IR-Only Regimes

This section provides a brief outline of my article A three-hourly data set of the state of tropical convection based on cloud regimes in Geophysical Research Letters.

A shortcoming of the original ISCCP cloud regimes is their time resolutions. The optical thickness retrievals from the visible channel is not available at night, hence the regimes defined only during sunlit hours. As a result, the regimes are either of three-hour resolution but unavailable at night or of daily resolution by averaging over daytime.

In Tan and Jakob (2013), we proposed a method to construct these regimes using the infrared-only information (i.e. only with cloud top pressure histograms). This allows us to have three-hourly cloud regimes at all times of the day which are associable with the original regimes. We call them IR-only regimes. The method is

1. At each three-hour sunlit period, assign the joint-histograms to the original cloud regime.
2. For each original regime, find the corresponding IR-only CTP-histogram.
3. Average all the CTP-histograms within each original regime to obtain the CTP-centroids (Figure 1).
4. Assign every CTP-histogram to its new regime by the shortest Euclidean distance.

A chief drawback of this technique is that the regime named IC has to be discarded. This is because of a severe mismatch between its CTP-histogram and its joint-histogram (Figure 1), stemming from the fact that the thin cirrus which dominates in this regime is primarily due to a correction algorithm that adjusts the CTP based on the optical thickness.

Some of the evaluation and applications of the regimes can be found in Tan and Jakob (2013). To download the IR-only regimes, see below.

Details

Figure 1: The centroids for the IR-only regimes overlaying the centroids of the original regimes.

Original regime IR-only regime

Figure 2: Geographical distribution of the IR-only regimes compared to the original regimes. Click for larger image.

Download

The full dataset of the IR-only regimes can be downloaded at the Monash Weather and Climate dataset repository. Please contact me if you encounter any technical difficulties.

More information:

• Article published in Geophysical Research Letters: Tan and Jakob (2013)
• Presentation slides at the AMOS Annual Conference (13th Feb 2013): AMOS (2013) Slides