stable layers heights.

It is a well known fact that environmental static stability influences the vertical profiles of detrainment from cumulus convection as well as the top heights of cumulus convection. It is therefore reasonable to expect that clouds detrained from cumulus convection (“detrainment shelves” of Mapes and Zuidema, 1996) would be frequently observed at around the specific layers as well as cloud tops.

In the present study, detrainment shelves are considered to be thin cloudy layers composed of small condensate particles of negligible terminal velocity (<0.1 mm in radius). The frequency distribution of thin cloud in the middle troposphere over the tropical western Pacific is examined with the use of 95 GHz cloud profiling radar and lidar.

2.Data

The data utilized in the present study is obtained over the tropical western Pacific by Res/V Mirai of Japan Marine Science and Technology Center (JAMSTEC). Stationary observation was conducted over the tropical western Pacific (around 1.85 N, 138 E) for the period of one month from November 9 to December 9, 2001 by Res/V Mirai. An active phase of convection of MJO passed over the observational area in the last 10 days of the period.

3.Procedures

Radar and lidar data were interpolated to produce consistent vertical and temporal resolution. The combined data used in the present analysis has a vertical resolution of 82.5 m and the time resolution of 1 minute. The base height of cloudy layers with little or no falling condensate particles was determined via the radar reflectivity factor and lidar backscattering coefficient, according to the following three steps.

I.The provisional base level of the cloudy layer (Zl) is defined as the level at which the lidar backscattering coefficient first exceeds a certain threshold (Lc) during a vertical scan from the height of 2 km. The lidar has two wavelengths, and the lower level is selected as Zl. The starting height of the vertical scan, 2 km, was selected in order to avoid aerosols and clouds within the boundary layer.

II.The provisional base level of the cloudy layer (Zr) is defined as the level at which the radar reflectivity factor first exceeds a certain threshold (Rc) during a vertical scan from a height of 2 km.

III.The ultimate base level of the cloudy layer (Zb) is defined by Zl for the case where the height of Zl is less than that of Zr, or where Zl is defined and Zr cannot be determined. Zb is considered to be indefinite in the case of the height of Zr being equal to or lower than that of Zl, or when Zl cannot be determined.

Radar is more sensitive than lidar in terms of falling condensate particles, and is able to detect such large particles at lower levels than lidar, provided an adequate Rc value is chosen. Therefore, when the height of Zr is equal to or lower than that of Zl, we can conclude that falling condensate particles exist. This forms the basis of defining Zb as indefinite in the third step outlined above. Conversely, lidar is more sensitive to small particles of negligible terminal velocity than radar. Accordingly, when the height of Zl is lower than that of Zr, or when Zr is indefinite, Zl can be considered to represent the base height of a cloudy layer with little or no falling condensate particles. As the absence of falling condensate particles indicates an absence of vigorous diabatic heating within a cloud, the vertical scale of the cloud would not be so large as the convective cloud or the stratiform cloud accompanied by a lot of falling condensate particles.

4.Results

Figure 1 shows the frequency distribution of the measured base heights of cloudy layers. Lc varies from -4.25 to -5.25 (= log10 â, where â (m-1 sr-1) is the backscattering coefficient) at 0.25 intervals, while Rc varies from -10 to -35 dBZe at 5 dBZe intervals. Values averaged over 30 (5 x 6) samples are displayed in Fig. 1. The error bars indicate the ranges within one standard deviation as calculated from the 30 samples.

A peak exists between the heights of 4.5 and 6.5 km. Although the characteristics of precipitation changes during the observation, the peak is apparent in the entire period. The mid-level peak apparent in Fig. 1, therefore, does not reflect a specific event. Moreover, the peak is located near the stable layer often observed. Accordingly, part of the thin cloudy layers probably represents detrainment shelves from the convective cloud that was enhanced by the stable layer.